An Introduction to
Programming in Emacs Lisp

An Introduction to
Programming in Emacs Lispby Robert J. ChassellThis is An Introduction to Programming in Emacs Lisp, for people who are notprogrammers.

Distributed with Emacs version 29.1.90.
Copyright c 1990–1995, 1997, 2001–2024 Free Software Foundation, Inc.
Published by the:GNU Press,
a division of theFree Software Foundation, Inc.
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ISBN 1-882114-43-4
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any later version published by the Free Software Foundation; there being
no Invariant Section, with the Front-Cover Texts being “A GNU Manual”,
and with the Back-Cover Texts as in (a) below. A copy of the license isincluded in the section entitled “GNU Free Documentation License”.
(a) The FSF’s Back-Cover Text is: “You have the freedom to copy andmodify this GNU manual. Buying copies from the FSF supports it indeveloping GNU and promoting software freedom.”

i

Short Contents
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1

List Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2Practicing Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3
4

How To Write Function Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
A Few Buffer-Related Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5

A Few More Complex Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6

Narrowing and Widening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

78car, cdr, cons: Fundamental Functions . . . . . . . . . . . . . . . . . . . . . . 77
Cutting and Storing Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

9How Lists are Implemented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

10 Yanking Text Back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
11 Loops and Recursion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
12 Regular Expression Searches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
13 Counting via Repetition and Regexps . . . . . . . . . . . . . . . . . . . . . . . 155
14 Counting Words in a defun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
15 Readying a Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
16 Your .emacs File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
17 Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
18 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
A
B

The the-the Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
Handling the Kill Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

C

A Graph with Labeled Axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

DFree Software and Free Manuals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254

E GNU Free Documentation License . . . . . . . . . . . . . . . . . . . . . . . . . . 256
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

ii

Table of Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
On Reading this Text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
For Whom This is Written. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Lisp History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
A Note for Novices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Thank You . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1

List Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1

Lisp Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Lisp Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Whitespace in Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.3 GNU Emacs Helps You Type Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Run a Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Generate an Error Message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Symbol Names and Function Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 The Lisp Interpreter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5.1 Byte Compiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.6 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.6.1 Evaluating Inner Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.7 Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.7.1 Error Message for a Symbol Without a Function . . . . . . . . . . . . . . . . . 10
1.7.2 Error Message for a Symbol Without a Value . . . . . . . . . . . . . . . . . . . . 10
1.8 Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.8.1 Arguments’ Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.8.2 An Argument as the Value of a Variable or List . . . . . . . . . . . . . . . . . 12
1.8.3 Variable Number of Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.8.4 Using the Wrong Type Object as an Argument . . . . . . . . . . . . . . . . . . 13
1.8.5 The message Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.9 Setting the Value of a Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.9.1 Using setq. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.9.2 Counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.11 Exercises. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2

Practicing Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.1
2.2
2.3
2.4
2.5

Buffer Names . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Getting Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Switching Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Buffer Size and the Location of Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Exercise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

iii

3

How To Write Function Definitions . . . . . . . . . . . . . . . . . . . . . . 26
3.1
3.2

The defun Macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Install a Function Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2.1 Change a Function Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3 Make a Function Interactive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.3.1 An Interactive multiply-by-seven. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4 Different Options for interactive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Install Code Permanently . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.6 let . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.6.1 The Parts of a let Expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.6.2 Sample let Expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.6.3 Uninitialized Variables in a let Statement . . . . . . . . . . . . . . . . . . . . . . . 35
3.6.4 How let Binds Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Differences Between Lexical and Dynamic Binding . . . . . . . . . . . . . . . . . . 36
Example of Lexical vs. Dynamic Binding . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7 The if Special Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.7.1 The type-of-animal Function in Detail . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.8 If–then–else Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.9 Truth and Falsehood in Emacs Lisp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.10 save-excursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.10.1 Template for a save-excursion Expression . . . . . . . . . . . . . . . . . . . . 43
3.11 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.12 Exercises. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4

A Few Buffer-Related Functions . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.1
4.2
4.3

Finding More Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
A Simplified beginning-of-buffer Definition . . . . . . . . . . . . . . . . . . . . . . . . 49
The Definition of mark-whole-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.3.1 Body of mark-whole-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4 The Definition of append-to-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.4.1 The append-to-buffer Interactive Expression . . . . . . . . . . . . . . . . . . 53
4.4.2 The Body of append-to-buffer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.4.3 save-excursion in append-to-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.5 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.6 Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

5

A Few More Complex Functions . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.1
5.2

The Definition of copy-to-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
The Definition of insert-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2.1 The Interactive Expression in insert-buffer . . . . . . . . . . . . . . . . . . . 61
A Read-only Buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
‘b’ in an Interactive Expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.2.2 The Body of the insert-buffer Function . . . . . . . . . . . . . . . . . . . . . . . 61
5.2.3 insert-buffer With an if Instead of an or . . . . . . . . . . . . . . . . . . . . 62

iv
5.2.4 The or in the Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.2.5 The let Expression in insert-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.2.6 New Body for insert-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.3 Complete Definition of beginning-of-buffer . . . . . . . . . . . . . . . . . . . . . . . . 65
5.3.1 Optional Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.3.2 beginning-of-buffer with an Argument . . . . . . . . . . . . . . . . . . . . . . . 67
What happens in a large buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
What happens in a small buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.3.3 The Complete beginning-of-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.4 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
5.5 optional Argument Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

6

Narrowing and Widening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.1
6.2
6.3

7

The save-restriction Special Form. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
what-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Exercise with Narrowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

car, cdr, cons: Fundamental Functions . . . . . . . . . . . . . . . . . . . 77
7.1
7.2

car and cdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
cons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
7.2.1 Find the Length of a List: length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
7.3 nthcdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
7.4 nth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
7.5 setcar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7.6 setcdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
7.7 Exercise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

8

Cutting and Storing Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
8.1

zap-to-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
8.1.1 The interactive Expression. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.1.2 The Body of zap-to-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8.1.3 The search-forward Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8.1.4 The progn Special Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
8.1.5 Summing up zap-to-char . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
8.2 kill-region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
8.2.1 condition-case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.2.2 Lisp macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
8.3 copy-region-as-kill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
8.3.1 The Body of copy-region-as-kill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
The kill-append function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
The kill-new function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
8.4 Digression into C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
8.5 Initializing a Variable with defvar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
8.5.1 defvar and an asterisk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

v
8.6
8.7

9

How Lists are Implemented . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
9.1
9.2

10

Symbols as a Chest of Drawers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Yanking Text Back . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
10.1
10.2
10.3

11

Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Searching Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Kill Ring Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
The kill-ring-yank-pointer Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Exercises with yank and nthcdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

Loops and Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
11.1 while. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
11.1.1 A while Loop and a List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
11.1.2 An Example: print-elements-of-list . . . . . . . . . . . . . . . . . . . . . . 117
11.1.3 A Loop with an Incrementing Counter . . . . . . . . . . . . . . . . . . . . . . . . 118
Example with incrementing counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
The parts of the function definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Putting the function definition together . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
11.1.4 Loop with a Decrementing Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
Example with decrementing counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
The parts of the function definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Putting the function definition together . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
11.2 Save your time: dolist and dotimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
The dolist Macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
The dotimes Macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
11.3 Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
11.3.1 Building Robots: Extending the Metaphor . . . . . . . . . . . . . . . . . . . . 127
11.3.2 The Parts of a Recursive Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
11.3.3 Recursion with a List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
11.3.4 Recursion in Place of a Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
An argument of 3 or 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
11.3.5 Recursion Example Using cond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
11.3.6 Recursive Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
Recursive Pattern: every . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
Recursive Pattern: accumulate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Recursive Pattern: keep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
11.3.7 Recursion without Deferments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
11.3.8 No Deferment Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
11.4 Looping Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

vi

12

Regular Expression Searches . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
12.1 The Regular Expression for sentence-end . . . . . . . . . . . . . . . . . . . . . . . . . 141
12.2 The re-search-forward Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
12.3 forward-sentence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
The while loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
The regular expression search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
12.4 forward-paragraph: a Goldmine of Functions . . . . . . . . . . . . . . . . . . . . . 147
The let* expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
The forward motion while loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
12.5 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
12.6 Exercises with re-search-forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

13

Counting via Repetition and Regexps . . . . . . . . . . . . . . . . . 155
13.1 The count-words-example Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
13.1.1 The Whitespace Bug in count-words-example . . . . . . . . . . . . . . . . 158
13.2 Count Words Recursively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
13.3 Exercise: Counting Punctuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

14Counting Words in a defun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
14.1 What to Count? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
14.2 What Constitutes a Word or Symbol? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
14.3 The count-words-in-defun Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
14.4 Count Several defuns Within a File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
14.5 Find a File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
14.6 lengths-list-file in Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
14.7 Count Words in defuns in Different Files. . . . . . . . . . . . . . . . . . . . . . . . . . . 175
14.7.1 The append Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
14.8 Recursively Count Words in Different Files . . . . . . . . . . . . . . . . . . . . . . . . . 177
14.9 Prepare the Data for Display in a Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
14.9.1 Sorting Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
14.9.2 Making a List of Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
14.9.3 Counting function definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

15

Readying a Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
15.1
15.2
15.3
15.4

The graph-body-print Function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
The recursive-graph-body-print Function . . . . . . . . . . . . . . . . . . . . . . . 194
Need for Printed Axes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

vii

16

Your .emacs File. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
16.1 Site-wide Initialization Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
16.2 Specifying Variables using defcustom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
16.3 Beginning a .emacs File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
16.4 Text and Auto Fill Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
16.5 Mail Aliases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
16.6 Indent Tabs Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
16.7 Some Key Bindings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
16.8 Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
16.9 Loading Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
16.10 Autoloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
16.11 A Simple Extension: line-to-top-of-window . . . . . . . . . . . . . . . . . . . . 207
16.12 X11 Colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
16.13 Miscellaneous Settings for a .emacs File . . . . . . . . . . . . . . . . . . . . . . . . . . . 210
16.14 A Modified Mode Line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212

17

Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
17.1 debug. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
17.2 debug-on-entry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
17.3 debug-on-quit and (debug) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
17.4 The edebug Source Level Debugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
17.5 Debugging Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

18

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

Appendix A

The the-the Function . . . . . . . . . . . . . . . . . . . . . . . . 223

Appendix B

Handling the Kill Ring . . . . . . . . . . . . . . . . . . . . . . 225

B.1 The current-kill Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
B.2 yank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
B.3 yank-pop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
B.4 The ring.el File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

Appendix C
C.1
C.2

A Graph with Labeled Axes . . . . . . . . . . . . . . . . 233

The print-graph Varlist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
The print-Y-axis Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234
C.2.1 Side Trip: Compute a Remainder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
C.2.2 Construct a Y Axis Element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
C.2.3 Create a Y Axis Column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
C.2.4 The Not Quite Final Version of print-Y-axis . . . . . . . . . . . . . . . . . 240
C.3 The print-X-axis Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
C.3.1 X Axis Tic Marks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

viii
C.4

Printing the Whole Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
C.4.1 Testing print-graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
C.4.2 Graphing Numbers of Words and Symbols . . . . . . . . . . . . . . . . . . . . . 248
C.4.3 A lambda Expression: Useful Anonymity . . . . . . . . . . . . . . . . . . . . . . . 249
C.4.4 The mapcar Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
C.4.5 Another Bug . . . Most Insidious . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
C.4.6 The Printed Graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253

Appendix D Free Software and Free Manuals . . . . . . . . . . . . 254
Appendix E

GNU Free Documentation License . . . . . . . . . 256

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

On Reading this Text

1

Preface
Most of the GNU Emacs integrated environment is written in the programming
language called Emacs Lisp. The code written in this programming language is thesoftware—the sets of instructions—that tell the computer what to do when you giveit commands. Emacs is designed so that you can write new code in Emacs Lisp andeasily install it as an extension to the editor.
(GNU Emacs is sometimes called an “extensible editor”, but it does much morethan provide editing capabilities. It is better to refer to Emacs as an “extensible
computing environment”. However, that phrase is quite a mouthful. It is easier torefer to Emacs simply as an editor. Moreover, everything you do in Emacs—findthe Mayan date and phases of the moon, simplify polynomials, debug code, manage
files, read letters, write books—all these activities are kinds of editing in the mostgeneral sense of the word.)
Although Emacs Lisp is usually thought of in association only with Emacs, it isa full computer programming language. You can use Emacs Lisp as you would anyother programming language.
Perhaps you want to understand programming; perhaps you want to extend
Emacs; or perhaps you want to become a programmer. This introduction to Emacs
Lisp is designed to get you started: to guide you in learning the fundamentals ofprogramming, and more importantly, to show you how you can teach yourself to go
further.

On Reading this Text
All through this document, you will see little sample programs you can run insideof Emacs. If you read this document in Info inside of GNU Emacs, you can run the
programs as they appear. (This is easy to do and is explained when the examples
are presented.) Alternatively, you can read this introduction as a printed bookwhile sitting beside a computer running Emacs. (This is what I like to do; I likeprinted books.) If you don’t have a running Emacs beside you, you can still readthis book, but in this case, it is best to treat it as a novel or as a travel guide to a
country not yet visited: interesting, but not the same as being there.
Much of this introduction is dedicated to walkthroughs or guided tours of codeused in GNU Emacs. These tours are designed for two purposes: first, to give
you familiarity with real, working code (code you use every day); and, second,to give you familiarity with the way Emacs works. It is interesting to see how aworking environment is implemented. Also, I hope that you will pick up the habit ofbrowsing through source code. You can learn from it and mine it for ideas. Having
GNU Emacs is like having a dragon’s cave of treasures.
In addition to learning about Emacs as an editor and Emacs Lisp as a programming language, the examples and guided tours will give you an opportunity to get
acquainted with Emacs as a Lisp programming environment. GNU Emacs supports
programming and provides tools that you will want to become comfortable using,such as M-. (the key which invokes the xref-find-definitions command). You
will also learn about buffers and other objects that are part of the environment.

2

Preface

Learning about these features of Emacs is like learning new routes around yourhome town.
Finally, I hope to convey some of the skills for using Emacs to learn aspects ofprogramming that you don’t know. You can often use Emacs to help you understandwhat puzzles you or to find out how to do something new. This self-reliance is not
only a pleasure, but an advantage.

For Whom This is WrittenThis text is written as an elementary introduction for people who are not programmers. If you are a programmer, you may not be satisfied with this primer. The
reason is that you may have become expert at reading reference manuals and be
put off by the way this text is organized.
An expert programmer who reviewed this text said to me:
I prefer to learn from reference manuals. I “dive into” each paragraph,and “come up for air” between paragraphs.
When I get to the end of a paragraph, I assume that subject is done, finished, that I know everything I need (with the possible exception of thecase when the next paragraph starts talking about it in more detail). I
expect that a well written reference manual will not have a lot of redundancy, and that it will have excellent pointers to the (one) place where theinformation I want is.
This introduction is not written for this person!
Firstly, I try to say everything at least three times: first, to introduce it; second,
to show it in context; and third, to show it in a different context, or to review it.
Secondly, I hardly ever put all the information about a subject in one place,much less in one paragraph. To my way of thinking, that imposes too heavy aburden on the reader. Instead I try to explain only what you need to know at thetime. (Sometimes I include a little extra information so you won’t be surprised later
when the additional information is formally introduced.)
When you read this text, you are not expected to learn everything the firsttime. Frequently, you need make only a nodding acquaintance with some of theitems mentioned. My hope is that I have structured the text and given you enoughhints that you will be alert to what is important, and concentrate on it.
You will need to dive into some paragraphs; there is no other way to read them.
But I have tried to keep down the number of such paragraphs. This book is intended
as an approachable hill, rather than as a daunting mountain.
This book, An Introduction to Programming in Emacs Lisp, has a companion
document, The GNU Emacs Lisp Reference Manual. The reference manual has moredetail than this introduction. In the reference manual, all the information about
one topic is concentrated in one place. You should turn to it if you are like theprogrammer quoted above. And, of course, after you have read this Introduction,
you will find the Reference Manual useful when you are writing your own programs.

A Note for Novices

3

Lisp History
Lisp was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language
makes it superior for other purposes as well, such as writing editor commands andintegrated environments.
GNU Emacs Lisp is largely inspired by Maclisp, which was written at MIT in
the 1960s. It is somewhat inspired by Common Lisp, which became a standard
in the 1980s. However, Emacs Lisp is much simpler than Common Lisp. (Thestandard Emacs distribution contains an optional extensions file, cl-lib.el, that
adds many Common Lisp features to Emacs Lisp.)

A Note for Novices
If you don’t know GNU Emacs, you can still read this document profitably. However, I recommend you learn Emacs, if only to learn to move around your computer
screen. You can teach yourself how to use Emacs with the built-in tutorial. To use
it, type C-h t. (This means you press and release the CTRL key and the h at thesame time, and then press and release t.)Also, I often refer to one of Emacs’s standard commands by listing the keys
which you press to invoke the command and then giving the name of the commandin parentheses, like this: M-C-\ (indent-region). What this means is that theindent-region command is customarily invoked by typing M-C-\. (You can, if you
wish, change the keys that are typed to invoke the command; this is called rebinding.
See Section 16.8 “Keymaps”, page 204.) The abbreviation M-C-\ means that you
type your META key, CTRL key and \ key all at the same time. (On many modern
keyboards the META key is labeled ALT.) Sometimes a combination like this is calleda keychord, since it is similar to the way you play a chord on a piano. If yourkeyboard does not have a META key, the ESC key prefix is used in place of it. In thiscase, M-C-\ means that you press and release your ESC key and then type the CTRL
key and the \ key at the same time. But usually M-C-\ means press the CTRL key
along with the key that is labeled ALT and, at the same time, press the \ key.
In addition to typing a lone keychord, you can prefix what you type with C-u,which is called the universal argument. The C-u keychord passes an argument to thesubsequent command. Thus, to indent a region of plain text by 6 spaces, mark theregion, and then type C-u 6 M-C-\. (If you do not specify a number, Emacs either
passes the number 4 to the command or otherwise runs the command differently
than it would otherwise.) See Section “Numeric Arguments” in The GNU Emacs
Manual.
If you are reading this in Info using GNU Emacs, you can read through thiswhole document just by pressing the space bar, SPC. (To learn about Info, type C-hi and then select Info.)
A note on terminology: when I use the word Lisp alone, I often am referringto the various dialects of Lisp in general, but when I speak of Emacs Lisp, I am
referring to GNU Emacs Lisp in particular.

4

PrefaceThank YouMy thanks to all who helped me with this book. My especial thanks to
Jim Blandy, Noah Friedman, Jim Kingdon, Roland McGrath, Frank Ritter,
Randy Smith, Richard M. Stallman, and Melissa Weisshaus. My thanks also go
to both Philip Johnson and David Stampe for their patient encouragement. My
mistakes are my own.
Robert J. Chassell
bob@gnu.org

Section 1.1: Lisp Lists

1

1 List Processing
To the untutored eye, Lisp is a strange programming language. In Lisp code there
are parentheses everywhere. Some people even claim that the name stands for“Lots of Isolated Silly Parentheses”. But the claim is unwarranted. Lisp stands for
LISt Processing, and the programming language handles lists (and lists of lists) by
putting them between parentheses. The parentheses mark the boundaries of thelist. Sometimes a list is preceded by an apostrophe ‘'’, called a single-quote in
Lisp.1 Lists are the basis of Lisp.

1.1 Lisp Lists
In Lisp, a list looks like this: '(rose violet daisy buttercup). This list is preceded by a single apostrophe. It could just as well be written as follows, which looks
more like the kind of list you are likely to be familiar with:'(rosevioletdaisy
buttercup)

The elements of this list are the names of the four different flowers, separated fromeach other by whitespace and surrounded by parentheses, like flowers in a field witha stone wall around them.
Lists can also have numbers in them, as in this list: (+ 2 2). This list has aplus-sign, ‘+’, followed by two ‘2’s, each separated by whitespace.
In Lisp, both data and programs are represented the same way; that is, they areboth lists of words, numbers, or other lists, separated by whitespace and surroundedby parentheses. (Since a program looks like data, one program may easily serveas data for another; this is a very powerful feature of Lisp.) (Incidentally, thesetwo parenthetical remarks are not Lisp lists, because they contain ‘;’ and ‘.’ aspunctuation marks.)
Here is another list, this time with a list inside of it:'(this list has (a list inside of it))

The components of this list are the words ‘this’, ‘list’, ‘has’, and the list ‘(alist inside of it)’. The interior list is made up of the words ‘a’, ‘list’, ‘inside’,‘of’, ‘it’.
1.1.1 Lisp Atoms
In Lisp, what we have been calling words are called atoms. This term comes fromthe historical meaning of the word atom, which means “indivisible”. As far as
Lisp is concerned, the words we have been using in the lists cannot be divided into
any smaller parts and still mean the same thing as part of a program; likewise withnumbers and single character symbols like ‘+’. On the other hand, unlike an ancient
1

A single-quote is an abbreviation for the special form quote; you need not think about
special forms now. See Section 1.5 “Lisp Interpreter”, page 6.

2

Chapter 1: List Processing

atom, a list can be split into parts. (See Chapter 7 “car cdr & cons Fundamental
Functions”, page 77.)
In a list, atoms are separated from each other by whitespace. They can be rightnext to a parenthesis.
Technically speaking, a list in Lisp consists of parentheses surrounding atoms
separated by whitespace or surrounding other lists or surrounding both atoms andother lists. A list can have just one atom in it or have nothing in it at all. A list
with nothing in it looks like this: (), and is called the empty list. Unlike anythingelse, an empty list is considered both an atom and a list at the same time.
The printed representation of both atoms and lists are called symbolic expressions or, more concisely, s-expressions. The word expression by itself can refer toeither the printed representation, or to the atom or list as it is held internally in thecomputer. Often, people use the term expression indiscriminately. (Also, in manytexts, the word form is used as a synonym for expression.)
Incidentally, the atoms that make up our universe were named such when they
were thought to be indivisible; but it has been found that physical atoms are notindivisible. Parts can split off an atom or it can fission into two parts of roughly
equal size. Physical atoms were named prematurely, before their truer nature wasfound. In Lisp, certain kinds of atom, such as an array, can be separated into parts;
but the mechanism for doing this is different from the mechanism for splitting alist. As far as list operations are concerned, the atoms of a list are unsplittable.
As in English, the meanings of the component letters of a Lisp atom are different
from the meaning the letters make as a word. For example, the word for the SouthAmerican sloth, the ‘ai’, is completely different from the two words, ‘a’, and ‘i’.
There are many kinds of atom in nature but only a few in Lisp: for example, numbers, such as 37, 511, or 1729, and symbols, such as ‘+’, ‘foo’, or‘forward-line’. The words we have listed in the examples above are all symbols.
In everyday Lisp conversation, the word “atom” is not often used, because programmers usually try to be more specific about what kind of atom they are dealing with.
Lisp programming is mostly about symbols (and sometimes numbers) within lists.
(Incidentally, the preceding three word parenthetical remark is a proper list in Lisp,
since it consists of atoms, which in this case are symbols, separated by whitespaceand enclosed by parentheses, without any non-Lisp punctuation.)
Text between double quotation marks—even sentences or paragraphs—is also
an atom. Here is an example:'(this list includes "text between quotation marks.")

In Lisp, all of the quoted text including the punctuation mark and the blank spaces
is a single atom. This kind of atom is called a string (for “string of characters”)
and is the sort of thing that is used for messages that a computer can print for ahuman to read. Strings are a different kind of atom than numbers or symbols andare used differently.
1.1.2 Whitespace in Lists

Section 1.2: Run a Program

3

The amount of whitespace in a list does not matter. From the point of view of theLisp language,'(this list
looks like this)is exactly the same as this:'(this list looks like this)Both examples show what to Lisp is the same list, the list made up of thesymbols ‘this’, ‘list’, ‘looks’, ‘like’, and ‘this’ in that order.
Extra whitespace and newlines are designed to make a list more readable byhumans. When Lisp reads the expression, it gets rid of all the extra whitespace(but it needs to have at least one space between atoms in order to tell them apart.)
Odd as it seems, the examples we have seen cover almost all of what Lisp lists
look like! Every other list in Lisp looks more or less like one of these examples,
except that the list may be longer and more complex. In brief, a list is between
parentheses, a string is between quotation marks, a symbol looks like a word, anda number looks like a number. (For certain situations, square brackets, dots anda few other special characters may be used; however, we will go quite far withoutthem.)
1.1.3 GNU Emacs Helps You Type Lists
When you type a Lisp expression in GNU Emacs using either Lisp Interaction modeor Emacs Lisp mode, you have available to you several commands to format the Lisp
expression so it is easy to read. For example, pressing the TAB key automaticallyindents the line the cursor is on by the right amount. A command to properly
indent the code in a region is customarily bound to M-C-\. Indentation is designed
so that you can see which elements of a list belong to which list—elements of a
sub-list are indented more than the elements of the enclosing list.
In addition, when you type a closing parenthesis, Emacs momentarily jumps the
cursor back to the matching opening parenthesis, so you can see which one it is.
This is very useful, since every list you type in Lisp must have its closing parenthesis
match its opening parenthesis. (See Section “Major Modes” in The GNU Emacs
Manual, for more information about Emacs’s modes.)1.2 Run a Program
A list in Lisp—any list—is a program ready to run. If you run it (for which the Lisp
jargon is evaluate), the computer will do one of three things: do nothing except
return to you the list itself; send you an error message; or, treat the first symbol in
the list as a command to do something. (Usually, of course, it is the last of thesethree things that you really want!)
The single apostrophe, ', that I put in front of some of the example lists inpreceding sections is called a quote; when it precedes a list, it tells Lisp to do
nothing with the list, other than take it as it is written. But if there is no quotepreceding a list, the first item of the list is special: it is a command for the computer
to obey. (In Lisp, these commands are called functions.) The list (+ 2 2) shown

4

Chapter 1: List Processing

above did not have a quote in front of it, so Lisp understands that the + is an
instruction to do something with the rest of the list: add the numbers that follow.
If you are reading this inside of GNU Emacs in Info, here is how you can evaluate
such a list: place your cursor immediately after the right hand parenthesis of thefollowing list and then type C-x C-e:(+ 2 2)

You will see the number 4 appear in the echo area2 . (What you have just done isevaluate the list. The echo area is the line at the bottom of the screen that displays
or echoes text.) Now try the same thing with a quoted list: place the cursor rightafter the following list and type C-x C-e:'(this is a quoted list)

You will see (this is a quoted list) appear in the echo area.
In both cases, what you are doing is giving a command to the program insideof GNU Emacs called the Lisp interpreter—giving the interpreter a command toevaluate the expression. The name of the Lisp interpreter comes from the word forthe task done by a human who comes up with the meaning of an expression—who
interprets it.
You can also evaluate an atom that is not part of a list—one that is not surrounded by parentheses; again, the Lisp interpreter translates from the humanly
readable expression to the language of the computer. But before discussing this(see Section 1.7 “Variables”, page 9), we will discuss what the Lisp interpreter
does when you make an error.

1.3 Generate an Error Message
Partly so you won’t worry if you do it accidentally, we will now give a command to
the Lisp interpreter that generates an error message. This is a harmless activity;and indeed, we will often try to generate error messages intentionally. Once you
understand the jargon, error messages can be informative. Instead of being called“error” messages, they should be called “help” messages. They are like signposts toa traveler in a strange country; deciphering them can be hard, but once understood,
they can point the way.
The error message is generated by a built-in GNU Emacs debugger. We willenter the debugger. You get out of the debugger by typing q.
What we will do is evaluate a list that is not quoted and does not have ameaningful command as its first element. Here is a list almost exactly the same as
the one we just used, but without the single-quote in front of it. Position the cursor
right after it and type C-x C-e:
(this is an unquoted list)

A *Backtrace* window will open up and you should see the following in it:
2

Emacs shows integer values in decimal, in octal and in hex, and also as a character,but let’s ignore this convenience feature for now.

Section 1.3: Generate an Error Message

5---------- Buffer: *Backtrace* ---------Debugger entered--Lisp error: (void-function this)(this is an unquoted list)
eval((this is an unquoted list) nil)elisp--eval-last-sexp(nil)eval-last-sexp(nil)funcall-interactively(eval-last-sexp nil)call-interactively(eval-last-sexp nil nil)command-execute(eval-last-sexp)---------- Buffer: *Backtrace* ----------Your cursor will be in this window (you may have to wait a few seconds before it
becomes visible). To quit the debugger and make the debugger window go away,
type:q

Please type q right now, so you become confident that you can get out of the
debugger. Then, type C-x C-e again to re-enter it.
Based on what we already know, we can almost read this error message.
You read the *Backtrace* buffer from the bottom up; it tells you what Emacs
did. When you typed C-x C-e, you made an interactive call to the commandeval-last-sexp. eval is an abbreviation for “evaluate” and sexp is an abbreviation for “symbolic expression”. The command means “evaluate last symbolic
expression”, which is the expression just before your cursor.
Each line above tells you what the Lisp interpreter evaluated next. The mostrecent action is at the top. The buffer is called the *Backtrace* buffer because it
enables you to track Emacs backwards.
At the top of the *Backtrace* buffer, you see the line:
Debugger entered--Lisp error: (void-function this)The Lisp interpreter tried to evaluate the first atom of the list, the word ‘this’. It
is this action that generated the error message ‘void-function this’.
The message contains the words ‘void-function’ and ‘this’.
The word ‘function’ was mentioned once before. It is a very important word.
For our purposes, we can define it by saying that a function is a set of instructions
to the computer that tell the computer to do something.
Now we can begin to understand the error message: ‘void-function this’.
The function (that is, the word ‘this’) does not have a definition of any set of
instructions for the computer to carry out.
The slightly odd word, ‘void-function’, is designed to cover the way Emacs
Lisp is implemented, which is that when a symbol does not have a function definitionattached to it, the place that should contain the instructions is void.
On the other hand, since we were able to add 2 plus 2 successfully, by evaluating(+ 2 2), we can infer that the symbol + must have a set of instructions for thecomputer to obey and those instructions must be to add the numbers that follow
the +.
It is possible to prevent Emacs entering the debugger in cases like this. We do
not explain how to do that here, but we will mention what the result looks like,because you may encounter a similar situation if there is a bug in some Emacs code

6

Chapter 1: List Processingthat you are using. In such cases, you will see only one line of error message; it willappear in the echo area and look like this:Symbol's function definition is void: thisThe message goes away as soon as you type a key, even just to move the cursor.
We know the meaning of the word ‘Symbol’. It refers to the first atom of thelist, the word ‘this’. The word ‘function’ refers to the instructions that tell thecomputer what to do. (Technically, the symbol tells the computer where to find the
instructions, but this is a complication we can ignore for the moment.)
The error message can be understood: ‘Symbol's function definition is
void: this’. The symbol (that is, the word ‘this’) lacks instructions for the computer to carry out.

1.4 Symbol Names and Function Definitions
We can articulate another characteristic of Lisp based on what we have discussed so
far—an important characteristic: a symbol, like +, is not itself the set of instructions
for the computer to carry out. Instead, the symbol is used, perhaps temporarily,
as a way of locating the definition or set of instructions. What we see is the namethrough which the instructions can be found. Names of people work the same way.
I can be referred to as ‘Bob’; however, I am not the letters ‘B’, ‘o’, ‘b’ but am, or was,the consciousness consistently associated with a particular life-form. The name is
not me, but it can be used to refer to me.
In Lisp, one set of instructions can be attached to several names. For example,
the computer instructions for adding numbers can be linked to the symbol plus aswell as to the symbol + (and are in some dialects of Lisp). Among humans, I canbe referred to as ‘Robert’ as well as ‘Bob’ and by other words as well.
On the other hand, a symbol can have only one function definition attached to it
at a time. Otherwise, the computer would be confused as to which definition to use.
If this were the case among people, only one person in the world could be named‘Bob’. However, the function definition to which the name refers can be changed
readily. (See Section 3.2 “Install a Function Definition”, page 28.)
Since Emacs Lisp is large, it is customary to name symbols in a way that identifies the part of Emacs to which the function belongs. Thus, all the names forfunctions that deal with Texinfo start with ‘texinfo-’ and those for functions thatdeal with reading mail start with ‘rmail-’.

1.5 The Lisp Interpreter
Based on what we have seen, we can now start to figure out what the Lisp interpreter
does when we command it to evaluate a list. First, it looks to see whether there isa quote before the list; if there is, the interpreter just gives us the list. On the other
hand, if there is no quote, the interpreter looks at the first element in the list andsees whether it has a function definition. If it does, the interpreter carries out the
instructions in the function definition. Otherwise, the interpreter prints an error
message.

Section 1.5: The Lisp Interpreter

7

This is how Lisp works. Simple. There are added complications which we will get
to in a minute, but these are the fundamentals. Of course, to write Lisp programs,
you need to know how to write function definitions and attach them to names, andhow to do this without confusing either yourself or the computer.
Now, for the first complication. In addition to lists, the Lisp interpreter canevaluate a symbol that is not quoted and does not have parentheses around it. The
Lisp interpreter will attempt to determine the symbol’s value as a variable. Thissituation is described in the section on variables. (See Section 1.7 “Variables”,
page 9.)The second complication occurs because some functions are unusual and do notwork in the usual manner. Those that don’t are called special forms. They are used
for special jobs, like defining a function, and there are not many of them. In thenext few chapters, you will be introduced to several of the more important special
forms.
As well as special forms, there are also macros. A macro is a construct definedin Lisp, which differs from a function in that it translates a Lisp expression intoanother expression that is to be evaluated in place of the original expression. (See
Section 8.2.2 “Lisp macro”, page 93.)
For the purposes of this introduction, you do not need to worry too much about
whether something is a special form, macro, or ordinary function. For example, if isa special form (see Section 3.7 “if”, page 37), but when is a macro (see Section 8.2.2“Lisp macro”, page 93). In earlier versions of Emacs, defun was a special form, but
now it is a macro (see Section 3.1 “defun”, page 26). It still behaves in the sameway.
The final complication is this: if the function that the Lisp interpreter is looking
at is not a special form, and if it is part of a list, the Lisp interpreter looks to see
whether the list has a list inside of it. If there is an inner list, the Lisp interpreter
first figures out what it should do with the inside list, and then it works on theoutside list. If there is yet another list embedded inside the inner list, it works onthat one first, and so on. It always works on the innermost list first. The interpreter
works on the innermost list first, to evaluate the result of that list. The result may
be used by the enclosing expression.
Otherwise, the interpreter works left to right, from one expression to the next.
1.5.1 Byte Compiling
One other aspect of interpreting: the Lisp interpreter is able to interpret two kindsof entity: humanly readable code, on which we will focus exclusively, and speciallyprocessed code, called byte compiled code, which is not humanly readable. Byte
compiled code runs faster than humanly readable code.
You can transform humanly readable code into byte compiled code by runningone of the compile commands such as byte-compile-file. Byte compiled code isusually stored in a file that ends with a .elc extension rather than a .el extension.
You will see both kinds of file in the emacs/lisp directory; the files to read arethose with .el extensions.

8

Chapter 1: List Processing

As a practical matter, for most things you might do to customize or extend
Emacs, you do not need to byte compile; and I will not discuss the topic here. See
Section “Byte Compilation” in The GNU Emacs Lisp Reference Manual, for a fulldescription of byte compilation.

1.6 Evaluation
When the Lisp interpreter works on an expression, the term for the activity is calledevaluation. We say that the interpreter “evaluates the expression”. I’ve used thisterm several times before. The word comes from its use in everyday language,“to ascertain the value or amount of; to appraise”, according to Webster’s New
Collegiate Dictionary.
After evaluating an expression, the Lisp interpreter will most likely return thevalue that the computer produces by carrying out the instructions it found in thefunction definition, or perhaps it will give up on that function and produce an error
message. (The interpreter may also find itself tossed, so to speak, to a different
function or it may attempt to repeat continually what it is doing for ever and everin an infinite loop. These actions are less common; and we can ignore them.) Most
frequently, the interpreter returns a value.
At the same time the interpreter returns a value, it may do something else as
well, such as move a cursor or copy a file; this other kind of action is called a side
effect. Actions that we humans think are important, such as printing results, areoften side effects to the Lisp interpreter. It is fairly easy to learn to use side effects.
In summary, evaluating a symbolic expression most commonly causes the Lisp
interpreter to return a value and perhaps carry out a side effect; or else produce anerror.
1.6.1 Evaluating Inner Lists
If evaluation applies to a list that is inside another list, the outer list may use thevalue returned by the first evaluation as information when the outer list is evaluated.
This explains why inner expressions are evaluated first: the values they return areused by the outer expressions.

Section 1.7: Variables

9

We can investigate this process by evaluating another addition example. Place
your cursor after the following expression and type C-x C-e:(+ 2 (+ 3 3))

The number 8 will appear in the echo area.
What happens is that the Lisp interpreter first evaluates the inner expression,(+ 3 3), for which the value 6 is returned; then it evaluates the outer expression
as if it were written (+ 2 6), which returns the value 8. Since there are no more
enclosing expressions to evaluate, the interpreter prints that value in the echo area.
Now it is easy to understand the name of the command invoked by the keystrokes
C-x C-e: the name is eval-last-sexp. The letters sexp are an abbreviation for“symbolic expression”, and eval is an abbreviation for “evaluate”. The commandevaluates the last symbolic expression.
As an experiment, you can try evaluating the expression by putting the cursor
at the beginning of the next line immediately following the expression, or inside theexpression.
Here is another copy of the expression:(+ 2 (+ 3 3))

If you place the cursor at the beginning of the blank line that immediately follows
the expression and type C-x C-e, you will still get the value 8 printed in the echoarea. Now try putting the cursor inside the expression. If you put it right after
the next to last parenthesis (so it appears to sit on top of the last parenthesis), youwill get a 6 printed in the echo area! This is because the command evaluates theexpression (+ 3 3).
Now put the cursor immediately after a number. Type C-x C-e and you will get
the number itself. In Lisp, if you evaluate a number, you get the number itself—thisis how numbers differ from symbols. If you evaluate a list starting with a symbol
like +, you will get a value returned that is the result of the computer carrying out
the instructions in the function definition attached to that name. If a symbol byitself is evaluated, something different happens, as we will see in the next section.

1.7 Variables
In Emacs Lisp, a symbol can have a value attached to it just as it can have afunction definition attached to it. The two are different. The function definition
is a set of instructions that a computer will obey. A value, on the other hand, issomething, such as number or a name, that can vary (which is why such a symbol
is called a variable). The value of a symbol can be any expression in Lisp, such as asymbol, number, list, or string. A symbol that has a value is often called a variable.
A symbol can have both a function definition and a value attached to it at thesame time. Or it can have just one or the other. The two are separate. This is
somewhat similar to the way the name Cambridge can refer to the city in Massachusetts and have some information attached to the name as well, such as “great
programming center”.
Another way to think about this is to imagine a symbol as being a chest of
drawers. The function definition is put in one drawer, the value in another, and so10

Chapter 1: List Processing

on. What is put in the drawer holding the value can be changed without affecting
the contents of the drawer holding the function definition, and vice versa.
The variable fill-column illustrates a symbol with a value attached to it: inevery GNU Emacs buffer, this symbol is set to some value, usually 72 or 70, butsometimes to some other value. To find the value of this symbol, evaluate it byitself. If you are reading this in Info inside of GNU Emacs, you can do this byputting the cursor after the symbol and typing C-x C-e:
fill-column

After I typed C-x C-e, Emacs printed the number 72 in my echo area. This is thevalue for which fill-column is set for me as I write this. It may be different for you
in your Info buffer. Notice that the value returned as a variable is printed in exactly
the same way as the value returned by a function carrying out its instructions. Fromthe point of view of the Lisp interpreter, a value returned is a value returned. Whatkind of expression it came from ceases to matter once the value is known.
A symbol can have any value attached to it or, to use the jargon, we can bind thevariable to a value: to a number, such as 72; to a string, "such as this"; to a list,
such as (spruce pine oak); we can even bind a variable to a function definition.
A symbol can be bound to a value in several ways. See Section 1.9 “Setting theValue of a Variable”, page 16, for information about one way to do this.
1.7.1 Error Message for a Symbol Without a Function
When we evaluated fill-column to find its value as a variable, we did not placeparentheses around the word. This is because we did not intend to use it as afunction name.
If fill-column were the first or only element of a list, the Lisp interpreter
would attempt to find the function definition attached to it. But fill-column has
no function definition. Try evaluating this:(fill-column)

You will create a *Backtrace* buffer that says:---------- Buffer: *Backtrace* ---------Debugger entered--Lisp error: (void-function fill-column)(fill-column)eval((fill-column) nil)elisp--eval-last-sexp(nil)eval-last-sexp(nil)funcall-interactively(eval-last-sexp nil)call-interactively(eval-last-sexp nil nil)command-execute(eval-last-sexp)---------- Buffer: *Backtrace* ----------(Remember, to quit the debugger and make the debugger window go away, type q
in the *Backtrace* buffer.)
1.7.2 Error Message for a Symbol Without a Value
If you attempt to evaluate a symbol that does not have a value bound to it, youwill receive an error message. You can see this by experimenting with our 2 plus 2

Section 1.8: Arguments

11

addition. In the following expression, put your cursor right after the +, before thefirst number 2, type C-x C-e:(+ 2 2)In GNU Emacs 22, you will create a *Backtrace* buffer that says:---------- Buffer: *Backtrace* ---------Debugger entered--Lisp error: (void-variable +)eval(+)
elisp--eval-last-sexp(nil)eval-last-sexp(nil)funcall-interactively(eval-last-sexp nil)call-interactively(eval-last-sexp nil nil)command-execute(eval-last-sexp)---------- Buffer: *Backtrace* ----------(Again, you can quit the debugger by typing q in the *Backtrace* buffer.)
This backtrace is different from the very first error message we saw, which said,‘Debugger entered--Lisp error: (void-function this)’. In this case, the function does not have a value as a variable; while in the other error message, thefunction (the word ‘this’) did not have a definition.
In this experiment with the +, what we did was cause the Lisp interpreter toevaluate the + and look for the value of the variable instead of the function definition.
We did this by placing the cursor right after the symbol rather than after theparenthesis of the enclosing list as we did before. As a consequence, the Lisp
interpreter evaluated the preceding s-expression, which in this case was + by itself.
Since + does not have a value bound to it, just the function definition, the error
message reported that the symbol’s value as a variable was void.

1.8 Arguments
To see how information is passed to functions, let’s look again at our old standby,
the addition of two plus two. In Lisp, this is written as follows:(+ 2 2)

If you evaluate this expression, the number 4 will appear in your echo area.
What the Lisp interpreter does is add the numbers that follow the +.
The numbers added by + are called the arguments of the function +. These
numbers are the information that is given to or passed to the function.
The word “argument” comes from the way it is used in mathematics and does
not refer to a disputation between two people; instead it refers to the information
presented to the function, in this case, to the +. In Lisp, the arguments to afunction are the atoms or lists that follow the function. The values returned by theevaluation of these atoms or lists are passed to the function. Different functions
require different numbers of arguments; some functions require none at all.3
3

It is curious to track the path by which the word “argument” came to have twodifferent meanings, one in mathematics and the other in everyday English. According
to the Oxford English Dictionary, the word derives from the Latin for ‘to make clear,prove’; thus it came to mean, by one thread of derivation, “the evidence offered as
proof”, which is to say, “the information offered”, which led to its meaning in Lisp.

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1.8.1 Arguments’ Data Types
The type of data that should be passed to a function depends on what kind of
information it uses. The arguments to a function such as + must have values thatare numbers, since + adds numbers. Other functions use different kinds of data for
their arguments.
For example, the concat function links together or unites two or more stringsof text to produce a string. The arguments are strings. Concatenating the two
character strings abc, def produces the single string abcdef. This can be seen byevaluating the following:(concat "abc" "def")

The value produced by evaluating this expression is "abcdef".
A function such as substring uses both a string and numbers as arguments.
The function returns a part of the string, a substring of the first argument. Thisfunction takes three arguments. Its first argument is the string of characters, thesecond and third arguments are numbers that indicate the beginning (inclusive)and end (exclusive) of the substring. The numbers are a count of the number ofcharacters (including spaces and punctuation) from the beginning of the string.
Note that the characters in a string are numbered from zero, not one.
For example, if you evaluate the following:(substring "The quick brown fox jumped." 16 19)

you will see "fox" appear in the echo area. The arguments are the string and thetwo numbers.
Note that the string passed to substring is a single atom even though it ismade up of several words separated by spaces. Lisp counts everything between thetwo quotation marks as part of the string, including the spaces. You can thinkof the substring function as a kind of atom smasher since it takes an otherwise
indivisible atom and extracts a part. However, substring is only able to extract a
substring from an argument that is a string, not from another type of atom such as
a number or symbol.
1.8.2 An Argument as the Value of a Variable or List
An argument can be a symbol that returns a value when it is evaluated. For
example, when the symbol fill-column by itself is evaluated, it returns a number.
This number can be used in an addition.

But in the other thread of derivation, it came to mean “to assert in a manner against
which others may make counter assertions”, which led to the meaning of the word as
a disputation. (Note here that the English word has two different definitions attached
to it at the same time. By contrast, in Emacs Lisp, a symbol cannot have two different
function definitions at the same time.)

Section 1.8: Arguments

13

Position the cursor after the following expression and type C-x C-e:(+ 2 fill-column)

The value will be a number two more than what you get by evaluating fill-column
alone. For me, this is 74, because my value of fill-column is 72.
As we have just seen, an argument can be a symbol that returns a value whenevaluated. In addition, an argument can be a list that returns a value when it isevaluated. For example, in the following expression, the arguments to the function
concat are the strings "The " and " red foxes." and the list (number-to-string(+ 2 fill-column)).
(concat "The " (number-to-string (+ 2 fill-column)) " red foxes.")If you evaluate this expression—and if, as with my Emacs, fill-column evaluates
to 72—"The 74 red foxes." will appear in the echo area. (Note that you must put
spaces after the word ‘The’ and before the word ‘red’ so they will appear in the
final string. The function number-to-string converts the integer that the addition
function returns to a string. number-to-string is also known as int-to-string.)
1.8.3 Variable Number of Arguments
Some functions, such as concat, + or *, take any number of arguments. (The * isthe symbol for multiplication.) This can be seen by evaluating each of the following
expressions in the usual way. What you will see in the echo area is printed in thistext after ‘⇒’, which you may read as “evaluates to”.
In the first set, the functions have no arguments:
(+)

⇒ 0

(*)

⇒ 1

In this set, the functions have one argument each:
(+ 3)

⇒ 3

(* 3)

⇒ 3

In this set, the functions have three arguments each:(+ 3 4 5) ⇒ 12(* 3 4 5) ⇒ 60

1.8.4 Using the Wrong Type Object as an Argument
When a function is passed an argument of the wrong type, the Lisp interpreter
produces an error message. For example, the + function expects the values of its
arguments to be numbers. As an experiment we can pass it the quoted symbol
hello instead of a number. Position the cursor after the following expression andtype C-x C-e:(+ 2 'hello)

When you do this you will generate an error message. What has happened is that+ has tried to add the 2 to the value returned by 'hello, but the value returned
by 'hello is the symbol hello, not a number. Only numbers can be added. So +could not carry out its addition.

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Chapter 1: List Processing
You will create and enter a *Backtrace* buffer that says:---------- Buffer: *Backtrace* ---------Debugger entered--Lisp error:(wrong-type-argument number-or-marker-p hello)+(2 hello)eval((+ 2 'hello) nil)elisp--eval-last-sexp(t)eval-last-sexp(nil)funcall-interactively(eval-print-last-sexp nil)call-interactively(eval-print-last-sexp nil nil)command-execute(eval-print-last-sexp)---------- Buffer: *Backtrace* ----------As usual, the error message tries to be helpful and makes sense after you learn
how to read it.4
The first part of the error message is straightforward; it says ‘wrong typeargument’. Next comes the mysterious jargon word ‘number-or-marker-p’. This
word is trying to tell you what kind of argument the + expected.
The symbol number-or-marker-p says that the Lisp interpreter is trying todetermine whether the information presented it (the value of the argument) is anumber or a marker (a special object representing a buffer position). What it doesis test to see whether the + is being given numbers to add. It also tests to see
whether the argument is something called a marker, which is a specific feature ofEmacs Lisp. (In Emacs, locations in a buffer are recorded as markers. When themark is set with the C-@ or C-SPC command, its position is kept as a marker. The
mark can be considered a number—the number of characters the location is fromthe beginning of the buffer.) In Emacs Lisp, + can be used to add the numeric valueof marker positions as numbers.
The ‘p’ of number-or-marker-p is the embodiment of a practice started in theearly days of Lisp programming. The ‘p’ stands for predicate. In the jargon usedby the early Lisp researchers, a predicate refers to a function to determine whether
some property is true or false. So the ‘p’ tells us that number-or-marker-p is thename of a function that determines whether it is true or false that the argument
supplied is a number or a marker. Other Lisp symbols that end in ‘p’ include
zerop, a function that tests whether its argument has the value of zero, and listp,
a function that tests whether its argument is a list.
Finally, the last part of the error message is the symbol hello. This is the valueof the argument that was passed to +. If the addition had been passed the correct
type of object, the value passed would have been a number, such as 37, rather thana symbol like hello. But then you would not have got the error message.
1.8.5 The message Function
Like +, the message function takes a variable number of arguments. It is used tosend messages to the user and is so useful that we will describe it here.

4(quote hello) is an expansion of the abbreviation 'hello.

Section 1.8: Arguments

15

A message is printed in the echo area. For example, you can print a message inyour echo area by evaluating the following list:(message "This message appears in the echo area!")

The whole string between double quotation marks is a single argument and is
printed in toto. (Note that in this example, the message itself will appear in theecho area within double quotes; that is because you see the value returned by themessage function. In most uses of message in programs that you write, the text will
be printed in the echo area as a side-effect, without the quotes. See Section 3.3.1“multiply-by-seven in detail”, page 30, for an example of this.)
However, if there is a ‘%s’ in the quoted string of characters, the message function
does not print the ‘%s’ as such, but looks to the argument that follows the string.
It evaluates the second argument and prints the value at the location in the string
where the ‘%s’ is.
You can see this by positioning the cursor after the following expression andtyping C-x C-e:(message "The name of this buffer is: %s." (buffer-name))

In Info, "The name of this buffer is: *info*." will appear in the echo area. The
function buffer-name returns the name of the buffer as a string, which the message
function inserts in place of %s.
To print a value as an integer, use ‘%d’ in the same way as ‘%s’. For example, toprint a message in the echo area that states the value of the fill-column, evaluate
the following:(message "The value of fill-column is %d." fill-column)

On my system, when I evaluate this list, "The value of fill-column is 72." appears in my echo area5 .
If there is more than one ‘%s’ in the quoted string, the value of the first argument
following the quoted string is printed at the location of the first ‘%s’ and the valueof the second argument is printed at the location of the second ‘%s’, and so on.

5

Actually, you can use %s to print a number. It is non-specific. %d prints only the partof a number left of a decimal point, and not anything that is not a number.

16

Chapter 1: List Processing
For example, if you evaluate the following,(message "There are %d %s in the office!"(- fill-column 14) "pink elephants")

a rather whimsical message will appear in your echo area. On my system it says,"There are 58 pink elephants in the office!".
The expression (- fill-column 14) is evaluated and the resulting number isinserted in place of the ‘%d’; and the string in double quotes, "pink elephants",
is treated as a single argument and inserted in place of the ‘%s’. (That is to say, astring between double quotes evaluates to itself, like a number.)
Finally, here is a somewhat complex example that not only illustrates the computation of a number, but also shows how you can use an expression within an
expression to generate the text that is substituted for ‘%s’:(message "He saw %d %s"(- fill-column 32)(concat "red "(substring"The quick brown foxes jumped." 16 21)" leaping."))In this example, message has three arguments: the string, "He saw %d %s", theexpression, (- fill-column 32), and the expression beginning with the function
concat. The value resulting from the evaluation of (- fill-column 32) is inserted
in place of the ‘%d’; and the value returned by the expression beginning with concat
is inserted in place of the ‘%s’.
When your fill column is 70 and you evaluate the expression, the message "Hesaw 38 red foxes leaping." appears in your echo area.

1.9 Setting the Value of a Variable
There are several ways by which a variable can be given a value. One of the ways
is to use the special form setq. Another way is to use let (see Section 3.6 “let”,
page 33). (The jargon for this process is to bind a variable to a value.)
The following sections not only describe how setq works but also illustrate how
arguments are passed.
1.9.1 Using setq
To set the value of the symbol flowers to the list (rose violet daisy buttercup),
evaluate the following expression by positioning the cursor after the expression andtyping C-x C-e.
(setq flowers '(rose violet daisy buttercup))

The list (rose violet daisy buttercup) will appear in the echo area. This iswhat is returned by the setq special form. As a side effect, the symbol flowers isbound to the list; that is, the symbol flowers, which can be viewed as a variable,
is given the list as its value. (This process, by the way, illustrates how a side effectto the Lisp interpreter, setting the value, can be the primary effect that we humans
are interested in. This is because every Lisp function must return a value if it does
not get an error, but it will only have a side effect if it is designed to have one.)

Section 1.9: Setting the Value of a Variable

17

After evaluating the setq expression, you can evaluate the symbol flowers andit will return the value you just set. Here is the symbol. Place your cursor after it
and type C-x C-e.
flowers

When you evaluate flowers, the list (rose violet daisy buttercup) appears in
the echo area.
Incidentally, if you evaluate 'flowers, the variable with a quote in front of it,
what you will see in the echo area is the symbol itself, flowers. Here is the quoted
symbol, so you can try this:'flowers

Also, as an added convenience, setq permits you to set several different variables
to different values, all in one expression.
To set the value of the variable carnivores to the list '(lion tiger leopard)using setq, the following expression is used:(setq carnivores '(lion tiger leopard))

Also, setq can be used to assign different values to different variables. The
first argument is bound to the value of the second argument, the third argument isbound to the value of the fourth argument, and so on. For example, you could use
the following to assign a list of trees to the symbol trees and a list of herbivores
to the symbol herbivores:(setq trees '(pine fir oak maple)
herbivores '(gazelle antelope zebra))(The expression could just as well have been on one line, but it might not have fit
on a page; and humans find it easier to read nicely formatted lists.)
Although I have been using the term “assign”, there is another way of thinking
about the workings of setq; and that is to say that setq makes the symbol point
to the list. This latter way of thinking is very common and in forthcoming chapters
we shall come upon at least one symbol that has “pointer” as part of its name. The
name is chosen because the symbol has a value, specifically a list, attached to it;or, expressed another way, the symbol is set to point to the list.
1.9.2 Counting
Here is an example that shows how to use setq in a counter. You might use this tocount how many times a part of your program repeats itself. First set a variable tozero; then add one to the number each time the program repeats itself. To do this,you need a variable that serves as a counter, and two expressions: an initial setq
expression that sets the counter variable to zero; and a second setq expression that
increments the counter each time it is evaluated.
(setq counter 0); Let’s call this the initializer.

(setq counter (+ counter 1)); This is the incrementer.

counter; This is the counter.

(The text following the ‘;’ are comments. See Section 3.2.1 “Change a Function
Definition”, page 29.)

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Chapter 1: List Processing

If you evaluate the first of these expressions, the initializer, (setq counter 0),
and then evaluate the third expression, counter, the number 0 will appear in theecho area. If you then evaluate the second expression, the incrementer, (setq
counter (+ counter 1)), the counter will get the value 1. So if you again evaluate
counter, the number 1 will appear in the echo area. Each time you evaluate thesecond expression, the value of the counter will be incremented.
When you evaluate the incrementer, (setq counter (+ counter 1)), the Lisp
interpreter first evaluates the innermost list; this is the addition. In order to evaluate
this list, it must evaluate the variable counter and the number 1. When it evaluates
the variable counter, it receives its current value. It passes this value and thenumber 1 to the + which adds them together. The sum is then returned as thevalue of the inner list and passed to the setq which sets the variable counter tothis new value. Thus, the value of the variable, counter, is changed.

1.10 Summary
Learning Lisp is like climbing a hill in which the first part is the steepest. You have
now climbed the most difficult part; what remains becomes easier as you progress
onwards.
In summary,• Lisp programs are made up of expressions, which are lists or single atoms.
• Lists are made up of zero or more atoms or inner lists, separated by whitespaceand surrounded by parentheses. A list can be empty.
• Atoms are multi-character symbols, like forward-paragraph, single character symbols like +, strings of characters between double quotation marks, or
numbers.
• A number evaluates to itself.
• A string between double quotes also evaluates to itself.
• When you evaluate a symbol by itself, its value is returned.
• When you evaluate a list, the Lisp interpreter looks at the first symbol in
the list and then at the function definition bound to that symbol. Then the
instructions in the function definition are carried out.
• A single-quote ‘'’ tells the Lisp interpreter that it should return the following
expression as written, and not evaluate it as it would if the quote were notthere.
• Arguments are the information passed to a function. The arguments to afunction are computed by evaluating the rest of the elements of the list ofwhich the function is the first element.
• A function always returns a value when it is evaluated (unless it gets an error);in addition, it may also carry out some action that is a side effect. In manycases, a function’s primary purpose is to create a side effect.

Section 1.11: Exercises

19

1.11 Exercises
A few simple exercises:• Generate an error message by evaluating an appropriate symbol that is notwithin parentheses.
• Generate an error message by evaluating an appropriate symbol that is between
parentheses.
• Create a counter that increments by two rather than one.
• Write an expression that prints a message in the echo area when evaluated.

20

Chapter 2: Practicing Evaluation

2 Practicing Evaluation
Before learning how to write a function definition in Emacs Lisp, it is useful tospend a little time evaluating various expressions that have already been written.
These expressions will be lists with the functions as their first (and often only)
element. Since some of the functions associated with buffers are both simple andinteresting, we will start with those. In this section, we will evaluate a few of these.
In another section, we will study the code of several other buffer-related functions,to see how they were written.
Whenever you give an editing command to Emacs Lisp, such as the command
to move the cursor or to scroll the screen, you are evaluating an expression, the firstelement of which is a function. This is how Emacs works.
When you type keys, you cause the Lisp interpreter to evaluate an expression
and that is how you get your results. Even typing plain text involves evaluating an Emacs Lisp function, in this case, one that uses self-insert-command,
which simply inserts the character you typed. The functions you evaluate by typing
keystrokes are called interactive functions, or commands; how you make a function
interactive will be illustrated in the chapter on how to write function definitions.
See Section 3.3 “Making a Function Interactive”, page 29.
In addition to typing keyboard commands, we have seen a second way to evaluate
an expression: by positioning the cursor after a list and typing C-x C-e. This iswhat we will do in the rest of this section. There are other ways to evaluate an
expression as well; these will be described as we come to them.
Besides being used for practicing evaluation, the functions shown in the next
few sections are important in their own right. A study of these functions makes
clear the distinction between buffers and files, how to switch to a buffer, and how
to determine a location within it.

2.1 Buffer Names
The two functions, buffer-name and buffer-file-name, show the difference
between a file and a buffer. When you evaluate the following expression,(buffer-name), the name of the buffer appears in the echo area. When you
evaluate (buffer-file-name), the name of the file to which the buffer refers
appears in the echo area. Usually, the name returned by (buffer-name) is
the same as the name of the file to which it refers, and the name returned by(buffer-file-name) is the full path-name of the file.
A file and a buffer are two different entities. A file is information recorded
permanently in the computer (unless you delete it). A buffer, on the other hand,is information inside of Emacs that will vanish at the end of the editing session(or when you kill the buffer). Usually, a buffer contains information that you havecopied from a file; we say the buffer is visiting that file. This copy is what you workon and modify. Changes to the buffer do not change the file, until you save thebuffer. When you save the buffer, the buffer is copied to the file and is thus saved
permanently.

Section 2.1: Buffer Names

21

If you are reading this in Info inside of GNU Emacs, you can evaluate each of
the following expressions by positioning the cursor after it and typing C-x C-e.
(buffer-name)(buffer-file-name)
When I do this in Info, the value returned by evaluating (buffer-name) is"*info*", and the value returned by evaluating (buffer-file-name) is nil.
On the other hand, while I am writing this document, the value returned byevaluating (buffer-name) is "introduction.texinfo", and the value returned byevaluating (buffer-file-name) is "/gnu/work/intro/introduction.texinfo".
The former is the name of the buffer and the latter is the name of the file. In
Info, the buffer name is "*info*". Info does not point to any file, so the result of
evaluating (buffer-file-name) is nil. The symbol nil is from the Latin word
for “nothing”; in this case, it means that the buffer is not associated with any file.
(In Lisp, nil is also used to mean “false” and is a synonym for the empty list, ().)
When I am writing, the name of my buffer is "introduction.texinfo". The
name of the file to which it points is "/gnu/work/intro/introduction.texinfo".
(In the expressions, the parentheses tell the Lisp interpreter to treat
buffer-name and buffer-file-name as functions; without the parentheses, theinterpreter would attempt to evaluate the symbols as variables. See Section 1.7“Variables”, page 9.)
In spite of the distinction between files and buffers, you will often find thatpeople refer to a file when they mean a buffer and vice versa. Indeed, most people
say, “I am editing a file,” rather than saying, “I am editing a buffer which I willsoon save to a file.” It is almost always clear from context what people mean. Whendealing with computer programs, however, it is important to keep the distinction
in mind, since the computer is not as smart as a person.
The word “buffer”, by the way, comes from the meaning of the word as a cushionthat deadens the force of a collision. In early computers, a buffer cushioned theinteraction between files and the computer’s central processing unit. The drums ortapes that held a file and the central processing unit were pieces of equipment thatwere very different from each other, working at their own speeds, in spurts. The
buffer made it possible for them to work together effectively. Eventually, the buffer
grew from being an intermediary, a temporary holding place, to being the place
where work is done. This transformation is rather like that of a small seaport thatgrew into a great city: once it was merely the place where cargo was warehoused
temporarily before being loaded onto ships; then it became a business and cultural
center in its own right.
Not all buffers are associated with files. For example, a *scratch* buffer does
not visit any file. Similarly, a *Help* buffer is not associated with any file.
In the old days, when you lacked a ~/.emacs file and started an Emacs session
by typing the command emacs alone, without naming any files, Emacs started withthe *scratch* buffer visible. Nowadays, you will see a splash screen. You canfollow one of the commands suggested on the splash screen, visit a file, or press q
to quit the splash screen and reach the *scratch* buffer.

22

Chapter 2: Practicing Evaluation

If you switch to the *scratch* buffer, type (buffer-name), position the cursor
after it, and then type C-x C-e to evaluate the expression. The name "*scratch*"will be returned and will appear in the echo area. "*scratch*" is the name of
the buffer. When you type (buffer-file-name) in the *scratch* buffer andevaluate that, nil will appear in the echo area, just as it does when you evaluate(buffer-file-name) in Info.
Incidentally, if you are in the *scratch* buffer and want the value returned by
an expression to appear in the *scratch* buffer itself rather than in the echo area,
type C-u C-x C-e instead of C-x C-e. This causes the value returned to appear
after the expression. The buffer will look like this:(buffer-name)"*scratch*"

You cannot do this in Info since Info is read-only and it will not allow you to change
the contents of the buffer. But you can do this in any buffer you can edit; and when
you write code or documentation (such as this book), this feature is very useful.

2.2 Getting Buffers
The buffer-name function returns the name of the buffer; to get the buffer itself, adifferent function is needed: the current-buffer function. If you use this function
in code, what you get is the buffer itself.
A name and the object or entity to which the name refers are different from eachother. You are not your name. You are a person to whom others refer by name.
If you ask to speak to George and someone hands you a card with the letters ‘G’,‘e’, ‘o’, ‘r’, ‘g’, and ‘e’ written on it, you might be amused, but you would not besatisfied. You do not want to speak to the name, but to the person to whom thename refers. A buffer is similar: the name of the scratch buffer is *scratch*, but
the name is not the buffer. To get a buffer itself, you need to use a function suchas current-buffer.
However, there is a slight complication: if you evaluate current-buffer in an
expression on its own, as we will do here, what you see is a printed representation of
the name of the buffer without the contents of the buffer. Emacs works this way for
two reasons: the buffer may be thousands of lines long—too long to be conveniently
displayed; and, another buffer may have the same contents but a different name,and it is important to distinguish between them.

Section 2.3: Switching Buffers

23

Here is an expression containing the function:(current-buffer)If you evaluate this expression in Info in Emacs in the usual way, #<buffer *info*>
will appear in the echo area. The special format indicates that the buffer itself isbeing returned, rather than just its name.
Incidentally, while you can type a number or symbol into a program, you cannot
do that with the printed representation of a buffer: the only way to get a buffer
itself is with a function such as current-buffer.
A related function is other-buffer. This returns the most recently selectedbuffer other than the one you are in currently, not a printed representation of its
name. If you have recently switched back and forth from the *scratch* buffer,other-buffer will return that buffer.
You can see this by evaluating the expression:(other-buffer)

You should see #<buffer *scratch*> appear in the echo area, or the name ofwhatever other buffer you switched back from most recently1 .

2.3 Switching Buffers
The other-buffer function actually provides a buffer when it is used as an argument to a function that requires one. We can see this by using other-buffer andswitch-to-buffer to switch to a different buffer.
But first, a brief introduction to the switch-to-buffer function. Whenyou switched back and forth from Info to the *scratch* buffer to evaluate(buffer-name), you most likely typed C-x b and then typed *scratch*2 when
prompted in the minibuffer for the name of the buffer to which you wanted toswitch. The keystrokes, C-x b, cause the Lisp interpreter to evaluate the interactivefunction switch-to-buffer. As we said before, this is how Emacs works: different
keystrokes call or run different functions. For example, C-f calls forward-char,M-e calls forward-sentence, and so on.
By writing switch-to-buffer in an expression, and giving it a buffer to switch
to, we can switch buffers just the way C-x b does:(switch-to-buffer (other-buffer))

The symbol switch-to-buffer is the first element of the list, so the Lisp interpreter
will treat it as a function and carry out the instructions that are attached to it. But
before doing that, the interpreter will note that other-buffer is inside parentheses
and work on that symbol first. other-buffer is the first (and in this case, theonly) element of this list, so the Lisp interpreter calls or runs the function. It
returns another buffer. Next, the interpreter runs switch-to-buffer, passing to
1

Actually, by default, if the buffer from which you just switched is visible to you in
another window, other-buffer will choose the most recent buffer that you cannot see;this is a subtlety that I often forget.
2
Or rather, to save typing, you probably only typed RET if the default buffer was*scratch*, or if it was different, then you typed just part of the name, such as *sc,pressed your TAB key to cause it to expand to the full name, and then typed RET.

24

Chapter 2: Practicing Evaluation

it, as an argument, the other buffer, which is what Emacs will switch to. If you arereading this in Info, try this now. Evaluate the expression. (To get back, type C-x
b RET.)3
In the programming examples in later sections of this document, you will see
the function set-buffer more often than switch-to-buffer. This is because of adifference between computer programs and humans: humans have eyes and expect
to see the buffer on which they are working on their computer terminals. This is so
obvious, it almost goes without saying. However, programs do not have eyes. Whena computer program works on a buffer, that buffer does not need to be visible on
the screen.
switch-to-buffer is designed for humans and does two different things: it
switches the buffer to which Emacs’s attention is directed; and it switches the buffer
displayed in the window to the new buffer. set-buffer, on the other hand, does
only one thing: it switches the attention of the computer program to a different
buffer. The buffer on the screen remains unchanged (of course, normally nothing
happens there until the command finishes running).
Also, we have just introduced another jargon term, the word call. When you
evaluate a list in which the first symbol is a function, you are calling that function.
The use of the term comes from the notion of the function as an entity that can dosomething for you if you call it—just as a plumber is an entity who can fix a leakif you call him or her.

2.4 Buffer Size and the Location of Point
Finally, let’s look at several rather simple functions, buffer-size, point,
point-min, and point-max. These give information about the size of a buffer andthe location of point within it.
The function buffer-size tells you the size of the current buffer; that is, thefunction returns a count of the number of characters in the buffer.
(buffer-size)

You can evaluate this in the usual way, by positioning the cursor after the expression
and typing C-x C-e.
In Emacs, the current position of the cursor is called point. The expression(point) returns a number that tells you where the cursor is located as a count of
the number of characters from the beginning of the buffer up to point.

3

Remember, this expression will move you to your most recent other buffer that you
cannot see. If you really want to go to your most recently selected buffer, even if you
can still see it, you need to evaluate the following more complex expression:(switch-to-buffer (other-buffer (current-buffer) t))
In this case, the first argument to other-buffer tells it which buffer to skip—thecurrent one—and the second argument tells other-buffer it is OK to switch to avisible buffer. In regular use, switch-to-buffer takes you to a buffer not visible inwindows since you would most likely use C-x o (other-window) to go to another visible
buffer.

Section 2.5: Exercise

25

You can see the character count for point in this buffer by evaluating the following expression in the usual way:(point)As I write this, the value of point is 65724. The point function is frequently usedin some of the examples later in this book.
The value of point depends, of course, on its location within the buffer. If you
evaluate point in this spot, the number will be larger:(point)For me, the value of point in this location is 66043, which means that there are 319
characters (including spaces) between the two expressions. (Doubtless, you will see
different numbers, since I will have edited this since I first evaluated point.)
The function point-min is somewhat similar to point, but it returns the value ofthe minimum permissible value of point in the current buffer. This is the number 1
unless narrowing is in effect. (Narrowing is a mechanism whereby you can restrict
yourself, or a program, to operations on just a part of a buffer. See Chapter 6“Narrowing and Widening”, page 73.) Likewise, the function point-max returns
the value of the maximum permissible value of point in the current buffer.

2.5 Exercise
Find a file with which you are working and move towards its middle. Find its buffer
name, file name, length, and your position in the file.

26

Chapter 3: How To Write Function Definitions

3 How To Write Function Definitions
When the Lisp interpreter evaluates a list, it looks to see whether the first symbolon the list has a function definition attached to it; or, put another way, whether
the symbol points to a function definition. If it does, the computer carries out the
instructions in the definition. A symbol that has a function definition is called,
simply, a function (although, properly speaking, the definition is the function andthe symbol refers to it.)All functions are defined in terms of other functions, except for a few primitive functions that are written in the C programming language. When you write
functions’ definitions, you will write them in Emacs Lisp and use other functions as
your building blocks. Some of the functions you will use will themselves be written
in Emacs Lisp (perhaps by you) and some will be primitives written in C. The
primitive functions are used exactly like those written in Emacs Lisp and behave
like them. They are written in C so we can easily run GNU Emacs on any computerthat has sufficient power and can run C.
Let me re-emphasize this: when you write code in Emacs Lisp, you do not
distinguish between the use of functions written in C and the use of functions
written in Emacs Lisp. The difference is irrelevant. I mention the distinction only
because it is interesting to know. Indeed, unless you investigate, you won’t knowwhether an already-written function is written in Emacs Lisp or C.

3.1 The defun Macro
In Lisp, a symbol such as mark-whole-buffer has code attached to it that tells thecomputer what to do when the function is called. This code is called the function
definition and is created by evaluating a Lisp expression that starts with the symbol
defun (which is an abbreviation for define function).
In subsequent sections, we will look at function definitions from the Emacs
source code, such as mark-whole-buffer. In this section, we will describe a simple function definition so you can see how it looks. This function definition uses
arithmetic because it makes for a simple example. Some people dislike examples
using arithmetic; however, if you are such a person, do not despair. Hardly any of
the code we will study in the remainder of this introduction involves arithmetic ormathematics. The examples mostly involve text in one way or another.
A function definition has up to five parts following the word defun:
1. The name of the symbol to which the function definition should be attached.
2. A list of the arguments that will be passed to the function. If no arguments
will be passed to the function, this is an empty list, ().
3. Documentation describing the function. (Technically optional, but strongly
recommended.)
4. Optionally, an expression to make the function interactive so you can use it
by typing M-x and then the name of the function; or by typing an appropriate
key or keychord.

Section 3.1: The defun Macro

27

5. The code that instructs the computer what to do: the body of the function
definition.
It is helpful to think of the five parts of a function definition as being organized
in a template, with slots for each part:(defun function-name (arguments...)"optional-documentation..."(interactive argument-passing-info)
body...); optional

As an example, here is the code for a function that multiplies its argument by 7.
(This example is not interactive. See Section 3.3 “Making a Function Interactive”,
page 29, for that information.)(defun multiply-by-seven (number)"Multiply NUMBER by seven."(* 7 number))This definition begins with a parenthesis and the symbol defun, followed by thename of the function.
The name of the function is followed by a list that contains the arguments thatwill be passed to the function. This list is called the argument list. In this example,
the list has only one element, the symbol, number. When the function is used, thesymbol will be bound to the value that is used as the argument to the function.
Instead of choosing the word number for the name of the argument, I could havepicked any other name. For example, I could have chosen the word multiplicand.
I picked the word “number” because it tells what kind of value is intended for thisslot; but I could just as well have chosen the word “multiplicand” to indicate therole that the value placed in this slot will play in the workings of the function. I
could have called it foogle, but that would have been a bad choice because it would
not tell humans what it means. The choice of name is up to the programmer andshould be chosen to make the meaning of the function clear.
Indeed, you can choose any name you wish for a symbol in an argument list, eventhe name of a symbol used in some other function: the name you use in an argument
list is private to that particular definition. In that definition, the name refers toa different entity than any use of the same name outside the function definition.
Suppose you have a nick-name “Shorty” in your family; when your family members
refer to “Shorty”, they mean you. But outside your family, in a movie, for example,
the name “Shorty” refers to someone else. Because a name in an argument list is
private to the function definition, you can change the value of such a symbol inside
the body of a function without changing its value outside the function. The effectis similar to that produced by a let expression. (See Section 3.6 “let”, page 33.)
The argument list is followed by the documentation string that describes thefunction. This is what you see when you type C-h f and the name of a function.
Incidentally, when you write a documentation string like this, you should make thefirst line a complete sentence since some commands, such as apropos, print only
the first line of a multi-line documentation string. Also, you should not indent thesecond line of a documentation string, if you have one, because that looks odd when
you use C-h f (describe-function). The documentation string is optional, but itis so useful, it should be included in almost every function you write.

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Chapter 3: How To Write Function Definitions

The third line of the example consists of the body of the function definition.
(Most functions’ definitions, of course, are longer than this.) In this function, thebody is the list, (* 7 number), which says to multiply the value of number by 7.
(In Emacs Lisp, * is the function for multiplication, just as + is the function for
addition.)
When you use the multiply-by-seven function, the argument number evaluates to the actual number you want used. Here is an example that shows how
multiply-by-seven is used; but don’t try to evaluate this yet!
(multiply-by-seven 3)

The symbol number, specified in the function definition in the next section, is boundto the value 3 in the actual use of the function. Note that although number was inside
parentheses in the function definition, the argument passed to the multiply-byseven function is not in parentheses. The parentheses are written in the function
definition so the computer can figure out where the argument list ends and the restof the function definition begins.
If you evaluate this example, you are likely to get an error message. (Go ahead,try it!) This is because we have written the function definition, but not yet told
the computer about the definition—we have not yet loaded the function definition
in Emacs. Installing a function is the process that tells the Lisp interpreter thedefinition of the function. Installation is described in the next section.

3.2 Install a Function Definition
If you are reading this inside of Info in Emacs, you can try out the multiply-byseven function by first evaluating the function definition and then evaluating(multiply-by-seven 3). A copy of the function definition follows. Place the cursor after the last parenthesis of the function definition and type C-x C-e. Whenyou do this, multiply-by-seven will appear in the echo area. (What this means isthat when a function definition is evaluated, the value it returns is the name of thedefined function.) At the same time, this action installs the function definition.
(defun multiply-by-seven (number)"Multiply NUMBER by seven."(* 7 number))By evaluating this defun, you have just installed multiply-by-seven in Emacs.
The function is now just as much a part of Emacs as forward-word or any otherediting function you use. (multiply-by-seven will stay installed until you quit
Emacs. To reload code automatically whenever you start Emacs, see Section 3.5“Installing Code Permanently”, page 32.)
You can see the effect of installing multiply-by-seven by evaluating the following sample. Place the cursor after the following expression and type C-x C-e.
The number 21 will appear in the echo area.
(multiply-by-seven 3)

If you wish, you can read the documentation for the function by typing C-hf (describe-function) and then the name of the function, multiply-by-seven.
When you do this, a *Help* window will appear on your screen that says:

Section 3.3: Make a Function Interactive

29

multiply-by-seven is a Lisp function.
(multiply-by-seven NUMBER)
Multiply NUMBER by seven.

(To return to a single window on your screen, type C-x 1.)
3.2.1 Change a Function Definition
If you want to change the code in multiply-by-seven, just rewrite it. To install
the new version in place of the old one, evaluate the function definition again. Thisis how you modify code in Emacs. It is very simple.
As an example, you can change the multiply-by-seven function to add thenumber to itself seven times instead of multiplying the number by seven. It produces
the same answer, but by a different path. At the same time, we will add a comment
to the code; a comment is text that the Lisp interpreter ignores, but that a humanreader may find useful or enlightening. The comment is that this is the second
version.
(defun multiply-by-seven (number); Second version.
"Multiply NUMBER by seven."(+ number number number number number number number))

The comment follows a semicolon, ‘;’. In Lisp, everything on a line that follows
a semicolon is a comment. The end of the line is the end of the comment. To stretch
a comment over two or more lines, begin each line with a semicolon.
See Section 16.3 “Beginning a .emacs File”, page 199, and Section “Comments”
in The GNU Emacs Lisp Reference Manual, for more about comments.
You can install this version of the multiply-by-seven function by evaluating
it in the same way you evaluated the first function: place the cursor after the lastparenthesis and type C-x C-e.
In summary, this is how you write code in Emacs Lisp: you write a function;
install it; test it; and then make fixes or enhancements and install it again.

3.3 Make a Function Interactive
You make a function interactive by placing a list that begins with the special form
interactive immediately after the documentation. A user can invoke an interactive function by typing M-x and then the name of the function; or by typing thekeys to which it is bound, for example, by typing C-n for next-line or C-x h formark-whole-buffer.
Interestingly, when you call an interactive function interactively, the value returned is not automatically displayed in the echo area. This is because you often
call an interactive function for its side effects, such as moving forward by a wordor line, and not for the value returned. If the returned value were displayed in theecho area each time you typed a key, it would be very distracting.
Both the use of the special form interactive and one way to display a value inthe echo area can be illustrated by creating an interactive version of multiply-byseven.

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Chapter 3: How To Write Function Definitions
Here is the code:(defun multiply-by-seven (number); Interactive version.
"Multiply NUMBER by seven."(interactive "p")(message "The result is %d" (* 7 number)))

You can install this code by placing your cursor after it and typing C-x C-e. The
name of the function will appear in your echo area. Then, you can use this code bytyping C-u and a number and then typing M-x multiply-by-seven and pressing
RET. The phrase ‘The result is ...’ followed by the product will appear in theecho area.
Speaking more generally, you invoke a function like this in either of two ways:
1. By typing a prefix argument that contains the number to be passed, andthen typing M-x and the name of the function, as with C-u 3 M-x forwardsentence; or,
2. By typing whatever key or keychord the function is bound to, as with C-u 3
M-e.
Both the examples just mentioned work identically to move point forward threesentences. (Since multiply-by-seven is not bound to a key, it could not be used
as an example of key binding.)(See Section 16.7 “Some Key Bindings”, page 203, to learn how to bind acommand to a key.)
A prefix argument is passed to an interactive function by typing the META key
followed by a number, for example, M-3 M-e, or by typing C-u and then a number,for example, C-u 3 M-e (if you type C-u without a number, it defaults to 4).
3.3.1 An Interactive multiply-by-sevenLet’s look at the use of the special form interactive and then at the function
message in the interactive version of multiply-by-seven. You will recall that thefunction definition looks like this:(defun multiply-by-seven (number); Interactive version.
"Multiply NUMBER by seven."(interactive "p")(message "The result is %d" (* 7 number)))In this function, the expression, (interactive "p"), is a list of two elements.
The "p" tells Emacs to pass the prefix argument to the function and use its value
for the argument of the function.
The argument will be a number. This means that the symbol number will bebound to a number in the line:(message "The result is %d" (* 7 number))Section 3.4: Different Options for interactive

31

For example, if your prefix argument is 5, the Lisp interpreter will evaluate the line
as if it were:(message "The result is %d" (* 7 5))(If you are reading this in GNU Emacs, you can evaluate this expression yourself.)
First, the interpreter will evaluate the inner list, which is (* 7 5). This returns avalue of 35. Next, it will evaluate the outer list, passing the values of the secondand subsequent elements of the list to the function message.
As we have seen, message is an Emacs Lisp function especially designed forsending a one line message to a user. (See Section 1.8.5 “The message function”,
page 14.) In summary, the message function prints its first argument in the echoarea as is, except for occurrences of ‘%d’ or ‘%s’ (and various other %-sequences
which we have not mentioned). When it sees a control sequence, the function looks
to the second or subsequent arguments and prints the value of the argument in thelocation in the string where the control sequence is located.
In the interactive multiply-by-seven function, the control string is ‘%d’, which
requires a number, and the value returned by evaluating (* 7 5) is the number 35.
Consequently, the number 35 is printed in place of the ‘%d’ and the message is ‘The
result is 35’.
(Note that when you call the function multiply-by-seven, the message isprinted without quotes, but when you call message, the text is printed in double quotes. This is because the value returned by message is what appears in theecho area when you evaluate an expression whose first element is message; but whenembedded in a function, message prints the text as a side effect without quotes.)

3.4 Different Options for interactive
In the example, multiply-by-seven used "p" as the argument to interactive.
This argument told Emacs to interpret your typing either C-u followed by a number
or META followed by a number as a command to pass that number to the function
as its argument. Emacs has more than twenty characters predefined for use withinteractive. In almost every case, one of these options will enable you to pass
the right information interactively to a function. (See Section “Code Characters forinteractive” in The GNU Emacs Lisp Reference Manual.)

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Chapter 3: How To Write Function Definitions
Consider the function zap-to-char. Its interactive expression is(interactive "p\ncZap to char: ")The first part of the argument to interactive is ‘p’, with which you are alreadyfamiliar. This argument tells Emacs to interpret a prefix, as a number to be passed
to the function. You can specify a prefix either by typing C-u followed by a number
or by typing META followed by a number. The prefix is the number of specified
characters. Thus, if your prefix is three and the specified character is ‘x’, then you
will delete all the text up to and including the third next ‘x’. If you do not set a
prefix, then you delete all the text up to and including the specified character, but
no more.
The ‘c’ tells the function the name of the character to which to delete.
More formally, a function with two or more arguments can have information
passed to each argument by adding parts to the string that follows interactive.
When you do this, the information is passed to each argument in the same order
it is specified in the interactive list. In the string, each part is separated fromthe next part by a ‘\n’, which is a newline. For example, you can follow ‘p’ witha ‘\n’ and an ‘cZap to char: ’. This causes Emacs to pass the value of the prefix
argument (if there is one) and the character.
In this case, the function definition looks like the following, where arg and charare the symbols to which interactive binds the prefix argument and the specified
character:(defun name-of-function (arg char)"documentation..."(interactive "p\ncZap to char: ")body-of-function...)(The space after the colon in the prompt makes it look better when you areprompted. See Section 5.1 “The Definition of copy-to-buffer”, page 59, for
an example.)
When a function does not take arguments, interactive does not require any.
Such a function contains the simple expression (interactive). The mark-wholebuffer function is like this.
Alternatively, if the special letter-codes are not right for your application, you
can pass your own arguments to interactive as a list.
See Section 4.4 “The Definition of append-to-buffer”, page 52, for an example.
See Section “Using Interactive” in The GNU Emacs Lisp Reference Manual, fora more complete explanation about this technique.

3.5 Install Code Permanently
When you install a function definition by evaluating it, it will stay installed until
you quit Emacs. The next time you start a new session of Emacs, the function will
not be installed unless you evaluate the function definition again.
At some point, you may want to have code installed automatically whenever
you start a new session of Emacs. There are several ways of doing this:• If you have code that is just for yourself, you can put the code for the func-

Section 3.6: let33tion definition in your .emacs initialization file. When you start Emacs, your.emacs file is automatically evaluated and all the function definitions within
it are installed. See Chapter 16 “Your .emacs File”, page 196.
• Alternatively, you can put the function definitions that you want installed in
one or more files of their own and use the load function to cause Emacs toevaluate and thereby install each of the functions in the files. See Section 16.9
“Loading Files”, page 205.
• Thirdly, if you have code that your whole site will use, it is usual to put it in afile called site-init.el that is loaded when Emacs is built. This makes thecode available to everyone who uses your machine. (See the INSTALL file thatis part of the Emacs distribution.)
Finally, if you have code that everyone who uses Emacs may want, you canpost it on a computer network or send a copy to the Free Software Foundation.
(When you do this, please license the code and its documentation under a licensethat permits other people to run, copy, study, modify, and redistribute the code
and which protects you from having your work taken from you.) If you send a copyof your code to the Free Software Foundation, and properly protect yourself andothers, it may be included in the next release of Emacs. In large part, this is how
Emacs has grown over the past years, by donations.

3.6 letThe let expression is a special form in Lisp that you will need to use in mostfunction definitions.
let is used to attach or bind a symbol to a value in such a way that the Lisp
interpreter will not confuse the variable with a variable of the same name that is
not part of the function.
To understand why the let special form is necessary, consider the situation inwhich you own a home that you generally refer to as “the house”, as in the sentence,“The house needs painting.” If you are visiting a friend and your host refers to “thehouse”, he is likely to be referring to his house, not yours, that is, to a different
house.
If your friend is referring to his house and you think he is referring to yourhouse, you may be in for some confusion. The same thing could happen in Lisp if avariable that is used inside of one function has the same name as a variable that isused inside of another function, and the two are not intended to refer to the samevalue. The let special form prevents this kind of confusion.
The let special form prevents confusion. let creates a name for a local variable
that overshadows any use of the same name outside the let expression (in computerscience jargon, we call this binding the variable). This is like understanding thatin your host’s home, whenever he refers to “the house”, he means his house, notyours. (The symbols used to name function arguments are bound as local variables
in exactly the same way. See Section 3.1 “The defun Macro”, page 26.)
Another way to think about let is that it defines a special region in your code:
within the body of the let expression, the variables you’ve named have their own

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Chapter 3: How To Write Function Definitions

local meaning. Outside of the let body, they have other meanings (or they may not
be defined at all). This means that inside the let body, calling setq for a variablenamed by the let expression will set the value of the local variable of that name.
However, outside of the let body (such as when calling a function that was definedelsewhere), calling setq for a variable named by the let expression will not affectthat local variable.1
let can create more than one variable at once. Also, let gives each variable itcreates an initial value, either a value specified by you, or nil. (In the jargon, this isbinding the variable to the value.) After let has created and bound the variables, it
executes the code in the body of the let, and returns the value of the last expression
in the body, as the value of the whole let expression. (“Execute” is a jargon termthat means to evaluate a list; it comes from the use of the word meaning “to givepractical effect to” (Oxford English Dictionary). Since you evaluate an expression
to perform an action, “execute” has evolved as a synonym to “evaluate”.)
3.6.1 The Parts of a let Expression
A let expression is a list of three parts. The first part is the symbol let. The
second part is a list, called a varlist, each element of which is either a symbol byitself or a two-element list, the first element of which is a symbol. The third partof the let expression is the body of the let. The body usually consists of one ormore lists.
A template for a let expression looks like this:(let varlist body...)The symbols in the varlist are the variables that are given initial values by the let
special form. Symbols by themselves are given the initial value of nil; and eachsymbol that is the first element of a two-element list is bound to the value that isreturned when the Lisp interpreter evaluates the second element.
Thus, a varlist might look like this: (thread (needles 3)). In this case, ina let expression, Emacs binds the symbol thread to an initial value of nil, andbinds the symbol needles to an initial value of 3.
When you write a let expression, what you do is put the appropriate expressions
in the slots of the let expression template.
If the varlist is composed of two-element lists, as is often the case, the template
for the let expression looks like this:(let ((variable value)(variable value)...)
body...)

3.6.2 Sample let Expression
The following expression creates and gives initial values to the two variables zebra
and tiger. The body of the let expression is a list which calls the message function.
1

This describes the behavior of let when using a style called “lexical binding” (seeSection 3.6.4 “How let Binds Variables”, page 36).

Section 3.6: let

35(let ((zebra "stripes")(tiger "fierce"))(message "One kind of animal has %s and another is %s."zebra tiger))

Here, the varlist is ((zebra "stripes") (tiger "fierce")).
The two variables are zebra and tiger. Each variable is the first element of
a two-element list and each value is the second element of its two-element list. In
the varlist, Emacs binds the variable zebra to the value "stripes"2 , and binds
the variable tiger to the value "fierce". In this example, both values are strings.
The values could just as well have been another list or a symbol. The body of thelet follows after the list holding the variables. In this example, the body is a listthat uses the message function to print a string in the echo area.
You may evaluate the example in the usual fashion, by placing the cursor after
the last parenthesis and typing C-x C-e. When you do this, the following will appear
in the echo area:"One kind of animal has stripes and another is fierce."

As we have seen before, the message function prints its first argument, except
for ‘%s’. In this example, the value of the variable zebra is printed at the locationof the first ‘%s’ and the value of the variable tiger is printed at the location of thesecond ‘%s’.
3.6.3 Uninitialized Variables in a let Statement
If you do not bind the variables in a let statement to specific initial values, they
will automatically be bound to an initial value of nil, as in the following expression:(let ((birch 3)pine
fir(oak 'some))(message"Here are %d variables with %s, %s, and %s value."birch pine fir oak))Here, the varlist is ((birch 3) pine fir (oak 'some)).
If you evaluate this expression in the usual way, the following will appear in yourecho area:"Here are 3 variables with nil, nil, and some value."In this example, Emacs binds the symbol birch to the number 3, binds the symbols
pine and fir to nil, and binds the symbol oak to the value some.
Note that in the first part of the let, the variables pine and fir stand alone
as atoms that are not surrounded by parentheses; this is because they are beingbound to nil, the empty list. But oak is bound to some and so is a part of the list
(oak 'some). Similarly, birch is bound to the number 3 and so is in a list withthat number. (Since a number evaluates to itself, the number does not need to bequoted. Also, the number is printed in the message using a ‘%d’ rather than a ‘%s’.)2

According to Jared Diamond in Guns, Germs, and Steel, “. . . zebras become impossibly dangerous as they grow older” but the claim here is that they do not become fierce
like a tiger. (1997, W. W. Norton and Co., ISBN 0-393-03894-2, page 171)

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Chapter 3: How To Write Function Definitions

The four variables as a group are put into a list to delimit them from the body ofthe let.
3.6.4 How let Binds Variables
Emacs Lisp supports two different ways of binding variable names to their values.
These ways affect the parts of your program where a particular binding is valid.
For historical reasons, Emacs Lisp uses a form of variable binding called dynamic
binding by default. However, in this manual we discuss the preferred form ofbinding, called lexical binding, unless otherwise noted (in the future, the Emacs
maintainers plan to change the default to lexical binding). If you have programmedin other languages before, you’re likely already familiar with how lexical binding
behaves.
In order to use lexical binding in a program, you should add this to the first lineof your Emacs Lisp file:;;; -*- lexical-binding: t -*For more information about this, see Section “Variable Scoping” in The Emacs
Lisp Reference Manual.
Differences Between Lexical and Dynamic Binding
As we discussed before (see hundefinedi “Prevent confusion”, page hundefinedi),
when you create local variables with let under lexical binding, those variables arevalid only within the body of the let expression. In other parts of your code, they
have other meanings, so if you call a function defined elsewhere within the let body,that function would be unable to “see” the local variables you’ve created. (On theother hand, if you call a function that was defined within a let body, that function
would be able to see—and modify—the local variables from that let expression.)
Under dynamic binding, the rules are different: instead, when you use let,the local variables you’ve created are valid during execution of the let expression.
This means that, if your let expression calls a function, that function can see these
local variables, regardless of where the function is defined (including in another file
entirely).
Another way to think about let when using dynamic binding is that every
variable name has a global “stack” of bindings, and whenever you use that variable’s
name, it refers to the binding on the top of the stack. (You can imagine this likea stack of papers on your desk with the values written on them.) When you binda variable dynamically with let, it puts the new binding you’ve specified on the
top of the stack, and then executes the let body. Once the let body finishes, ittakes that binding off of the stack, revealing the one it had (if any) before the let
expression.
Example of Lexical vs. Dynamic Binding
In some cases, both lexical and dynamic binding behave identically. However, inother cases, they can change the meaning of your program. For example, see what
happens in this code under lexical binding:;;; -*- lexical-binding: t -*-Section 3.7: The if Special Form

37

(setq x 0)
(defun getx ()x)(setq x 1)
(let ((x 2))(getx))⇒ 1
Here, the result of (getx) is 1. Under lexical binding, getx doesn’t see the value
from our let expression. That’s because the body of getx is outside of the body of
our let expression. Since getx is defined at the top, global level of our code (i.e.
not inside the body of any let expression), it looks for and finds x at the global
level as well. When executing getx, the current global value of x is 1, so that’s
what getx returns.
If we use dynamic binding instead, the behavior is different:;;; -*- lexical-binding: nil -*(setq x 0)
(defun getx ()x)(setq x 1)
(let ((x 2))(getx))⇒ 2
Now, the result of (getx) is 2! That’s because under dynamic binding, when
executing getx, the current binding for x at the top of our stack is the one from ourlet binding. This time, getx doesn’t see the global value for x, since its binding is
below the one from our let expression in the stack of bindings.
(Some variables are also “special”, and they are always dynamically bound evenwhen lexical-binding is t. See Section 8.5 “Initializing a Variable with defvar”,
page 104.)

3.7 The if Special Form
Another special form is the conditional if. This form is used to instruct the computer to make decisions. You can write function definitions without using if, but
it is used often enough, and is important enough, to be included here. It is used,
for example, in the code for the function beginning-of-buffer.

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The basic idea behind an if, is that if a test is true, then an expression isevaluated. If the test is not true, the expression is not evaluated. For example, you
might make a decision such as, “if it is warm and sunny, then go to the beach!”
An if expression written in Lisp does not use the word “then”; the test andthe action are the second and third elements of the list whose first element is if.
Nonetheless, the test part of an if expression is often called the if-part and thesecond argument is often called the then-part.
Also, when an if expression is written, the true-or-false-test is usually written
on the same line as the symbol if, but the action to carry out if the test is true,
the then-part, is written on the second and subsequent lines. This makes the if
expression easier to read.
(if true-or-false-test
action-to-carry-out-if-test-is-true)

The true-or-false-test will be an expression that is evaluated by the Lisp interpreter.
Here is an example that you can evaluate in the usual manner. The test iswhether the number 5 is greater than the number 4. Since it is, the message ‘5 isgreater than 4!’ will be printed.
(if (> 5 4)
(message "5 is greater than 4!")); if-part; then-part(The function > tests whether its first argument is greater than its second argument
and returns true if it is.)Of course, in actual use, the test in an if expression will not be fixed for alltime as it is by the expression (> 5 4). Instead, at least one of the variables usedin the test will be bound to a value that is not known ahead of time. (If the value
were known ahead of time, we would not need to run the test!)
For example, the value may be bound to an argument of a function definition.
In the following function definition, the character of the animal is a value that ispassed to the function. If the value bound to characteristic is "fierce", thenthe message, ‘It is a tiger!’ will be printed; otherwise, nil will be returned.
(defun type-of-animal (characteristic)"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the string \"fierce\",then warn of a tiger."(if (equal characteristic "fierce")(message "It is a tiger!")))

Section 3.7: The if Special Form

39

If you are reading this inside of GNU Emacs, you can evaluate the function definitionin the usual way to install it in Emacs, and then you can evaluate the following two
expressions to see the results:(type-of-animal "fierce")(type-of-animal "striped")

When you evaluate (type-of-animal "fierce"), you will see the following message printed in the echo area: "It is a tiger!"; and when you evaluate (type-ofanimal "striped") you will see nil printed in the echo area.
3.7.1 The type-of-animal Function in DetailLet’s look at the type-of-animal function in detail.
The function definition for type-of-animal was written by filling the slots of
two templates, one for a function definition as a whole, and a second for an if
expression.
The template for every function that is not interactive is:(defun name-of-function (argument-list)"documentation..."body...)The parts of the function that match this template look like this:(defun type-of-animal (characteristic)"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the string \"fierce\",then warn of a tiger."
body: the if expression)

The name of function is type-of-animal; it is passed the value of one argument. The argument list is followed by a multi-line documentation string. The
documentation string is included in the example because it is a good habit to write
documentation string for every function definition. The body of the function definition consists of the if expression.
The template for an if expression looks like this:(if true-or-false-test
action-to-carry-out-if-the-test-returns-true)

In the type-of-animal function, the code for the if looks like this:(if (equal characteristic "fierce")(message "It is a tiger!"))Here, the true-or-false-test is the expression:(equal characteristic "fierce")

In Lisp, equal is a function that determines whether its first argument is equal
to its second argument. The second argument is the string "fierce" and the firstargument is the value of the symbol characteristic—in other words, the argument
passed to this function.
In the first exercise of type-of-animal, the argument "fierce" is passed to
type-of-animal. Since "fierce" is equal to "fierce", the expression, (equalcharacteristic "fierce"), returns a value of true. When this happens, the40

Chapter 3: How To Write Function Definitions

if evaluates the second argument or then-part of the if: (message "It is atiger!").
On the other hand, in the second exercise of type-of-animal, the argument"striped" is passed to type-of-animal. "striped" is not equal to "fierce", sothe then-part is not evaluated and nil is returned by the if expression.

3.8 If–then–else Expressions
An if expression may have an optional third argument, called the else-part, forthe case when the true-or-false-test returns false. When this happens, the second
argument or then-part of the overall if expression is not evaluated, but the thirdor else-part is evaluated. You might think of this as the cloudy day alternative forthe decision “if it is warm and sunny, then go to the beach, else read a book!”.
The word “else” is not written in the Lisp code; the else-part of an if expression
comes after the then-part. In the written Lisp, the else-part is usually written to
start on a line of its own and is indented less than the then-part:(if true-or-false-test
action-to-carry-out-if-the-test-returns-true
action-to-carry-out-if-the-test-returns-false)

For example, the following if expression prints the message ‘4 is not greaterthan 5!’ when you evaluate it in the usual way:
(if (> 4 5)
; if-part(message "4 falsely greater than 5!") ; then-part(message "4 is not greater than 5!")); else-part

Note that the different levels of indentation make it easy to distinguish the thenpart from the else-part. (GNU Emacs has several commands that automaticallyindent if expressions correctly. See Section 1.1.3 “GNU Emacs Helps You Type
Lists”, page 3.)
We can extend the type-of-animal function to include an else-part by simply
incorporating an additional part to the if expression.

Section 3.9: Truth and Falsehood in Emacs Lisp

41

You can see the consequences of doing this if you evaluate the following versionof the type-of-animal function definition to install it and then evaluate the two
subsequent expressions to pass different arguments to the function.
(defun type-of-animal (characteristic) ; Second version.
"Print message in echo area depending on CHARACTERISTIC.
If the CHARACTERISTIC is the string \"fierce\",then warn of a tiger; else say it is not fierce."(if (equal characteristic "fierce")(message "It is a tiger!")(message "It is not fierce!")))(type-of-animal "fierce")(type-of-animal "striped")

When you evaluate (type-of-animal "fierce"), you will see the following message printed in the echo area: "It is a tiger!"; but when you evaluate (type-ofanimal "striped"), you will see "It is not fierce!".
(Of course, if the characteristic were "ferocious", the message "It is not
fierce!" would be printed; and it would be misleading! When you write code, youneed to take into account the possibility that some such argument will be tested by
the if and write your program accordingly.)

3.9 Truth and Falsehood in Emacs Lisp
There is an important aspect to the truth test in an if expression. So far, we havespoken of “true” and “false” as values of predicates as if they were new kinds ofEmacs Lisp objects. In fact, “false” is just our old friend nil. Anything else—
anything at all—is “true”.
The expression that tests for truth is interpreted as true if the result of evaluating
it is a value that is not nil. In other words, the result of the test is considered true
if the value returned is a number such as 47, a string such as "hello", or a symbol(other than nil) such as flowers, or a list (so long as it is not empty), or even abuffer!
Before illustrating a test for truth, we need an explanation of nil.
In Emacs Lisp, the symbol nil has two meanings. First, it means the empty list.
Second, it means false and is the value returned when a true-or-false-test tests false.
nil can be written as an empty list, (), or as nil. As far as the Lisp interpreteris concerned, () and nil are the same. Humans, however, tend to use nil for falseand () for the empty list.
In Emacs Lisp, any value that is not nil—is not the empty list—is considered
true. This means that if an evaluation returns something that is not an empty list,
an if expression will test true. For example, if a number is put in the slot for the
test, it will be evaluated and will return itself, since that is what numbers do whenevaluated. In this conditional, the if expression will test true. The expression tests
false only when nil, an empty list, is returned by evaluating the expression.
You can see this by evaluating the two expressions in the following examples.

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Chapter 3: How To Write Function Definitions

In the first example, the number 4 is evaluated as the test in the if expression
and returns itself; consequently, the then-part of the expression is evaluated andreturned: ‘true’ appears in the echo area. In the second example, the nil indicatesfalse; consequently, the else-part of the expression is evaluated and returned: ‘false’appears in the echo area.
(if 4
'true
'false)(if nil'true'false)Incidentally, if some other useful value is not available for a test that returns
true, then the Lisp interpreter will return the symbol t for true. For example, theexpression (> 5 4) returns t when evaluated, as you can see by evaluating it in theusual way:(> 5 4)On the other hand, this function returns nil if the test is false.
(> 4 5)3.10 save-excursion
The save-excursion function is the final special form that we will discuss in thischapter.
In Emacs Lisp programs used for editing, the save-excursion function is very
common. It saves the location of point, executes the body of the function, andthen restores point to its previous position if its location was changed. Its primary
purpose is to keep the user from being surprised and disturbed by unexpectedmovement of point.
Before discussing save-excursion, however, it may be useful first to review
what point and mark are in GNU Emacs. Point is the current location of the
cursor. Wherever the cursor is, that is point. More precisely, on terminals where
the cursor appears to be on top of a character, point is immediately before thecharacter. In Emacs Lisp, point is an integer. The first character in a buffer isnumber one, the second is number two, and so on. The function point returns thecurrent position of the cursor as a number. Each buffer has its own value for point.
The mark is another position in the buffer; its value can be set with a command such as C-SPC (set-mark-command). If a mark has been set, you can use thecommand C-x C-x (exchange-point-and-mark) to cause the cursor to jump to themark and set the mark to be the previous position of point. In addition, if you set
another mark, the position of the previous mark is saved in the mark ring. Manymark positions can be saved this way. You can jump the cursor to a saved mark by
typing C-u C-SPC one or more times.
The part of the buffer between point and mark is called the region. Numerous
commands work on the region, including center-region, count-words-region,
kill-region, and print-region.

Section 3.10: save-excursion

43

The save-excursion special form saves the location of point and restores thisposition after the code within the body of the special form is evaluated by the Lisp
interpreter. Thus, if point were in the beginning of a piece of text and some code
moved point to the end of the buffer, the save-excursion would put point back towhere it was before, after the expressions in the body of the function were evaluated.
In Emacs, a function frequently moves point as part of its internal workings eventhough a user would not expect this. For example, count-words-region moves
point. To prevent the user from being bothered by jumps that are both unexpectedand (from the user’s point of view) unnecessary, save-excursion is often used tokeep point in the location expected by the user. The use of save-excursion isgood housekeeping.
To make sure the house stays clean, save-excursion restores the value of point
even if something goes wrong in the code inside of it (or, to be more precise and touse the proper jargon, “in case of abnormal exit”). This feature is very helpful.
In addition to recording the value of point, save-excursion keeps track of thecurrent buffer, and restores it, too. This means you can write code that will change
the buffer and have save-excursion switch you back to the original buffer. Thisis how save-excursion is used in append-to-buffer. (See Section 4.4 “TheDefinition of append-to-buffer”, page 52.)
3.10.1 Template for a save-excursion Expression
The template for code using save-excursion is simple:(save-excursion
body...)The body of the function is one or more expressions that will be evaluated insequence by the Lisp interpreter. If there is more than one expression in the body,the value of the last one will be returned as the value of the save-excursion
function. The other expressions in the body are evaluated only for their side effects;
and save-excursion itself is used only for its side effect (which is restoring theposition of point).

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Chapter 3: How To Write Function Definitions
In more detail, the template for a save-excursion expression looks like this:(save-excursion
first-expression-in-bodysecond-expression-in-bodythird-expression-in-body...
last-expression-in-body)

An expression, of course, may be a symbol on its own or a list.
In Emacs Lisp code, a save-excursion expression often occurs within the bodyof a let expression. It looks like this:(let varlist(save-excursion
body...))

3.11 Review
In the last few chapters we have introduced a macro and a fair number of functions and special forms. Here they are described in brief, along with a few similarfunctions that have not been mentioned yet.
eval-last-sexp
Evaluate the last symbolic expression before the current location ofpoint. The value is printed in the echo area unless the function is
invoked with an argument; in that case, the output is printed in thecurrent buffer. This command is normally bound to C-x C-e.
defun

Define function. This macro has up to five parts: the name, a template
for the arguments that will be passed to the function, documentation,
an optional interactive declaration, and the body of the definition.
For example, in Emacs the function definition of dired-unmark-allmarks is as follows.
(defun dired-unmark-all-marks ()"Remove all marks from all files in the Dired buffer."(interactive)(dired-unmark-all-files ?\r))interactive
Declare to the interpreter that the function can be used interactively.
This special form may be followed by a string with one or more parts
that pass the information to the arguments of the function, in sequence. These parts may also tell the interpreter to prompt for information. Parts of the string are separated by newlines, ‘\n’.
Common code characters are:b

The name of an existing buffer.

f

The name of an existing file.

p

The numeric prefix argument. (Note that this p is lower
case.)

Section 3.11: Reviewr45
Point and the mark, as two numeric arguments, smallest first. This is the only code letter that specifies two
successive arguments rather than one.

See Section “Code Characters for ‘interactive’” in The GNU Emacs
Lisp Reference Manual, for a complete list of code characters.
letDeclare that a list of variables is for use within the body of the let andgive them an initial value, either nil or a specified value; then evaluate
the rest of the expressions in the body of the let and return the valueof the last one. Inside the body of the let, the Lisp interpreter does
not see the values of the variables of the same names that are boundoutside of the let.
For example,(let ((foo (buffer-name))(bar (buffer-size)))(message"This buffer is %s and has %d characters."
foo bar))save-excursion
Record the values of point and the current buffer before evaluating
the body of this special form. Restore the value of point and buffer
afterward.
For example,(message "We are %d characters into this buffer."(- (point)(save-excursion(goto-char (point-min)) (point))))if

Evaluate the first argument to the function; if it is true, evaluate thesecond argument; else evaluate the third argument, if there is one.
The if special form is called a conditional. There are other conditionals in Emacs Lisp, but if is perhaps the most commonly used.
For example,(if (= 22 emacs-major-version)(message "This is version 22 Emacs")(message "This is not version 22 Emacs"))

<
>
<=
>=

The < function tests whether its first argument is smaller than its second argument. A corresponding function, >, tests whether the firstargument is greater than the second. Likewise, <= tests whether the
first argument is less than or equal to the second and >= tests whether
the first argument is greater than or equal to the second. In all cases,
both arguments must be numbers or markers (markers indicate positions in buffers).

46

Chapter 3: How To Write Function Definitions=

The = function tests whether two arguments, both numbers or markers,
are equal.

equal
eq

Test whether two objects are the same. equal uses one meaning of
the word “same” and eq uses another: equal returns true if the two
objects have a similar structure and contents, such as two copies ofthe same book. On the other hand, eq, returns true if both arguments
are actually the same object.

string<string-lessp
string=string-equal
The string-lessp function tests whether its first argument is smaller
than the second argument. A shorter, alternative name for the samefunction (a defalias) is string<.
The arguments to string-lessp must be strings or symbols; the ordering is lexicographic, so case is significant. The print names of symbols are used instead of the symbols themselves.
An empty string, ‘""’, a string with no characters in it, is smaller thanany string of characters.
string-equal provides the corresponding test for equality. Its shorter,
alternative name is string=. There are no string test functions that
correspond to >, >=, or <=.
messagesetq
set

Print a message in the echo area. The first argument is a string thatcan contain ‘%s’, ‘%d’, or ‘%c’ to print the value of arguments that
follow the string. The argument used by ‘%s’ must be a string or asymbol; the argument used by ‘%d’ must be a number. The argumentused by ‘%c’ must be an ascii code number; it will be printed as thecharacter with that ascii code. (Various other %-sequences have notbeen mentioned.)
The setq special form sets the value of its first argument to the valueof the second argument. The first argument is automatically quoted
by setq. It does the same for succeeding pairs of arguments.

buffer-nameWithout an argument, return the name of the buffer, as a string.
buffer-file-nameWithout an argument, return the name of the file the buffer is visiting.
current-buffer
Return the buffer in which Emacs is active; it may not be the bufferthat is visible on the screen.

Section 3.12: Exercises

47

other-buffer
Return the most recently selected buffer (other than the buffer passed
to other-buffer as an argument and other than the current buffer).
switch-to-buffer
Select a buffer for Emacs to be active in and display it in the current
window so users can look at it. Usually bound to C-x b.
set-buffer
Switch Emacs’s attention to a buffer on which programs will run.
Don’t alter what the window is showing.
buffer-size
Return the number of characters in the current buffer.
point

Return the value of the current position of the cursor, as an integer
counting the number of characters from the beginning of the buffer.

point-min

Return the minimum permissible value of point in the current buffer.
This is 1, unless narrowing is in effect.

point-max

Return the value of the maximum permissible value of point in thecurrent buffer. This is the end of the buffer, unless narrowing is ineffect.

3.12 Exercises• Write a non-interactive function that doubles the value of its argument, a
number. Make that function interactive.
• Write a function that tests whether the current value of fill-column is greaterthan the argument passed to the function, and if so, prints an appropriate
message.

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Chapter 4: A Few Buffer-Related Functions

4 A Few Buffer-Related Functions
In this chapter we study in detail several of the functions used in GNU Emacs.
This is called a “walk-through”. These functions are used as examples of Lisp code,
but are not imaginary examples; with the exception of the first, simplified function
definition, these functions show the actual code used in GNU Emacs. You can learna great deal from these definitions. The functions described here are all related tobuffers. Later, we will study other functions.

4.1 Finding More Information
In this walk-through, I will describe each new function as we come to it, sometimes
in detail and sometimes briefly. If you are interested, you can get the full documentation of any Emacs Lisp function at any time by typing C-h f and then the nameof the function (and then RET). Similarly, you can get the full documentation for avariable by typing C-h v and then the name of the variable (and then RET).
Also, describe-function will tell you the location of the function definition.
Put point into the name of the file that contains the function and press the RET
key. In this case, RET means push-button rather than “return” or “enter”. Emacs
will take you directly to the function definition.
More generally, if you want to see a function in its original source file, you can
use the xref-find-definitions function to jump to it. xref-find-definitions
works with a wide variety of languages, not just Lisp, and C, and it works with
non-programming text as well. For example, xref-find-definitions will jump to
the various nodes in the Texinfo source file of this document (provided that you’ve
run the etags utility to record all the nodes in the manuals that come with Emacs;
see Section “Create Tags Table” in The GNU Emacs Manual).
To use the xref-find-definitions command, type M-. (i.e., press the period
key while holding down the META key, or else type the ESC key and then type theperiod key), and then, at the prompt, type in the name of the function whose
source code you want to see, such as mark-whole-buffer, and then type RET. (Ifthe command doesn’t prompt, invoke it with an argument: C-u M-.; see Section 3.4“Interactive Options”, page 31.) Emacs will switch buffers and display the sourcecode for the function on your screen1 . To switch back to your current buffer, typeM-, or C-x b RET. (On some keyboards, the META key is labeled ALT.)
Incidentally, the files that contain Lisp code are conventionally called libraries.
The metaphor is derived from that of a specialized library, such as a law library or
an engineering library, rather than a general library. Each library, or file, contains
functions that relate to a particular topic or activity, such as abbrev.el for handlingabbreviations and other typing shortcuts, and help.el for help. (Sometimes several
libraries provide code for a single activity, as the various rmail... files provide codefor reading electronic mail.) In The GNU Emacs Manual, you will see sentences
1

If instead of showing the source code for a Lisp function, Emacs asks you whichtags table to visit, invoke M-. from a buffer whose major mode is Emacs Lisp or Lisp
Interaction.

Section 4.2: A Simplified beginning-of-buffer Definition

49

such as “The C-h p command lets you search the standard Emacs Lisp libraries by
topic keywords.”

4.2 A Simplified beginning-of-buffer Definition
The beginning-of-buffer command is a good function to start with since you arelikely to be familiar with it and it is easy to understand. Used as an interactive
command, beginning-of-buffer moves the cursor to the beginning of the buffer,leaving the mark at the previous position. It is generally bound to M-<.
In this section, we will discuss a shortened version of the function that shows
how it is most frequently used. This shortened function works as written, but it does
not contain the code for a complex option. In another section, we will describe theentire function. (See Section 5.3 “Complete Definition of beginning-of-buffer”,
page 65.)
Before looking at the code, let’s consider what the function definition has tocontain: it must include an expression that makes the function interactive so it canbe called by typing M-x beginning-of-buffer or by typing a keychord such as M-<;it must include code to leave a mark at the original position in the buffer; and it
must include code to move the cursor to the beginning of the buffer.
Here is the complete text of the shortened version of the function:(defun simplified-beginning-of-buffer ()"Move point to the beginning of the buffer;
leave mark at previous position."(interactive)(push-mark)(goto-char (point-min)))Like all function definitions, this definition has five parts following the macro
defun:
1. The name: in this example, simplified-beginning-of-buffer.
2. A list of the arguments: in this example, an empty list, (),3. The documentation string.
4. The interactive expression.
5. The body.
In this function definition, the argument list is empty; this means that this function
does not require any arguments. (When we look at the definition for the complete
function, we will see that it may be passed an optional argument.)
The interactive expression tells Emacs that the function is intended to be usedinteractively. In this example, interactive does not have an argument because
simplified-beginning-of-buffer does not require one.

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Chapter 4: A Few Buffer-Related Functions
The body of the function consists of the two lines:(push-mark)(goto-char (point-min))

The first of these lines is the expression, (push-mark). When this expression
is evaluated by the Lisp interpreter, it sets a mark at the current position of the
cursor, wherever that may be. The position of this mark is saved in the mark ring.
The next line is (goto-char (point-min)). This expression jumps the cursor
to the minimum point in the buffer, that is, to the beginning of the buffer (or to
the beginning of the accessible portion of the buffer if it is narrowed. See Chapter 6“Narrowing and Widening”, page 73.)
The push-mark command sets a mark at the place where the cursor was locatedbefore it was moved to the beginning of the buffer by the (goto-char (pointmin)) expression. Consequently, you can, if you wish, go back to where you wereoriginally by typing C-x C-x.
That is all there is to the function definition!
When you are reading code such as this and come upon an unfamiliar function,such as goto-char, you can find out what it does by using the describe-functioncommand. To use this command, type C-h f and then type in the name of thefunction and press RET. The describe-function command will print the function’s
documentation string in a *Help* window. For example, the documentation forgoto-char is:
Set point to POSITION, a number or marker.
Beginning of buffer is position (point-min), end is (point-max).

The function’s one argument is the desired position.
(The prompt for describe-function will offer you the symbol under or preceding
the cursor, so you can save typing by positioning the cursor right over or after thefunction and then typing C-h f RET.)
The end-of-buffer function definition is written in the same way as the
beginning-of-buffer definition except that the body of the function contains theexpression (goto-char (point-max)) in place of (goto-char (point-min)).

4.3 The Definition of mark-whole-buffer
The mark-whole-buffer function is no harder to understand than thesimplified-beginning-of-buffer function. In this case, however, we will lookat the complete function, not a shortened version.
The mark-whole-buffer function is not as commonly used as the
beginning-of-buffer function, but is useful nonetheless: it marks a whole buffer
as a region by putting point at the beginning and a mark at the end of the buffer.
It is generally bound to C-x h.

Section 4.3: The Definition of mark-whole-buffer

51

In GNU Emacs 22, the code for the complete function looks like this:(defun mark-whole-buffer ()"Put point at beginning and mark at end of buffer.
You probably should not use this function in Lisp programs;
it is usually a mistake for a Lisp function to use any subroutinethat uses or sets the mark."(interactive)(push-mark (point))(push-mark (point-max) nil t)(goto-char (point-min)))Like all other functions, the mark-whole-buffer function fits into the template
for a function definition. The template looks like this:(defun name-of-function (argument-list)"documentation..."(interactive-expression...)body...)

Here is how the function works: the name of the function is mark-whole-buffer;
it is followed by an empty argument list, ‘()’, which means that the function does
not require arguments. The documentation comes next.
The next line is an (interactive) expression that tells Emacs that the function
will be used interactively. These details are similar to the simplified-beginningof-buffer function described in the previous section.
4.3.1 Body of mark-whole-buffer
The body of the mark-whole-buffer function consists of three lines of code:(push-mark (point))(push-mark (point-max) nil t)(goto-char (point-min))

The first of these lines is the expression, (push-mark (point)).
This line does exactly the same job as the first line of the body of thesimplified-beginning-of-buffer function, which is written (push-mark). In
both cases, the Lisp interpreter sets a mark at the current position of the cursor.
I don’t know why the expression in mark-whole-buffer is written (push-mark(point)) and the expression in beginning-of-buffer is written (push-mark).
Perhaps whoever wrote the code did not know that the arguments for push-markare optional and that if push-mark is not passed an argument, the function automatically sets mark at the location of point by default. Or perhaps the expression
was written so as to parallel the structure of the next line. In any case, the line
causes Emacs to determine the position of point and set a mark there.
In earlier versions of GNU Emacs, the next line of mark-whole-buffer was(push-mark (point-max)). This expression sets a mark at the point in the bufferthat has the highest number. This will be the end of the buffer (or, if the buffer isnarrowed, the end of the accessible portion of the buffer. See Chapter 6 “Narrowing
and Widening”, page 73, for more about narrowing.) After this mark has been set,
the previous mark, the one set at point, is no longer set, but Emacs remembers its
position, just as all other recent marks are always remembered. This means thatyou can, if you wish, go back to that position by typing C-u C-SPC twice.

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Chapter 4: A Few Buffer-Related Functions

In GNU Emacs 22, the (point-max) is slightly more complicated. The line
reads
(push-mark (point-max) nil t)

The expression works nearly the same as before. It sets a mark at the highest
numbered place in the buffer that it can. However, in this version, push-mark has
two additional arguments. The second argument to push-mark is nil. This tells
the function it should display a message that says “Mark set” when it pushes themark. The third argument is t. This tells push-mark to activate the mark whenTransient Mark mode is turned on. Transient Mark mode highlights the currently
active region. It is often turned off.
Finally, the last line of the function is (goto-char (point-min))). This is written exactly the same way as it is written in beginning-of-buffer. The expression
moves the cursor to the minimum point in the buffer, that is, to the beginning of
the buffer (or to the beginning of the accessible portion of the buffer). As a resultof this, point is placed at the beginning of the buffer and mark is set at the end of
the buffer. The whole buffer is, therefore, the region.

4.4 The Definition of append-to-buffer
The append-to-buffer command is more complex than the mark-whole-buffer
command. What it does is copy the region (that is, the part of the buffer between
point and mark) from the current buffer to a specified buffer.
The append-to-buffer command uses the insert-buffer-substring function
to copy the region. insert-buffer-substring is described by its name: it takes asubstring from a buffer, and inserts it into another buffer.
Most of append-to-buffer is concerned with setting up the conditions for
insert-buffer-substring to work: the code must specify both the buffer to which
the text will go, the window it comes from and goes to, and the region that will becopied.

Section 4.4: The Definition of append-to-buffer

53

Here is a possible implementation of the function:(defun append-to-buffer (buffer start end)
"Append to specified buffer the text of the region.
It is inserted into that buffer before its point.
When calling from a program, give three arguments:
BUFFER (or buffer name), START and END.
START and END specify the portion of the current buffer to be copied."(interactive(list (read-buffer "Append to buffer: " (other-buffer(current-buffer) t))(region-beginning) (region-end)))(let ((oldbuf (current-buffer)))(save-excursion(let* ((append-to (get-buffer-create buffer))(windows (get-buffer-window-list append-to t t))
point)(set-buffer append-to)(setq point (point))(barf-if-buffer-read-only)(insert-buffer-substring oldbuf start end)
(dolist (window windows)(when (= (window-point window) point)(set-window-point window (point))))))))

The function can be understood by looking at it as a series of filled-in templates.
The outermost template is for the function definition. In this function, it looks
like this (with several slots filled in):(defun append-to-buffer (buffer start end)
"documentation..."(interactive ...)
body...)

The first line of the function includes its name and three arguments. The arguments are the buffer to which the text will be copied, and the start and end of
the region in the current buffer that will be copied.
The next part of the function is the documentation, which is clear and complete.
As is conventional, the three arguments are written in upper case so you will noticethem easily. Even better, they are described in the same order as in the argument
list.
Note that the documentation distinguishes between a buffer and its name. (Thefunction can handle either.)

4.4.1 The append-to-buffer Interactive Expression
Since the append-to-buffer function will be used interactively, the function musthave an interactive expression. (For a review of interactive, see Section 3.3“Making a Function Interactive”, page 29.)
The expression reads as follows:

54

Chapter 4: A Few Buffer-Related Functions(interactive(list (read-buffer"Append to buffer: "(other-buffer (current-buffer) t))(region-beginning)(region-end)))This expression is not one with letters standing for parts, as described earlier.
Instead, it starts a list with these parts:
The first part of the list is an expression to read the name of a buffer and returnit as a string. That is read-buffer. The function requires a prompt as its firstargument, ‘"Append to buffer: "’. Its second argument tells the command whatvalue to provide if you don’t specify anything.
In this case that second argument is an expression containing the function
other-buffer, an exception, and a ‘t’, standing for true.
The first argument to other-buffer, the exception, is yet another function,
current-buffer. That is not going to be returned. The second argument is the
symbol for true, t. That tells other-buffer that it may show visible buffers (except
in this case, it will not show the current buffer, which makes sense).
The expression looks like this:(other-buffer (current-buffer) t)

The second and third arguments to the list expression are(region-beginning) and (region-end).
These two functions specify the
beginning and end of the text to be appended.
Originally, the command used the letters ‘B’ and ‘r’. The whole interactive
expression looked like this:(interactive "BAppend to buffer: \nr")But when that was done, the default value of the buffer switched to was invisible.
That was not wanted.
(The prompt was separated from the second argument with a newline, ‘\n’. It
was followed by an ‘r’ that told Emacs to bind the two arguments that follow thesymbol buffer in the function’s argument list (that is, start and end) to the valuesof point and mark. That argument worked fine.)
4.4.2 The Body of append-to-buffer
The body of the append-to-buffer function begins with let.
As we have seen before (see Section 3.6 “let”, page 33), the purpose of a let
expression is to create and give initial values to one or more variables that will only
be used within the body of the let. This means that such a variable will not be
confused with any variable of the same name outside the let expression.
We can see how the let expression fits into the function as a whole by showing
a template for append-to-buffer with the let expression in outline:(defun append-to-buffer (buffer start end)
"documentation..."(interactive ...)
(let ((variable value))body...))

Section 4.4: The Definition of append-to-buffer

55

The let expression has three elements:
1. The symbol let;
2. A varlist containing, in this case, a single two-element list, (variable value);
3. The body of the let expression.
In the append-to-buffer function, the varlist looks like this:(oldbuf (current-buffer))

In this part of the let expression, the one variable, oldbuf, is bound to the value
returned by the (current-buffer) expression. The variable, oldbuf, is used tokeep track of the buffer in which you are working and from which you will copy.
The element or elements of a varlist are surrounded by a set of parentheses so
the Lisp interpreter can distinguish the varlist from the body of the let. As a consequence, the two-element list within the varlist is surrounded by a circumscribing
set of parentheses. The line looks like this:(let ((oldbuf (current-buffer)))... )The two parentheses before oldbuf might surprise you if you did not realizethat the first parenthesis before oldbuf marks the boundary of the varlist andthe second parenthesis marks the beginning of the two-element list, (oldbuf(current-buffer)).
4.4.3 save-excursion in append-to-buffer
The body of the let expression in append-to-buffer consists of a save-excursion
expression.
The save-excursion function saves the location of point, and restores it tothat position after the expressions in the body of the save-excursion complete
execution. In addition, save-excursion keeps track of the original buffer, andrestores it. This is how save-excursion is used in append-to-buffer.
Incidentally, it is worth noting here that a Lisp function is normally formatted
so that everything that is enclosed in a multi-line spread is indented more to theright than the first symbol. In this function definition, the let is indented morethan the defun, and the save-excursion is indented more than the let, like this:(defun ...
...
...
(let...
(save-excursion
...

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Chapter 4: A Few Buffer-Related Functions

This formatting convention makes it easy to see that the lines in the body of thesave-excursion are enclosed by the parentheses associated with save-excursion,
just as the save-excursion itself is enclosed by the parentheses associated withthe let:(let ((oldbuf (current-buffer)))(save-excursion...
(set-buffer ...)
(insert-buffer-substring oldbuf start end)...))

The use of the save-excursion function can be viewed as a process of filling inthe slots of a template:(save-excursion
first-expression-in-bodysecond-expression-in-body...
last-expression-in-body)In this function, the body of the save-excursion contains only one expression, thelet* expression. You know about a let function. The let* function is different.
It enables Emacs to set each variable in its varlist in sequence, one after another;such that variables in the latter part of the varlist can make use of the values towhich Emacs set variables earlier in the varlist.
Looking at the let* expression in append-to-buffer:(let* ((append-to (get-buffer-create buffer))(windows (get-buffer-window-list append-to t t))
point)
BODY...)

we see that append-to is bound to the value returned by the(get-buffer-create buffer).
On the next line, append-to is used as
an argument to get-buffer-window-list; this would not be possible with thelet expression. Note that point is automatically bound to nil, the same way as
it would be done in the let statement.
Now let’s focus on the functions set-buffer and insert-buffer-substringin the body of the let* expression.
In the old days, the set-buffer expression was simply(set-buffer (get-buffer-create buffer))but now it is(set-buffer append-to)This is because append-to was bound to (get-buffer-create buffer) earlier onin the let* expression.
The append-to-buffer function definition inserts text from the buffer in which
you are currently to a named buffer. It happens that insert-buffer-substring
does just the reverse—it copies text from another buffer to the current buffer—that
is why the append-to-buffer definition starts out with a let that binds the local
symbol oldbuf to the value returned by current-buffer.

Section 4.5: Review

57

The insert-buffer-substring expression looks like this:(insert-buffer-substring oldbuf start end)

The insert-buffer-substring function copies a string from the buffer specified
as its first argument and inserts the string into the present buffer. In this case, theargument to insert-buffer-substring is the value of the variable created andbound by the let, namely the value of oldbuf, which was the current buffer when
you gave the append-to-buffer command.
After insert-buffer-substring has done its work, save-excursion will restore the action to the original buffer and append-to-buffer will have done its
job.
Written in skeletal form, the workings of the body look like this:(let (bind-oldbuf-to-value-of-current-buffer)(save-excursion; Keep track of buffer.
change-buffer
insert-substring-from-oldbuf-into-buffer)change-back-to-original-buffer-when-finished
let-the-local-meaning-of-oldbuf-disappear-when-finished

In summary, append-to-buffer works as follows: it saves the value of thecurrent buffer in the variable called oldbuf. It gets the new buffer (creating one
if need be) and switches Emacs’s attention to it. Using the value of oldbuf, it
inserts the region of text from the old buffer into the new buffer; and then using
save-excursion, it brings you back to your original buffer.
In looking at append-to-buffer, you have explored a fairly complex function.
It shows how to use let and save-excursion, and how to change to and comeback from another buffer. Many function definitions use let, save-excursion,and set-buffer this way.

4.5 Review
Here is a brief summary of the various functions discussed in this chapter.
describe-function
describe-variable
Print the documentation for a function or variable. Conventionallybound to C-h f and C-h v.
xref-find-definitions
Find the file containing the source for a function or variable and switch
buffers to it, positioning point at the beginning of the item. Conventionally bound to M-. (that’s a period following the META key).
save-excursion
Save the location of point and restore its value after the arguments
to save-excursion have been evaluated. Also, remember the current
buffer and return to it.

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Chapter 4: A Few Buffer-Related Functions

push-markSet mark at a location and record the value of the previous mark on
the mark ring. The mark is a location in the buffer that will keep its
relative position even if text is added to or removed from the buffer.

goto-charSet point to the location specified by the value of the argument, which
can be a number, a marker, or an expression that returns the numberof a position, such as (point-min).

insert-buffer-substring
Copy a region of text from a buffer that is passed to the function as
an argument and insert the region into the current buffer.
mark-whole-bufferMark the whole buffer as a region. Normally bound to C-x h.
let*

Declare a list of variables and give them an initial value; then evaluate
the rest of the expressions in the body of let*. The values of thevariables can be used to bind ensuing variables in the list.

set-buffer
Switch the attention of Emacs to another buffer, but do not change
the window being displayed. Used when the program rather than ahuman is to work on a different buffer.
get-buffer-create
get-buffer
Find a named buffer or create one if a buffer of that name does notexist. The get-buffer function returns nil if the named buffer does
not exist.

4.6 Exercises• Write your own simplified-end-of-buffer function definition; then test it
to see whether it works.
• Use if and get-buffer to write a function that prints a message telling youwhether a buffer exists.
• Using xref-find-definitions, find the source for the copy-to-buffer function.

Section 5.1: The Definition of copy-to-buffer

59

5 A Few More Complex Functions
In this chapter, we build on what we have learned in previous chapters by looking
at more complex functions. The copy-to-buffer function illustrates use of two
save-excursion expressions in one definition, while the insert-buffer function
illustrates use of an asterisk in an interactive expression, use of or, and theimportant distinction between a name and the object to which the name refers.

5.1 The Definition of copy-to-buffer
After understanding how append-to-buffer works, it is easy to understandcopy-to-buffer. This function copies text into a buffer, but instead of adding
to the second buffer, it replaces all the previous text in the second buffer.
The body of copy-to-buffer looks like this,...
(interactive "BCopy to buffer: \nr")(let ((oldbuf (current-buffer)))(with-current-buffer (get-buffer-create buffer)(barf-if-buffer-read-only)(erase-buffer)(save-excursion(insert-buffer-substring oldbuf start end)))))

The copy-to-buffer function has a simpler interactive expression thanappend-to-buffer.
The definition then says
(with-current-buffer (get-buffer-create buffer) ...

First, look at the earliest inner expression; that is evaluated first. That expression starts with get-buffer-create buffer. The function tells the computer to
use the buffer with the name specified as the one to which you are copying, or ifsuch a buffer does not exist, to create it. Then, the with-current-buffer function
evaluates its body with that buffer temporarily current, after which it will switch
back to the buffer we are at now1 .
(This demonstrates another way to shift the computer’s attention but not
the user’s. The append-to-buffer function showed how to do the same withsave-excursion and set-buffer. with-current-buffer is a newer, and arguably
easier, mechanism.)
The barf-if-buffer-read-only function sends you an error message saying
the buffer is read-only if you cannot modify it.
The next line has the erase-buffer function as its sole contents. That function
erases the buffer.
Finally, the last two lines contain the save-excursion expression withinsert-buffer-substring as its body.
The insert-buffer-substring
expression copies the text from the buffer you are in (and you have not seen the1

It is like calling (save-excursion (set-buffer ...) ...) in one go, though it is defined slightly differently which interested reader can find out using describe-function.

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Chapter 5: A Few More Complex Functions

computer shift its attention, so you don’t know that that buffer is now called
oldbuf).
Incidentally, this is what is meant by “replacement”. To replace text, Emacs
erases the previous text and then inserts new text.
In outline, the body of copy-to-buffer looks like this:(let (bind-oldbuf-to-value-of-current-buffer)(with-the-buffer-you-are-copying-to(but-do-not-erase-or-copy-to-a-read-only-buffer)(erase-buffer)(save-excursion
insert-substring-from-oldbuf-into-buffer)))

5.2 The Definition of insert-buffer
insert-buffer is yet another buffer-related function. This command copies another buffer into the current buffer. It is the reverse of append-to-buffer or
copy-to-buffer, since they copy a region of text from the current buffer to another buffer.
Here is a discussion based on the original code. The code was simplified in 2003
and is harder to understand.
(See Section 5.2.6 “New Body for insert-buffer”, page 65, to see a discussionof the new body.)
In addition, this code illustrates the use of interactive with a buffer that might
be read-only and the important distinction between the name of an object and theobject actually referred to.
Here is the earlier code:(defun insert-buffer (buffer)"Insert after point the contents of BUFFER.
Puts mark after the inserted text.
BUFFER may be a buffer or a buffer name."(interactive "*bInsert buffer: ")(or (bufferp buffer)(setq buffer (get-buffer buffer)))(let (start end newmark)(save-excursion(save-excursion(set-buffer buffer)(setq start (point-min) end (point-max)))(insert-buffer-substring buffer start end)
(setq newmark (point)))(push-mark newmark)))

Section 5.2: The Definition of insert-buffer

61

As with other function definitions, you can use a template to see an outline of
the function:(defun insert-buffer (buffer)"documentation..."(interactive "*bInsert buffer: ")body...)

5.2.1 The Interactive Expression in insert-buffer
In insert-buffer, the argument to the interactive declaration has two parts, anasterisk, ‘*’, and ‘bInsert buffer: ’.
A Read-only Buffer
The asterisk is for the situation when the current buffer is a read-only buffer—abuffer that cannot be modified. If insert-buffer is called when the current buffer
is read-only, a message to this effect is printed in the echo area and the terminal
may beep or blink at you; you will not be permitted to insert anything into current
buffer. The asterisk does not need to be followed by a newline to separate it fromthe next argument.
‘b’ in an Interactive Expression
The next argument in the interactive expression starts with a lower case ‘b’. (Thisis different from the code for append-to-buffer, which uses an upper-case ‘B’. See
Section 4.4 “The Definition of append-to-buffer”, page 52.) The lower-case ‘b’tells the Lisp interpreter that the argument for insert-buffer should be an existing
buffer or else its name. (The upper-case ‘B’ option provides for the possibility thatthe buffer does not exist.) Emacs will prompt you for the name of the buffer,offering you a default buffer, with name completion enabled. If the buffer does notexist, you receive a message that says “No match”; your terminal may beep at you
as well.
The new and simplified code generates a list for interactive. It uses thebarf-if-buffer-read-only and read-buffer functions with which we are alreadyfamiliar and the progn special form with which we are not. (It will be describedlater.)
5.2.2 The Body of the insert-buffer Function
The body of the insert-buffer function has two major parts: an or expression anda let expression. The purpose of the or expression is to ensure that the argument
buffer is bound to a buffer and not just the name of a buffer. The body of the let
expression contains the code which copies the other buffer into the current buffer.

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Chapter 5: A Few More Complex Functions
In outline, the two expressions fit into the insert-buffer function like this:(defun insert-buffer (buffer)"documentation..."(interactive "*bInsert buffer: ")(or ...
...
(let (varlist)
body-of-let... )To understand how the or expression ensures that the argument buffer is boundto a buffer and not to the name of a buffer, it is first necessary to understand theor function.
Before doing this, let me rewrite this part of the function using if so that you
can see what is done in a manner that will be familiar.
5.2.3 insert-buffer With an if Instead of an or
The job to be done is to make sure the value of buffer is a buffer itself and not thename of a buffer. If the value is the name, then the buffer itself must be got.
You can imagine yourself at a conference where an usher is wandering aroundholding a list with your name on it and looking for you: the usher is bound to yourname, not to you; but when the usher finds you and takes your arm, the usher
becomes bound to you.
In Lisp, you might describe this situation like this:(if (not (holding-on-to-guest))(find-and-take-arm-of-guest))

We want to do the same thing with a buffer—if we do not have the buffer itself,
we want to get it.
Using a predicate called bufferp that tells us whether we have a buffer (rather
than its name), we can write the code like this:(if (not (bufferp buffer))(setq buffer (get-buffer buffer))); if-part; then-part

Here, the true-or-false-test of the if expression is (not (bufferp buffer)); andthe then-part is the expression (setq buffer (get-buffer buffer)).
In the test, the function bufferp returns true if its argument is a buffer—butfalse if its argument is the name of the buffer. (The last character of the function
name bufferp is the character ‘p’; as we saw earlier, such use of ‘p’ is a convention
that indicates that the function is a predicate, which is a term that means that thefunction will determine whether some property is true or false. See Section 1.8.4“Using the Wrong Type Object as an Argument”, page 13.)
The function not precedes the expression (bufferp buffer), so the true-orfalse-test looks like this:(not (bufferp buffer))

not is a function that returns true if its argument is false and false if its argument
is true. So if (bufferp buffer) returns true, the not expression returns false andvice versa.
Using this test, the if expression works as follows: when the value of the variablebuffer is actually a buffer rather than its name, the true-or-false-test returns false

Section 5.2: The Definition of insert-buffer63

and the if expression does not evaluate the then-part. This is fine, since we do notneed to do anything to the variable buffer if it really is a buffer.
On the other hand, when the value of buffer is not a buffer itself, but the nameof a buffer, the true-or-false-test returns true and the then-part of the expression
is evaluated. In this case, the then-part is (setq buffer (get-buffer buffer)).
This expression uses the get-buffer function to return an actual buffer itself, given
its name. The setq then sets the variable buffer to the value of the buffer itself,
replacing its previous value (which was the name of the buffer).
5.2.4 The or in the Body
The purpose of the or expression in the insert-buffer function is to ensure thatthe argument buffer is bound to a buffer and not just to the name of a buffer. The
previous section shows how the job could have been done using an if expression.
However, the insert-buffer function actually uses or. To understand this, it isnecessary to understand how or works.
An or function can have any number of arguments. It evaluates each argument
in turn and returns the value of the first of its arguments that is not nil. Also, andthis is a crucial feature of or, it does not evaluate any subsequent arguments after
returning the first non-nil value.
The or expression looks like this:(or (bufferp buffer)(setq buffer (get-buffer buffer)))

The first argument to or is the expression (bufferp buffer). This expression
returns true (a non-nil value) if the buffer is actually a buffer, and not just thename of a buffer. In the or expression, if this is the case, the or expression returns
this true value and does not evaluate the next expression—and this is fine with us,
since we do not want to do anything to the value of buffer if it really is a buffer.
On the other hand, if the value of (bufferp buffer) is nil, which it will be if
the value of buffer is the name of a buffer, the Lisp interpreter evaluates the next
element of the or expression. This is the expression (setq buffer (get-buffer
buffer)). This expression returns a non-nil value, which is the value to which it
sets the variable buffer—and this value is a buffer itself, not the name of a buffer.
The result of all this is that the symbol buffer is always bound to a buffer itself
rather than to the name of a buffer. All this is necessary because the set-buffer
function in a following line only works with a buffer itself, not with the name to abuffer.

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Chapter 5: A Few More Complex Functions
Incidentally, using or, the situation with the usher would be written like this:(or (holding-on-to-guest) (find-and-take-arm-of-guest))

5.2.5 The let Expression in insert-buffer
After ensuring that the variable buffer refers to a buffer itself and not just to thename of a buffer, the insert-buffer function continues with a let expression.
This specifies three local variables, start, end, and newmark and binds them tothe initial value nil. These variables are used inside the remainder of the let andtemporarily hide any other occurrence of variables of the same name in Emacs until
the end of the let.
The body of the let contains two save-excursion expressions. First, we will
look at the inner save-excursion expression in detail. The expression looks likethis:(save-excursion(set-buffer buffer)(setq start (point-min) end (point-max)))

The expression (set-buffer buffer) changes Emacs’s attention from the current
buffer to the one from which the text will copied. In that buffer, the variables
start and end are set to the beginning and end of the buffer, using the commands
point-min and point-max. Note that we have here an illustration of how setq isable to set two variables in the same expression. The first argument of setq is set
to the value of its second, and its third argument is set to the value of its fourth.
After the body of the inner save-excursion is evaluated, the save-excursion
restores the original buffer, but start and end remain set to the values of the
beginning and end of the buffer from which the text will be copied.
The outer save-excursion expression looks like this:(save-excursion(inner-save-excursion-expression(go-to-new-buffer-and-set-start-and-end)(insert-buffer-substring buffer start end)
(setq newmark (point)))

The insert-buffer-substring function copies the text into the current bufferfrom the region indicated by start and end in buffer. Since the whole of thesecond buffer lies between start and end, the whole of the second buffer is copied
into the buffer you are editing. Next, the value of point, which will be at the endof the inserted text, is recorded in the variable newmark.
After the body of the outer save-excursion is evaluated, point is relocated toits original place.
However, it is convenient to locate a mark at the end of the newly inserted
text and locate point at its beginning. The newmark variable records the end of
the inserted text. In the last line of the let expression, the (push-mark newmark)expression function sets a mark to this location. (The previous location of the markis still accessible; it is recorded on the mark ring and you can go back to it with C-u
C-SPC.) Meanwhile, point is located at the beginning of the inserted text, which iswhere it was before you called the insert function, the position of which was savedby the first save-excursion.

Section 5.3: Complete Definition of beginning-of-buffer

65

The whole let expression looks like this:(let (start end newmark)(save-excursion(save-excursion(set-buffer buffer)(setq start (point-min) end (point-max)))(insert-buffer-substring buffer start end)
(setq newmark (point)))(push-mark newmark))

Like the append-to-buffer function, the insert-buffer function uses let,save-excursion, and set-buffer. In addition, the function illustrates one way touse or. All these functions are building blocks that we will find and use again andagain.
5.2.6 New Body for insert-buffer
The body in the GNU Emacs 22 version is more confusing than the original.
It consists of two expressions,(push-mark(save-excursion(insert-buffer-substring (get-buffer buffer))(point)))nil

except, and this is what confuses novices, very important work is done inside thepush-mark expression.
The get-buffer function returns a buffer with the name provided. You willnote that the function is not called get-buffer-create; it does not create a buffer
if one does not already exist. The buffer returned by get-buffer, an existing buffer,is passed to insert-buffer-substring, which inserts the whole of the buffer (since
you did not specify anything else).
The location into which the buffer is inserted is recorded by push-mark. Thenthe function returns nil, the value of its last command. Put another way, theinsert-buffer function exists only to produce a side effect, inserting anotherbuffer, not to return any value.

5.3 Complete Definition of beginning-of-buffer
The basic structure of the beginning-of-buffer function has already been discussed. (See Section 4.2 “A Simplified beginning-of-buffer Definition”, page 49.)
This section describes the complex part of the definition.
As previously described, when invoked without an argument, beginning-ofbuffer moves the cursor to the beginning of the buffer (in truth, the beginningof the accessible portion of the buffer), leaving the mark at the previous position.
However, when the command is invoked with a number between one and ten, thefunction considers that number to be a fraction of the length of the buffer, measured
in tenths, and Emacs moves the cursor that fraction of the way from the beginningof the buffer. Thus, you can either call this function with the key command M-<,

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Chapter 5: A Few More Complex Functions

which will move the cursor to the beginning of the buffer, or with a key commandsuch as C-u 7 M-< which will move the cursor to a point 70% of the way throughthe buffer. If a number bigger than ten is used for the argument, it moves to theend of the buffer.
The beginning-of-buffer function can be called with or without an argument.
The use of the argument is optional.
5.3.1 Optional Arguments
Unless told otherwise, Lisp expects that a function with an argument in its function
definition will be called with a value for that argument. If that does not happen,
you get an error and a message that says ‘Wrong number of arguments’.
However, optional arguments are a feature of Lisp: a particular keyword is used
to tell the Lisp interpreter that an argument is optional. The keyword is &optional.
(The ‘&’ in front of ‘optional’ is part of the keyword.) In a function definition,
if an argument follows the keyword &optional, no value need be passed to thatargument when the function is called.
The first line of the function definition of beginning-of-buffer therefore looks
like this:(defun beginning-of-buffer (&optional arg)

Section 5.3: Complete Definition of beginning-of-buffer

67

In outline, the whole function looks like this:(defun beginning-of-buffer (&optional arg)
"documentation..."(interactive "P")(or (is-the-argument-a-cons-cell arg)
(and are-both-transient-mark-mode-and-mark-active-true)(push-mark))(let (determine-size-and-set-it)(goto-char(if-there-is-an-argument
figure-out-where-to-goelse-go-to(point-min))))do-nicety

The function is similar to the simplified-beginning-of-buffer function except that the interactive expression has "P" as an argument and the goto-charfunction is followed by an if-then-else expression that figures out where to put the
cursor if there is an argument that is not a cons cell.
(Since I do not explain a cons cell for many more chapters, please consider ignoring the function consp. See Chapter 9 “How Lists are Implemented”, page 108, andSection “Cons Cell and List Types” in The GNU Emacs Lisp Reference Manual.)
The "P" in the interactive expression tells Emacs to pass a prefix argument,
if there is one, to the function in raw form. A prefix argument is made by typing
the META key followed by a number, or by typing C-u and then a number. (If you
don’t type a number, C-u defaults to a cons cell with a 4. A lowercase "p" in theinteractive expression causes the function to convert a prefix arg to a number.)
The true-or-false-test of the if expression looks complex, but it is not: it checks
whether arg has a value that is not nil and whether it is a cons cell. (That iswhat consp does; it checks whether its argument is a cons cell.) If arg has a valuethat is not nil (and is not a cons cell), which will be the case if beginning-ofbuffer is called with a numeric argument, then this true-or-false-test will return
true and the then-part of the if expression will be evaluated. On the other hand,
if beginning-of-buffer is not called with an argument, the value of arg will be
nil and the else-part of the if expression will be evaluated. The else-part is simply point-min, and when this is the outcome, the whole goto-char expression is(goto-char (point-min)), which is how we saw the beginning-of-buffer function in its simplified form.

5.3.2 beginning-of-buffer with an Argument
When beginning-of-buffer is called with an argument, an expression is evaluated which calculates what value to pass to goto-char. This expression is rather
complicated at first sight. It includes an inner if expression and much arithmetic.
It looks like this:

68

Chapter 5: A Few More Complex Functions(if (> (buffer-size) 10000);; Avoid overflow for large buffer sizes!
(* (prefix-numeric-value arg)(/ size 10))(/(+ 10(* size(prefix-numeric-value arg)))
10))

Like other complex-looking expressions, the conditional expression within
beginning-of-buffer can be disentangled by looking at it as parts of a template,in this case, the template for an if-then-else expression. In skeletal form, theexpression looks like this:(if (buffer-is-large
divide-buffer-size-by-10-and-multiply-by-arg
else-use-alternate-calculation

The true-or-false-test of this inner if expression checks the size of the buffer. The
reason for this is that the old version 18 Emacs used numbers that are no bigger thaneight million or so and in the computation that followed, the programmer fearedthat Emacs might try to use over-large numbers if the buffer were large. The term
“overflow”, mentioned in the comment, means numbers that are over large. Morerecent versions of Emacs use larger numbers, but this code has not been touched,
if only because people now look at buffers that are far, far larger than ever before.
There are two cases: if the buffer is large and if it is not.
What happens in a large buffer
In beginning-of-buffer, the inner if expression tests whether the size of thebuffer is greater than 10,000 characters. To do this, it uses the > function and thecomputation of size that comes from the let expression.
In the old days, the function buffer-size was used. Not only was that function
called several times, it gave the size of the whole buffer, not the accessible part. The
computation makes much more sense when it handles just the accessible part. (See
Chapter 6 “Narrowing and Widening”, page 73, for more information on focusing
attention to an accessible part.)
The line looks like this:(if (> size 10000)

Section 5.3: Complete Definition of beginning-of-buffer

69

When the buffer is large, the then-part of the if expression is evaluated. It reads
like this (after formatting for easy reading):(*(prefix-numeric-value arg)(/ size 10))

This expression is a multiplication, with two arguments to the function *.
The first argument is (prefix-numeric-value arg). When "P" is used as theargument for interactive, the value passed to the function as its argument ispassed a raw prefix argument, and not a number. (It is a number in a list.) To
perform the arithmetic, a conversion is necessary, and prefix-numeric-value does
the job.
The second argument is (/ size 10). This expression divides the numeric value
by ten—the numeric value of the size of the accessible portion of the buffer. This
produces a number that tells how many characters make up one tenth of the buffer
size. (In Lisp, / is used for division, just as * is used for multiplication.)In the multiplication expression as a whole, this amount is multiplied by thevalue of the prefix argument—the multiplication looks like this:(* numeric-value-of-prefix-arg
number-of-characters-in-one-tenth-of-the-accessible-buffer)

If, for example, the prefix argument is ‘7’, the one-tenth value will be multiplied by
7 to give a position 70% of the way through.
The result of all this is that if the accessible portion of the buffer is large, thegoto-char expression reads like this:(goto-char (* (prefix-numeric-value arg)(/ size 10)))

This puts the cursor where we want it.
What happens in a small buffer
If the buffer contains fewer than 10,000 characters, a slightly different computationis performed. You might think this is not necessary, since the first computation
could do the job. However, in a small buffer, the first method may not put the
cursor on exactly the desired line; the second method does a better job.
The code looks like this:(/ (+ 10 (* size (prefix-numeric-value arg))) 10)

This is code in which you figure out what happens by discovering how the functions
are embedded in parentheses. It is easier to read if you reformat it with each
expression indented more deeply than its enclosing expression:
(/
(+ 10
(*
size
(prefix-numeric-value arg)))
10)

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Chapter 5: A Few More Complex Functions

Looking at parentheses, we see that the innermost operation is (prefix-numericvalue arg), which converts the raw argument to a number. In the following expression, this number is multiplied by the size of the accessible portion of the buffer:(* size (prefix-numeric-value arg))

This multiplication creates a number that may be larger than the size of the buffer—seven times larger if the argument is 7, for example. Ten is then added to thisnumber and finally the large number is divided by ten to provide a value that isone character larger than the percentage position in the buffer.
The number that results from all this is passed to goto-char and the cursor is
moved to that point.
5.3.3 The Complete beginning-of-buffer
Here is the complete text of the beginning-of-buffer function:(defun beginning-of-buffer (&optional arg)
"Move point to the beginning of the buffer;
leave mark at previous position.
With \\[universal-argument] prefix,
do not set mark at previous position.
With numeric arg N,
put point N/10 of the way from the beginning.
If the buffer is narrowed,
this command uses the beginning and sizeof the accessible part of the buffer.
Don't use this command in Lisp programs!
\(goto-char (point-min)) is fasterand avoids clobbering the mark."(interactive "P")(or (consp arg)
(and transient-mark-mode mark-active)(push-mark))(let ((size (- (point-max) (point-min))))(goto-char (if (and arg (not (consp arg)))(+ (point-min)(if (> size 10000);; Avoid overflow for large buffer sizes!
(* (prefix-numeric-value arg)(/ size 10))(/ (+ 10 (* size (prefix-numeric-value arg)))
10)))(point-min))))(if (and arg (not (consp arg))) (forward-line 1)))

Except for two small points, the previous discussion shows how this function works.
The first point deals with a detail in the documentation string, and the second point
concerns the last line of the function.

Section 5.4: Review

71

In the documentation string, there is reference to an expression:\\[universal-argument]

A ‘\\’ is used before the first square bracket of this expression. This ‘\\’ tells the
Lisp interpreter to substitute whatever key is currently bound to the ‘[...]’. In
the case of universal-argument, that is usually C-u, but it might be different.
(See Section “Tips for Documentation Strings” in The GNU Emacs Lisp Reference
Manual, for more information.)
Finally, the last line of the beginning-of-buffer command says to move point
to the beginning of the next line if the command is invoked with an argument:(if (and arg (not (consp arg))) (forward-line 1))

This puts the cursor at the beginning of the first line after the appropriate tenths
position in the buffer. This is a flourish that means that the cursor is always locatedat least the requested tenths of the way through the buffer, which is a nicety thatis, perhaps, not necessary, but which, if it did not occur, would be sure to draw
complaints. (The (not (consp arg)) portion is so that if you specify the commandwith a C-u, but without a number, that is to say, if the raw prefix argument is simply
a cons cell, the command does not put you at the beginning of the second line.)

5.4 Review
Here is a brief summary of some of the topics covered in this chapter.
or

Evaluate each argument in sequence, and return the value of the firstargument that is not nil; if none return a value that is not nil, returnnil. In brief, return the first true value of the arguments; return atrue value if one or any of the others are true.

andEvaluate each argument in sequence, and if any are nil, return nil;
if none are nil, return the value of the last argument. In brief, return
a true value only if all the arguments are true; return a true value if
one and each of the others is true.

&optional

A keyword used to indicate that an argument to a function definitionis optional; this means that the function can be evaluated without theargument, if desired.

prefix-numeric-value
Convert the raw prefix argument produced by (interactive "P") toa numeric value.
forward-line
Move point forward to the beginning of the next line, or if the argument
is greater than one, forward that many lines. If it can’t move as far
forward as it is supposed to, forward-line goes forward as far as it
can and then returns a count of the number of additional lines it was
supposed to move but couldn’t.
erase-buffer
Delete the entire contents of the current buffer.

72
bufferp

Chapter 5: A Few More Complex Functions
Return t if its argument is a buffer; otherwise return nil.

5.5 optional Argument Exercise
Write an interactive function with an optional argument that tests whether its
argument, a number, is greater than or equal to, or else, less than the value offill-column, and tells you which, in a message. However, if you do not pass anargument to the function, use 56 as a default value.

Section 6.1: The save-restriction Special Form

73

6 Narrowing and Widening
Narrowing is a feature of Emacs that makes it possible for you to focus on a specificpart of a buffer, and work without accidentally changing other parts. Narrowing isnormally disabled since it can confuse novices.
With narrowing, the rest of a buffer is made invisible, as if it weren’t there. Thisis an advantage if, for example, you want to replace a word in one part of a buffer
but not in another: you narrow to the part you want and the replacement is carriedout only in that section, not in the rest of the buffer. Searches will only work withina narrowed region, not outside of one, so if you are fixing a part of a document,
you can keep yourself from accidentally finding parts you do not need to fix bynarrowing just to the region you want. (The key binding for narrow-to-region is
C-x n n.)However, narrowing does make the rest of the buffer invisible, which can scarepeople who inadvertently invoke narrowing and think they have deleted a part oftheir file. Moreover, the undo command (which is usually bound to C-x u) does notturn off narrowing (nor should it), so people can become quite desperate if they
do not know that they can return the rest of a buffer to visibility with the widen
command. (The key binding for widen is C-x n w.)
Narrowing is just as useful to the Lisp interpreter as to a human. Often, an
Emacs Lisp function is designed to work on just part of a buffer; or conversely, an
Emacs Lisp function needs to work on all of a buffer that has been narrowed. The
what-line function, for example, removes the narrowing from a buffer, if it has
any narrowing and when it has finished its job, restores the narrowing to what itwas. On the other hand, the count-lines function uses narrowing to restrict itself
to just that portion of the buffer in which it is interested and then restores theprevious situation.

6.1 The save-restriction Special Form
In Emacs Lisp, you can use the save-restriction special form to keep trackof whatever narrowing is in effect, if any. When the Lisp interpreter meets withsave-restriction, it executes the code in the body of the save-restriction
expression, and then undoes any changes to narrowing that the code caused. If,
for example, the buffer is narrowed and the code that follows save-restriction
gets rid of the narrowing, save-restriction returns the buffer to its narrowed
region afterwards. In the what-line command, any narrowing the buffer may haveis undone by the widen command that immediately follows the save-restrictioncommand. Any original narrowing is restored just before the completion of thefunction.

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Chapter 6: Narrowing and Widening
The template for a save-restriction expression is simple:(save-restriction
body... )The body of the save-restriction is one or more expressions that will be evaluatedin sequence by the Lisp interpreter.
Finally, a point to note: when you use both save-excursion andsave-restriction, one right after the other, you should use save-excursion
outermost. If you write them in reverse order, you may fail to record narrowing
in the buffer to which Emacs switches after calling save-excursion. Thus, when
written together, save-excursion and save-restriction should be written likethis:(save-excursion(save-restriction
body...))

In other circumstances, when not written together, the save-excursion andsave-restriction special forms must be written in the order appropriate to thefunction.
For example,(save-restriction(widen)(save-excursion
body...))

6.2 what-line
The what-line command tells you the number of the line in which the cursor is located. The function illustrates the use of the save-restriction andsave-excursion commands. Here is the original text of the function:(defun what-line ()"Print the current line number (in the buffer) of point."(interactive)(save-restriction(widen)(save-excursion(beginning-of-line)(message "Line %d"(1+ (count-lines 1 (point)))))))(In modern versions of GNU Emacs, the what-line function has been expanded
to tell you your line number in a narrowed buffer as well as your line number in awidened buffer. The modern version is more complex than the version shown here.
If you feel adventurous, you might want to look at it after figuring out how thisversion works. You will probably need to use C-h f (describe-function). The
newer version uses a conditional to determine whether the buffer has been narrowed.
Also, the modern version of what-line uses line-number-at-pos, which among
other simple expressions, such as (goto-char (point-min)), moves point to thebeginning of the current line with (forward-line 0) rather than beginning-ofline.)Section 6.2: what-line

75

The what-line function as shown here has a documentation line and is interactive, as you would expect. The next two lines use the functions save-restriction
and widen.
The save-restriction special form notes whatever narrowing is in effect, if
any, in the current buffer and restores that narrowing after the code in the body ofthe save-restriction has been evaluated.
The save-restriction special form is followed by widen. This function undoes
any narrowing the current buffer may have had when what-line was called. (Thenarrowing that was there is the narrowing that save-restriction remembers.)
This widening makes it possible for the line counting commands to count from the
beginning of the buffer. Otherwise, they would have been limited to counting within
the accessible region. Any original narrowing is restored just before the completionof the function by the save-restriction special form.
The call to widen is followed by save-excursion, which saves the locationof the cursor (i.e., of point), and restores it after the code in the body of thesave-excursion uses the beginning-of-line function to move point.
(Note that the (widen) expression comes between the save-restriction andsave-excursion special forms. When you write the two save- ... expressions insequence, write save-excursion outermost.)
The last two lines of the what-line function are functions to count the numberof lines in the buffer and then print the number in the echo area.
(message "Line %d"(1+ (count-lines 1 (point)))))))The message function prints a one-line message at the bottom of the Emacs
screen. The first argument is inside of quotation marks and is printed as a string ofcharacters. However, it may contain a ‘%d’ expression to print a following argument.
‘%d’ prints the argument as a decimal, so the message will say something such as‘Line 243’.

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Chapter 6: Narrowing and Widening

The number that is printed in place of the ‘%d’ is computed by the last line of
the function:(1+ (count-lines 1 (point)))

What this does is count the lines from the first position of the buffer, indicated by
the 1, up to (point), and then add one to that number. (The 1+ function adds
one to its argument.) We add one to it because line 2 has only one line before it,
and count-lines counts only the lines before the current line.
After count-lines has done its job, and the message has been printed in
the echo area, the save-excursion restores point to its original position; andsave-restriction restores the original narrowing, if any.

6.3 Exercise with Narrowing
Write a function that will display the first 60 characters of the current buffer, even
if you have narrowed the buffer to its latter half so that the first line is inaccessible.
Restore point, mark, and narrowing. For this exercise, you need to use a whole potpourri of functions, including save-restriction, widen, goto-char, point-min,message, and buffer-substring.
(buffer-substring is a previously unmentioned function you will have to investigate yourself; or perhaps you will have to use buffer-substring-no-properties
or filter-buffer-substring . . . , yet other functions. Text properties are a feature otherwise not discussed here. See Section “Text Properties” in The GNU
Emacs Lisp Reference Manual.)
Additionally, do you really need goto-char or point-min? Or can you write
the function without them?

Section 7.1: car and cdr

77

7 car, cdr, cons: Fundamental Functions
In Lisp, car, cdr, and cons are fundamental functions. The cons function is used
to construct lists, and the car and cdr functions are used to take them apart.
In the walk through of the copy-region-as-kill function, we will see cons aswell as two variants on cdr, namely, setcdr and nthcdr. (See Section 8.3 “copyregion-as-kill”, page 93.)
The name of the cons function is not unreasonable: it is an abbreviation of theword “construct”. The origins of the names for car and cdr, on the other hand,are esoteric: car is an acronym from the phrase “Contents of the Address partof the Register”; and cdr (pronounced “could-er”) is an acronym from the phrase“Contents of the Decrement part of the Register”. These phrases refer to the IBM
704 computer on which the original Lisp was developed.
The IBM 704 is a footnote in history, but these names are now beloved traditionsof Lisp.

7.1 car and cdr
The car of a list is, quite simply, the first item in the list. Thus the car of the list
(rose violet daisy buttercup) is rose.
If you are reading this in Info in GNU Emacs, you can see this by evaluating
the following:(car '(rose violet daisy buttercup))

After evaluating the expression, rose will appear in the echo area.
car does not remove the first item from the list; it only reports what it is. Aftercar has been applied to a list, the list is still the same as it was. In the jargon, car
is “non-destructive”. This feature turns out to be important.
The cdr of a list is the rest of the list, that is, the cdr function returns thepart of the list that follows the first item. Thus, while the car of the list '(roseviolet daisy buttercup) is rose, the rest of the list, the value returned by the
cdr function, is (violet daisy buttercup).
You can see this by evaluating the following in the usual way:
(cdr '(rose violet daisy buttercup))

When you evaluate this, (violet daisy buttercup) will appear in the echo area.
Like car, cdr does not remove any elements from the list—it just returns areport of what the second and subsequent elements are.
Incidentally, in the example, the list of flowers is quoted. If it were not, theLisp interpreter would try to evaluate the list by calling rose as a function. In thisexample, we do not want to do that.
For operating on lists, the names first and rest would make more sense than
the names car and cdr. Indeed, some programmers define first and rest as
aliases for car and cdr, then write first and rest in their code.
However, lists in Lisp are built using a lower-level structure known as “conscells” (see Chapter 9 “List Implementation”, page 108), in which there is no suchthing as “first” or “rest”, and the car and the cdr are symmetrical. Lisp does not

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Chapter 7: car, cdr, cons: Fundamental Functions

try to hide the existence of cons cells, and programs do use them for things otherthan lists. For this reason, the names are helpful for reminding programmers thatcar and cdr are in fact symmetrical, despite the asymmetrical way they are usedin lists.
When car and cdr are applied to a list made up of symbols, such as the list
(pine fir oak maple), the element of the list returned by the function car is the
symbol pine without any parentheses around it. pine is the first element in thelist. However, the cdr of the list is a list itself, (fir oak maple), as you can see
by evaluating the following expressions in the usual way:(car '(pine fir oak maple))(cdr '(pine fir oak maple))On the other hand, in a list of lists, the first element is itself a list. car returns
this first element as a list. For example, the following list contains three sub-lists,
a list of carnivores, a list of herbivores and a list of sea mammals:(car '((lion tiger cheetah)(gazelle antelope zebra)(whale dolphin seal)))In this example, the first element or car of the list is the list of carnivores, (liontiger cheetah), and the rest of the list is ((gazelle antelope zebra) (whale
dolphin seal)).
(cdr '((lion tiger cheetah)(gazelle antelope zebra)(whale dolphin seal)))It is worth saying again that car and cdr are non-destructive—that is, they do
not modify or change lists to which they are applied. This is very important for
how they are used.
Also, in the first chapter, in the discussion about atoms, I said that in Lisp,
certain kinds of atom, such as an array, can be separated into parts; but the mechanism for doing this is different from the mechanism for splitting a list. As far as
Lisp is concerned, the atoms of a list are unsplittable. (See Section 1.1.1 “Lisp
Atoms”, page 1.) The car and cdr functions are used for splitting lists and areconsidered fundamental to Lisp. Since they cannot split or gain access to the partsof an array, an array is considered an atom. Conversely, the other fundamental
function, cons, can put together or construct a list, but not an array. (Arrays arehandled by array-specific functions. See Section “Arrays” in The GNU Emacs Lisp
Reference Manual.)

7.2 cons
The cons function constructs lists; it is the inverse of car and cdr. For example,
cons can be used to make a four element list from the three element list, (fir oakmaple):(cons 'pine '(fir oak maple))

Section 7.2: cons

79

After evaluating this list, you will see(pine fir oak maple)

appear in the echo area. cons causes the creation of a new list in which the element
is followed by the elements of the original list.
We often say that cons puts a new element at the beginning of a list, or thatit attaches or pushes elements onto the list, but this phrasing can be misleading,since cons does not change an existing list, but creates a new one.
Like car and cdr, cons is non-destructive.
cons must have a list to attach to.1 You cannot start from absolutely nothing.
If you are building a list, you need to provide at least an empty list at the beginning.
Here is a series of cons expressions that build up a list of flowers. If you are reading
this in Info in GNU Emacs, you can evaluate each of the expressions in the usual
way; the value is printed in this text after ‘⇒’, which you may read as “evaluatesto”.
(cons 'buttercup ())⇒ (buttercup)(cons 'daisy '(buttercup))⇒ (daisy buttercup)(cons 'violet '(daisy buttercup))⇒ (violet daisy buttercup)(cons 'rose '(violet daisy buttercup))⇒ (rose violet daisy buttercup)

In the first example, the empty list is shown as () and a list made up of buttercup
followed by the empty list is constructed. As you can see, the empty list is not shownin the list that was constructed. All that you see is (buttercup). The empty list
is not counted as an element of a list because there is nothing in an empty list.
Generally speaking, an empty list is invisible.
The second example, (cons 'daisy '(buttercup)) constructs a new, two element list by putting daisy in front of buttercup; and the third example constructs
a three element list by putting violet in front of daisy and buttercup.
7.2.1 Find the Length of a List: length
You can find out how many elements there are in a list by using the Lisp function
length, as in the following examples:(length '(buttercup))⇒ 1(length '(daisy buttercup))⇒ 2

1

Actually, you can cons an element to an atom to produce a dotted pair. Dotted pairs
are not discussed here; see Section “Dotted Pair Notation” in The GNU Emacs Lisp
Reference Manual.

80

Chapter 7: car, cdr, cons: Fundamental Functions(length (cons 'violet '(daisy buttercup)))⇒ 3

In the third example, the cons function is used to construct a three element list
which is then passed to the length function as its argument.
We can also use length to count the number of elements in an empty list:(length ())⇒ 0

As you would expect, the number of elements in an empty list is zero.
An interesting experiment is to find out what happens if you try to find thelength of no list at all; that is, if you try to call length without giving it anargument, not even an empty list:(length )What you see, if you evaluate this, is the error message
Lisp error: (wrong-number-of-arguments length 0)This means that the function receives the wrong number of arguments, zero, when
it expects some other number of arguments. In this case, one argument is expected,
the argument being a list whose length the function is measuring. (Note that one
list is one argument, even if the list has many elements inside it.)
The part of the error message that says ‘length’ is the name of the function.

7.3 nthcdr
The nthcdr function is associated with the cdr function. What it does is take thecdr of a list repeatedly.
If you take the cdr of the list (pine fir oak maple), you will be returned thelist (fir oak maple). If you repeat this on what was returned, you will be returned
the list (oak maple). (Of course, repeated cdring on the original list will just give
you the original cdr since the function does not change the list. You need toevaluate the cdr of the cdr and so on.) If you continue this, eventually you will
be returned an empty list, which in this case, instead of being shown as () is shownas nil.
For review, here is a series of repeated cdrs, the text following the ‘⇒’ shows
what is returned.
(cdr '(pine fir oak maple))⇒ (fir oak maple)(cdr '(fir oak maple))⇒ (oak maple)(cdr '(oak maple))⇒ (maple)(cdr '(maple))⇒ nil(cdr 'nil)⇒ nilSection 7.4: nth81(cdr ())⇒ nil

You can also do several cdrs without printing the values in between, like this:(cdr (cdr '(pine fir oak maple)))⇒ (oak maple)

In this example, the Lisp interpreter evaluates the innermost list first. The innermost list is quoted, so it just passes the list as it is to the innermost cdr. This cdr
passes a list made up of the second and subsequent elements of the list to the outermost cdr, which produces a list composed of the third and subsequent elementsof the original list. In this example, the cdr function is repeated and returns a listthat consists of the original list without its first two elements.
The nthcdr function does the same as repeating the call to cdr. In the following
example, the argument 2 is passed to the function nthcdr, along with the list, andthe value returned is the list without its first two items, which is exactly the sameas repeating cdr twice on the list:(nthcdr 2 '(pine fir oak maple))⇒ (oak maple)

Using the original four element list, we can see what happens when various
numeric arguments are passed to nthcdr, including 0, 1, and 5:;; Leave the list as it was.
(nthcdr 0 '(pine fir oak maple))⇒ (pine fir oak maple);; Return a copy without the first element.
(nthcdr 1 '(pine fir oak maple))⇒ (fir oak maple);; Return a copy of the list without three elements.
(nthcdr 3 '(pine fir oak maple))⇒ (maple);; Return a copy lacking all four elements.
(nthcdr 4 '(pine fir oak maple))⇒ nil;; Return a copy lacking all elements.
(nthcdr 5 '(pine fir oak maple))⇒ nil

7.4 nthThe nthcdr function takes the cdr of a list repeatedly. The nth function takes the
car of the result returned by nthcdr. It returns the Nth element of the list.

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Thus, if it were not defined in C for speed, the definition of nth would be:(defun nth (n list)"Returns the Nth element of LIST.
N counts from zero. If LIST is not that long, nil is returned."(car (nthcdr n list)))(Originally, nth was defined in Emacs Lisp in subr.el, but its definition was redone
in C in the 1980s.)
The nth function returns a single element of a list. This can be very convenient.
Note that the elements are numbered from zero, not one. That is to say, thefirst element of a list, its car is the zeroth element. This zero-based counting often
bothers people who are accustomed to the first element in a list being number one,which is one-based.
For example:(nth 0 '("one" "two" "three"))⇒ "one"(nth 1 '("one" "two" "three"))⇒ "two"It is worth mentioning that nth, like nthcdr and cdr, does not change theoriginal list—the function is non-destructive. This is in sharp contrast to the setcar
and setcdr functions.

7.5 setcar
As you might guess from their names, the setcar and setcdr functions set the caror the cdr of a list to a new value. They actually change the original list, unlike
car and cdr which leave the original list as it was. One way to find out how thisworks is to experiment. We will start with the setcar function.
First, we can make a list and then set the value of a variable to the list, using
the setq special form. Because we intend to use setcar to change the list, thissetq should not use the quoted form '(antelope giraffe lion tiger), as thatwould yield a list that is part of the program and bad things could happen if we
tried to change part of the program while running it. Generally speaking an Emacs
Lisp program’s components should be constant (or unchanged) while the programis running. So we instead construct an animal list by using the list function, as
follows:(setq animals (list 'antelope 'giraffe 'lion 'tiger))If you are reading this in Info inside of GNU Emacs, you can evaluate this expression
in the usual fashion, by positioning the cursor after the expression and typing C-x
C-e. (I’m doing this right here as I write this. This is one of the advantages of
having the interpreter built into the computing environment. Incidentally, when
there is nothing on the line after the final parentheses, such as a comment, point
can be on the next line. Thus, if your cursor is in the first column of the next line,
you do not need to move it. Indeed, Emacs permits any amount of white space
after the final parenthesis.)Section 7.6: setcdr

83

When we evaluate the variable animals, we see that it is bound to the list
(antelope giraffe lion tiger):
animals⇒ (antelope giraffe lion tiger)

Put another way, the variable animals points to the list (antelope giraffe lion
tiger).
Next, evaluate the function setcar while passing it two arguments, the variableanimals and the quoted symbol hippopotamus; this is done by writing the threeelement list (setcar animals 'hippopotamus) and then evaluating it in the usual
fashion:(setcar animals 'hippopotamus)

After evaluating this expression, evaluate the variable animals again. You will seethat the list of animals has changed:
animals⇒ (hippopotamus giraffe lion tiger)

The first element on the list, antelope is replaced by hippopotamus.
So we can see that setcar did not add a new element to the list as cons would
have; it replaced antelope with hippopotamus; it changed the list.

7.6 setcdr
The setcdr function is similar to the setcar function, except that the function
replaces the second and subsequent elements of a list rather than the first element.
(To see how to change the last element of a list, look ahead to “The kill-new
function”, page 97, which uses the nthcdr and setcdr functions.)
To see how this works, set the value of the variable to a list of domesticatedanimals by evaluating the following expression:(setq domesticated-animals (list 'horse 'cow 'sheep 'goat))

If you now evaluate the list, you will be returned the list (horse cow sheep goat):
domesticated-animals⇒ (horse cow sheep goat)

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Next, evaluate setcdr with two arguments, the name of the variable which has
a list as its value, and the list to which the cdr of the first list will be set;(setcdr domesticated-animals '(cat dog))

If you evaluate this expression, the list (cat dog) will appear in the echo area. Thisis the value returned by the function. The result we are interested in is the side
effect, which we can see by evaluating the variable domesticated-animals:
domesticated-animals⇒ (horse cat dog)

Indeed, the list is changed from (horse cow sheep goat) to (horse cat dog). The
cdr of the list is changed from (cow sheep goat) to (cat dog).

7.7 Exercise
Construct a list of four birds by evaluating several expressions with cons. Find out
what happens when you cons a list onto itself. Replace the first element of the listof four birds with a fish. Replace the rest of that list with a list of other fish.

Section 8.1: zap-to-char

85

8 Cutting and Storing Text
Whenever you cut or clip text out of a buffer with a kill command in GNU Emacs,
it is stored in a list and you can bring it back with a yank command.
(The use of the word “kill” in Emacs for processes which specifically do not
destroy the values of the entities is an unfortunate historical accident. A much
more appropriate word would be “clip” since that is what the kill commands do;they clip text out of a buffer and put it into storage from which it can be broughtback. I have often been tempted to replace globally all occurrences of “kill” in the
Emacs sources with “clip” and all occurrences of “killed” with “clipped”.)
When text is cut out of a buffer, it is stored on a list. Successive pieces of text
are stored on the list successively, so the list might look like this:("a piece of text" "previous piece")

The function cons can be used to create a new list from a piece of text (an “atom”,
to use the jargon) and an existing list, like this:(cons "another piece"'("a piece of text" "previous piece"))If you evaluate this expression, a list of three elements will appear in the echo area:("another piece" "a piece of text" "previous piece")With the car and nthcdr functions, you can retrieve whichever piece of text
you want. For example, in the following code, nthcdr 1 ... returns the list withthe first item removed; and the car returns the first element of that remainder—thesecond element of the original list:(car (nthcdr 1 '("another piece""a piece of text""previous piece")))⇒ "a piece of text"

The actual functions in Emacs are more complex than this, of course. The codefor cutting and retrieving text has to be written so that Emacs can figure out whichelement in the list you want—the first, second, third, or whatever. In addition,
when you get to the end of the list, Emacs should give you the first element of thelist, rather than nothing at all.
The list that holds the pieces of text is called the kill ring. This chapter leads
up to a description of the kill ring and how it is used by first tracing how thezap-to-char function works. This function calls a function that invokes a functionthat manipulates the kill ring. Thus, before reaching the mountains, we climb thefoothills.
A subsequent chapter describes how text that is cut from the buffer is retrieved.
See Chapter 10 “Yanking Text Back”, page 113.

8.1 zap-to-char
Let us look at the interactive zap-to-char function.
The zap-to-char function removes the text in the region between the locationof the cursor (i.e., of point) up to and including the next occurrence of a specified
character. The text that zap-to-char removes is put in the kill ring; and it can

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be retrieved from the kill ring by typing C-y (yank). If the command is given anargument, it removes text through that number of occurrences. Thus, if the cursor
were at the beginning of this sentence and the character were ‘s’, ‘Thus’ would be
removed. If the argument were two, ‘Thus, if the curs’ would be removed, up to
and including the ‘s’ in ‘cursor’.
If the specified character is not found, zap-to-char will say “Search failed”, tell
you the character you typed, and not remove any text.
In order to determine how much text to remove, zap-to-char uses a searchfunction. Searches are used extensively in code that manipulates text, and we will
focus attention on them as well as on the deletion command.
Here is the complete text of the version 22 implementation of the function:(defun zap-to-char (arg char)"Kill up to and including ARG'th occurrence of CHAR.
Case is ignored if `case-fold-search' is non-nil in the current buffer.
Goes backward if ARG is negative; error if CHAR not found."(interactive "p\ncZap to char: ")(if (char-table-p translation-table-for-input)(setq char (or (aref translation-table-for-input char) char)))(kill-region (point) (progn(search-forward (char-to-string char)nil nil arg)
(point))))

The documentation is thorough. You do need to know the jargon meaning of
the word “kill”.
The version 22 documentation string for zap-to-char uses ASCII grave accent
and apostrophe to quote a symbol, so it appears as ‘case-fold-search’. Thisquoting style was inspired by 1970s-era displays in which grave accent and apostrophe were often mirror images suitable for use as quotes. On most modern displays
this is no longer true, and when these two ASCII characters appear in documentation strings or diagnostic message formats, Emacs typically transliterates themto curved quotes (left and right single quotation marks), so that the abovequoted
symbol appears as ‘case-fold-search’. Source-code strings can also simply use
curved quotes directly.
8.1.1 The interactive Expression

Section 8.1: zap-to-char

87

The interactive expression in the zap-to-char command looks like this:(interactive "p\ncZap to char: ")The part within quotation marks, "p\ncZap to char: ", specifies two different
things. First, and most simply, is the ‘p’. This part is separated from the next partby a newline, ‘\n’. The ‘p’ means that the first argument to the function will bepassed the value of a processed prefix. The prefix argument is passed by typing C-u
and a number, or M- and a number. If the function is called interactively without
a prefix, 1 is passed to this argument.
The second part of "p\ncZap to char: " is ‘cZap to char: ’. In this part, thelower case ‘c’ indicates that interactive expects a prompt and that the argument
will be a character. The prompt follows the ‘c’ and is the string ‘Zap to char: ’(with a space after the colon to make it look good).
What all this does is prepare the arguments to zap-to-char so they are of theright type, and give the user a prompt.
In a read-only buffer, the zap-to-char function copies the text to the kill ring,
but does not remove it. The echo area displays a message saying that the buffer isread-only. Also, the terminal may beep or blink at you.
8.1.2 The Body of zap-to-char
The body of the zap-to-char function contains the code that kills (that is, removes)
the text in the region from the current position of the cursor up to and including
the specified character.
The first part of the code looks like this:(if (char-table-p translation-table-for-input)(setq char (or (aref translation-table-for-input char) char)))(kill-region (point) (progn(search-forward (char-to-string char) nil nil arg)
(point)))char-table-p is a hitherto unseen function. It determines whether its argument isa character table. When it is, it sets the character passed to zap-to-char to one ofthem, if that character exists, or to the character itself. (This becomes importantfor certain characters in non-European languages. The aref function extracts anelement from an array. It is an array-specific function that is not described in thisdocument. See Section “Arrays” in The GNU Emacs Lisp Reference Manual.)(point) is the current position of the cursor.
The next part of the code is an expression using progn. The body of the progn
consists of calls to search-forward and point.
It is easier to understand how progn works after learning about
search-forward, so we will look at search-forward and then at progn.
8.1.3 The search-forward Function
The search-forward function is used to locate the zapped-for-character in zap-tochar. If the search is successful, search-forward leaves point immediately after
the last character in the target string. (In zap-to-char, the target string is justone character long. zap-to-char uses the function char-to-string to ensure

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that the computer treats that character as a string.) If the search is backwards,
search-forward leaves point just before the first character in the target. Also,
search-forward returns t for true. (Moving point is therefore a side effect.)In zap-to-char, the search-forward function looks like this:(search-forward (char-to-string char) nil nil arg)

The search-forward function takes four arguments:
1. The first argument is the target, what is searched for. This must be a string,such as ‘"z"’.
As it happens, the argument passed to zap-to-char is a single character. Because of the way computers are built, the Lisp interpreter may treat a single
character as being different from a string of characters. Inside the computer, asingle character has a different electronic format than a string of one character.
(A single character can often be recorded in the computer using exactly one
byte; but a string may be longer, and the computer needs to be ready for this.)
Since the search-forward function searches for a string, the character thatthe zap-to-char function receives as its argument must be converted inside
the computer from one format to the other; otherwise the search-forward
function will fail. The char-to-string function is used to make this conversion.
2. The second argument bounds the search; it is specified as a position in thebuffer. In this case, the search can go to the end of the buffer, so no bound is
set and the second argument is nil.
3. The third argument tells the function what it should do if the search fails—it
can signal an error (and print a message) or it can return nil. A nil as thethird argument causes the function to signal an error when the search fails.
4. The fourth argument to search-forward is the repeat count—how many occurrences of the string to look for. This argument is optional and if the function
is called without a repeat count, this argument is passed the value 1. If thisargument is negative, the search goes backwards.
In template form, a search-forward expression looks like this:(search-forward "target-string"
limit-of-search
what-to-do-if-search-fails
repeat-count)

We will look at progn next.
8.1.4 The progn Special Formprogn is a special form that causes each of its arguments to be evaluated in sequenceand then returns the value of the last one. The preceding expressions are evaluatedonly for the side effects they perform. The values produced by them are discarded.

Section 8.2: kill-region

89

The template for a progn expression is very simple:(prognbody...)

In zap-to-char, the progn expression has to do two things: put point in exactly
the right position; and return the location of point so that kill-region will know
how far to kill to.
The first argument to the progn is search-forward. When search-forward
finds the string, the function leaves point immediately after the last character inthe target string. (In this case the target string is just one character long.) If thesearch is backwards, search-forward leaves point just before the first character inthe target. The movement of point is a side effect.
The second and last argument to progn is the expression (point). This expression returns the value of point, which in this case will be the location to which it has
been moved by search-forward. (In the source, a line that tells the function to go
to the previous character, if it is going forward, was commented out in 1999; I don’t
remember whether that feature or mis-feature was ever a part of the distributed
source.) The value of point is returned by the progn expression and is passed to
kill-region as kill-region’s second argument.
8.1.5 Summing up zap-to-char
Now that we have seen how search-forward and progn work, we can see how thezap-to-char function works as a whole.
The first argument to kill-region is the position of the cursor when thezap-to-char command is given—the value of point at that time. Within the progn,
the search function then moves point to just after the zapped-to-character and point
returns the value of this location. The kill-region function puts together thesetwo values of point, the first one as the beginning of the region and the second one
as the end of the region, and removes the region.
The progn special form is necessary because the kill-region command takes
two arguments; and it would fail if search-forward and point expressions were
written in sequence as two additional arguments. The progn expression is a single
argument to kill-region and returns the one value that kill-region needs for
its second argument.

8.2 kill-region
The zap-to-char function uses the kill-region function. This function clips text
from a region and copies that text to the kill ring, from which it may be retrieved.
The Emacs 22 version of that function uses condition-case and copy-regionas-kill, both of which we will explain. condition-case is an important specialform.
In essence, the kill-region function calls condition-case, which takes threearguments. In this function, the first argument does nothing. The second argument
contains the code that does the work when all goes well. The third argument
contains the code that is called in the event of an error.

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We will go through the condition-case code in a moment. First, let us lookat the definition of kill-region, with comments added:(defun kill-region (beg end)
"Kill (\"cut\") text between point and mark.
This deletes the text from the buffer and saves it in the kill ring.
The command \\[yank] can retrieve it from there. ... ";; • Since order matters, pass point first.
(interactive (list (point) (mark)));; • And tell us if we cannot cut the text.
;; 'unless' is an 'if' without a then-part.
(unless (and beg end)
(error "The mark is not set now, so there is no region"));; • 'condition-case' takes three arguments.
;;
If the first argument is nil, as it is here,;;
information about the error signal is not;;stored for use by another function.
(condition-case nil;; • The second argument to 'condition-case' tells the;;
Lisp interpreter what to do when all goes well.
;;
;;
;;
;;
;;
;;
;;

It starts with a 'let' function that extracts the string
and tests whether it exists. If so (that is what the'when' checks), it calls an 'if' function that determines
whether the previous command was another call to'kill-region'; if it was, then the new text is appended tothe previous text; if not, then a different function,'kill-new', is called.

;;
;;
;;

The 'kill-append' function concatenates the new string andthe old. The 'kill-new' function inserts text into a newitem in the kill ring.

;;'when' is an 'if' without an else-part. The second 'when';;again checks whether the current string exists; in;;addition, it checks whether the previous command was;;another call to 'kill-region'. If one or the other;;condition is true, then it sets the current command to;;
be 'kill-region'.
(let ((string (filter-buffer-substring beg end t)))(when string;STRING is nil if BEG = END;; Add that string to the kill ring, one way or another.
(if (eq last-command 'kill-region)

Section 8.2: kill-region

91;;− 'yank-handler' is an optional argument to;;'kill-region' that tells the 'kill-append' and;;'kill-new' functions how deal with properties;;
added to the text, such as 'bold' or 'italics'.
(kill-append string (< end beg) yank-handler)(kill-new string nil yank-handler)))(when (or string (eq last-command 'kill-region))(setq this-command 'kill-region))nil);; • The third argument to 'condition-case' tells the interpreter;;
what to do with an error.
;;
The third argument has a conditions part and a body part.
;;If the conditions are met (in this case,;;if text or buffer are read-only);;
then the body is executed.
;;
The first part of the third argument is the following:((buffer-read-only text-read-only) ;; the if-part;; ... the then-part(copy-region-as-kill beg end);;
Next, also as part of the then-part, set this-command, so;;
it will be set in an error(setq this-command 'kill-region);;
Finally, in the then-part, send a message if you may copy;;
the text to the kill ring without signaling an error, but;;
don't if you may not.
(if kill-read-only-ok(progn (message "Read only text copied to kill ring") nil)(barf-if-buffer-read-only);; If the buffer isn't read-only, the text is.
(signal 'text-read-only (list (current-buffer)))))))

8.2.1 condition-case
As we have seen earlier (see Section 1.3 “Generate an Error Message”, page 4), when
the Emacs Lisp interpreter has trouble evaluating an expression, it provides you with
help; in the jargon, this is called “signaling an error”. Usually, the computer stops
the program and shows you a message.
However, some programs undertake complicated actions. They should not simply stop on an error. In the kill-region function, the most likely error is that you
will try to kill text that is read-only and cannot be removed. So the kill-region
function contains code to handle this circumstance. This code, which makes up the
body of the kill-region function, is inside of a condition-case special form.

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The template for condition-case looks like this:(condition-casevarbodyform
error-handler...)

The second argument, bodyform, is straightforward. The condition-case special form causes the Lisp interpreter to evaluate the code in bodyform. If no error
occurs, the special form returns the code’s value and produces the side-effects, ifany.
In short, the bodyform part of a condition-case expression determines whatshould happen when everything works correctly.
However, if an error occurs, among its other actions, the function generating theerror signal will define one or more error condition names.
An error handler is the third argument to condition-case. An error handler
has two parts, a condition-name and a body. If the condition-name part of an error
handler matches a condition name generated by an error, then the body part of theerror handler is run.
As you will expect, the condition-name part of an error handler may be eithera single condition name or a list of condition names.
Also, a complete condition-case expression may contain more than one error
handler. When an error occurs, the first applicable handler is run.
Lastly, the first argument to the condition-case expression, the var argument,
is sometimes bound to a variable that contains information about the error. However, if that argument is nil, as is the case in kill-region, that information is
discarded.

Section 8.3: copy-region-as-kill

93

In brief, in the kill-region function, the code condition-case works like this:If no errors, run only this code
but, if errors, run this other code.

8.2.2 Lisp macro
The part of the condition-case expression that is evaluated in the expectationthat all goes well has a when. The code uses when to determine whether the string
variable points to text that exists.
A when expression is simply a programmers’ convenience. It is like an if without
the possibility of an else clause. In your mind, you can replace when with if andunderstand what goes on. That is what the Lisp interpreter does.
Technically speaking, when is a Lisp macro. A Lisp macro enables you to define
new control constructs and other language features. It tells the interpreter how tocompute another Lisp expression which will in turn compute the value. In this case,the other expression is an if expression.
The kill-region function definition also has an unless macro; it is the oppositeof when. The unless macro is like an if except that it has no then-clause, and it
supplies an implicit nil for that.
For more about Lisp macros, see Section “Macros” in The GNU Emacs Lisp
Reference Manual. The C programming language also provides macros. These aredifferent, but also useful.
Regarding the when macro, in the condition-case expression, when the stringhas content, then another conditional expression is executed. This is an if withboth a then-part and an else-part.
(if (eq last-command 'kill-region)(kill-append string (< end beg) yank-handler)(kill-new string nil yank-handler))

The then-part is evaluated if the previous command was another call tokill-region; if not, the else-part is evaluated.
yank-handler is an optional argument to kill-region that tells thekill-append and kill-new functions how deal with properties added to the text,
such as bold or italics.
last-command is a variable that comes with Emacs that we have not seen before.
Normally, whenever a function is executed, Emacs sets the value of last-commandto the previous command.
In this segment of the definition, the if expression checks whether the previous
command was kill-region. If it was,(kill-append string (< end beg) yank-handler)

concatenates a copy of the newly clipped text to the just previously clipped text in
the kill ring.

8.3 copy-region-as-kill
The copy-region-as-kill function copies a region of text from a buffer and (viaeither kill-append or kill-new) saves it in the kill-ring.

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If you call copy-region-as-kill immediately after a kill-region command,
Emacs appends the newly copied text to the previously copied text. This means that
if you yank back the text, you get it all, from both this and the previous operation.
On the other hand, if some other command precedes the copy-region-as-kill,
the function copies the text into a separate entry in the kill ring.
Here is the complete text of the version 22 copy-region-as-kill function:(defun copy-region-as-kill (beg end)
"Save the region as if killed, but don't kill it.
In Transient Mark mode, deactivate the mark.
If `interprogram-cut-function' is non-nil, also save the text for a window
system cut and paste."(interactive "r")(if (eq last-command 'kill-region)(kill-append (filter-buffer-substring beg end) (< end beg))(kill-new (filter-buffer-substring beg end)))(if transient-mark-mode(setq deactivate-mark t))nil)

As usual, this function can be divided into its component parts:(defun copy-region-as-kill (argument-list)"documentation..."(interactive "r")body...)

The arguments are beg and end and the function is interactive with "r", so thetwo arguments must refer to the beginning and end of the region. If you have beenreading through this document from the beginning, understanding these parts of afunction is almost becoming routine.
The documentation is somewhat confusing unless you remember that theword “kill” has a meaning different from usual. The Transient Mark andinterprogram-cut-function comments explain certain side-effects.
After you once set a mark, a buffer always contains a region. If you wish, you
can use Transient Mark mode to highlight the region temporarily. (No one wants
to highlight the region all the time, so Transient Mark mode highlights it only atappropriate times. Many people turn off Transient Mark mode, so the region isnever highlighted.)Also, a windowing system allows you to copy, cut, and paste among different programs. In the X windowing system, for example, the interprogram-cut-function
function is x-select-text, which works with the windowing system’s equivalent of
the Emacs kill ring.
The body of the copy-region-as-kill function starts with an if clause. What
this clause does is distinguish between two different situations: whether or not thiscommand is executed immediately after a previous kill-region command. In thefirst case, the new region is appended to the previously copied text. Otherwise, it
is inserted into the beginning of the kill ring as a separate piece of text from theprevious piece.
The last two lines of the function prevent the region from lighting up if TransientMark mode is turned on.

Section 8.3: copy-region-as-kill

95

The body of copy-region-as-kill merits discussion in detail.
8.3.1 The Body of copy-region-as-kill
The copy-region-as-kill function works in much the same way as thekill-region function. Both are written so that two or more kills in a row combine
their text into a single entry. If you yank back the text from the kill ring, you get
it all in one piece. Moreover, kills that kill forward from the current position of
the cursor are added to the end of the previously copied text and commands thatcopy text backwards add it to the beginning of the previously copied text. Thisway, the words in the text stay in the proper order.
Like kill-region, the copy-region-as-kill function makes use of thelast-command variable that keeps track of the previous Emacs command.
Normally, whenever a function is executed, Emacs sets the value of
this-command to the function being executed (which in this case would becopy-region-as-kill). At the same time, Emacs sets the value of last-commandto the previous value of this-command.
In the first part of the body of the copy-region-as-kill function, an if expression determines whether the value of last-command is kill-region. If so, thethen-part of the if expression is evaluated; it uses the kill-append function to
concatenate the text copied at this call to the function with the text already inthe first element (the car) of the kill ring. On the other hand, if the value oflast-command is not kill-region, then the copy-region-as-kill function attaches a new element to the kill ring using the kill-new function.
The if expression reads as follows; it uses eq:(if (eq last-command 'kill-region);; then-part(kill-append (filter-buffer-substring beg end) (< end beg));; else-part(kill-new (filter-buffer-substring beg end)))(The filter-buffer-substring function returns a filtered substring of thebuffer, if any. Optionally—the arguments are not here, so neither is done—thefunction may delete the initial text or return the text without its properties; thisfunction is a replacement for the older buffer-substring function, which came
before text properties were implemented.)
The eq function tests whether its first argument is the same Lisp object as its
second argument. The eq function is similar to the equal function in that it is used
to test for equality, but differs in that it determines whether two representations
are actually the same object inside the computer, but with different names. equal
determines whether the structure and contents of two expressions are the same.
If the previous command was kill-region, then the Emacs Lisp interpreter
calls the kill-append function
The kill-append function

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The kill-append function looks like this:(defun kill-append (string before-p &optional yank-handler)"Append STRING to the end of the latest kill in the kill ring.
If BEFORE-P is non-nil, prepend STRING to the kill.
... "(let* ((cur (car kill-ring)))(kill-new (if before-p (concat string cur) (concat cur string))(or (= (length cur) 0)(equal yank-handler(get-text-property 0 'yank-handler cur)))
yank-handler)))

The kill-append function is fairly straightforward. It uses the kill-new function,
which we will discuss in more detail in a moment.
(Also, the function provides an optional argument called yank-handler; wheninvoked, this argument tells the function how to deal with properties added to thetext, such as bold or italics.)
It has a let* function to set the value of the first element of the kill ring to cur.
(I do not know why the function does not use let instead; only one value is set in
the expression. Perhaps this is a bug that produces no problems?)
Consider the conditional that is one of the two arguments to kill-new. It uses
concat to concatenate the new text to the car of the kill ring. Whether it prepends
or appends the text depends on the results of an if expression:(if before-p(concat string cur)(concat cur string)); if-part; then-part; else-part

If the region being killed is before the region that was killed in the last command,then it should be prepended before the material that was saved in the previous kill;and conversely, if the killed text follows what was just killed, it should be appended
after the previous text. The if expression depends on the predicate before-p todecide whether the newly saved text should be put before or after the previously
saved text.
The symbol before-p is the name of one of the arguments to kill-append.
When the kill-append function is evaluated, it is bound to the value returned byevaluating the actual argument. In this case, this is the expression (< end beg).
This expression does not directly determine whether the killed text in this command
is located before or after the kill text of the last command; what it does is determine
whether the value of the variable end is less than the value of the variable beg. If
it is, it means that the user is most likely heading towards the beginning of thebuffer. Also, the result of evaluating the predicate expression, (< end beg), will betrue and the text will be prepended before the previous text. On the other hand, if
the value of the variable end is greater than the value of the variable beg, the text
will be appended after the previous text.
When the newly saved text will be prepended, then the string with the new text
will be concatenated before the old text:(concat string cur)Section 8.3: copy-region-as-kill

97

But if the text will be appended, it will be concatenated after the old text:(concat cur string))

To understand how this works, we first need to review the concat function. The
concat function links together or unites two strings of text. The result is a string.
For example:(concat "abc" "def")⇒ "abcdef"(concat "new "(car '("first element" "second element")))⇒ "new first element"(concat (car'("first element" "second element")) " modified")⇒ "first element modified"

We can now make sense of kill-append: it modifies the contents of the killring. The kill ring is a list, each element of which is saved text. The kill-append
function uses the kill-new function which in turn uses the setcar function.
The kill-new function

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In version 22 the kill-new function looks like this:(defun kill-new (string &optional replace yank-handler)"Make STRING the latest kill in the kill ring.
Set `kill-ring-yank-pointer' to point to it.
If `interprogram-cut-function' is non-nil, apply it to STRING.
Optional second argument REPLACE non-nil means that STRING will replace
the front of the kill ring, rather than being added to the list.
..."(if (> (length string) 0)(if yank-handler(put-text-property 0 (length string)'yank-handler yank-handler string))(if yank-handler(signal 'args-out-of-range(list string "yank-handler specified for empty string"))))(if (fboundp 'menu-bar-update-yank-menu)(menu-bar-update-yank-menu string (and replace (car kill-ring))))(if (and replace kill-ring)(setcar kill-ring string)(push string kill-ring)(if (> (length kill-ring) kill-ring-max)(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))(setq kill-ring-yank-pointer kill-ring)(if interprogram-cut-function(funcall interprogram-cut-function string (not replace))))(Notice that the function is not interactive.)
As usual, we can look at this function in parts.
The function definition has an optional yank-handler argument, which wheninvoked tells the function how to deal with properties added to the text, such as
bold or italics. We will skip that.
The first line of the documentation makes sense:
Make STRING the latest kill in the kill ring.

Let’s skip over the rest of the documentation for the moment.
Also, let’s skip over the initial if expression and those lines of code involving
menu-bar-update-yank-menu. We will explain them below.

Section 8.3: copy-region-as-kill

99

The critical lines are these:(if (and replace kill-ring);; then(setcar kill-ring string);; else(push string kill-ring)(if (> (length kill-ring) kill-ring-max);; avoid overly long kill ring(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))(setq kill-ring-yank-pointer kill-ring)(if interprogram-cut-function(funcall interprogram-cut-function string (not replace))))

The conditional test is (and replace kill-ring). This will be true when two
conditions are met: the kill ring has something in it, and the replace variable istrue.
When the kill-append function sets replace to be true and when the kill ringhas at least one item in it, the setcar expression is executed:(setcar kill-ring string)

The setcar function actually changes the first element of the kill-ring list to
the value of string. It replaces the first element.
On the other hand, if the kill ring is empty, or replace is false, the else-part ofthe condition is executed:(push string kill-ring)push puts its first argument onto the second. It is similar to the older(setq kill-ring (cons string kill-ring))or the newer(add-to-list kill-ring string)

When it is false, the expression first constructs a new version of the kill ring by
prepending string to the existing kill ring as a new element (that is what the pushdoes). Then it executes a second if clause. This second if clause keeps the killring from growing too long.
Let’s look at these two expressions in order.
The push line of the else-part sets the new value of the kill ring to what resultsfrom adding the string being killed to the old kill ring.
We can see how this works with an example.
First,(setq example-list '("here is a clause" "another clause"))After evaluating this expression with C-x C-e, you can evaluate example-list and
see what it returns:
example-list⇒ ("here is a clause" "another clause")

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Now, we can add a new element on to this list by evaluating the following expression:(push "a third clause" example-list)

When we evaluate example-list, we find its value is:example-list⇒ ("a third clause" "here is a clause" "another clause")

Thus, the third clause is added to the list by push.
Now for the second part of the if clause. This expression keeps the kill ringfrom growing too long. It looks like this:(if (> (length kill-ring) kill-ring-max)(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil))

The code checks whether the length of the kill ring is greater than the maximum
permitted length. This is the value of kill-ring-max (which is 120, by default).
If the length of the kill ring is too long, then this code sets the last element of thekill ring to nil. It does this by using two functions, nthcdr and setcdr.
We looked at setcdr earlier (see Section 7.6 “setcdr”, page 83). It sets the
cdr of a list, just as setcar sets the car of a list. In this case, however, setcdr
will not be setting the cdr of the whole kill ring; the nthcdr function is used tocause it to set the cdr of the next to last element of the kill ring—this means thatsince the cdr of the next to last element is the last element of the kill ring, it will
set the last element of the kill ring.
The nthcdr function works by repeatedly taking the cdr of a list—it takes the
cdr of the cdr of the cdr . . . It does this N times and returns the results. (See
Section 7.3 “nthcdr”, page 80.)Thus, if we had a four element list that was supposed to be three elements long,
we could set the cdr of the next to last element to nil, and thereby shorten thelist. (If you set the last element to some other value than nil, which you could do,then you would not have shortened the list. See Section 7.6 “setcdr”, page 83.)
You can see shortening by evaluating the following three expressions in turn.
First set the value of trees to (maple oak pine birch), then set the cdr of its
second cdr to nil and then find the value of trees:(setq trees (list 'maple 'oak 'pine 'birch))⇒ (maple oak pine birch)(setcdr (nthcdr 2 trees) nil)
⇒ nil
trees
⇒ (maple oak pine)(The value returned by the setcdr expression is nil since that is what the cdr is
set to.)To repeat, in kill-new, the nthcdr function takes the cdr a number of times
that is one less than the maximum permitted size of the kill ring and setcdr sets
the cdr of that element (which will be the rest of the elements in the kill ring) tonil. This prevents the kill ring from growing too long.

Section 8.3: copy-region-as-kill

101

The next to last expression in the kill-new function is(setq kill-ring-yank-pointer kill-ring)

The kill-ring-yank-pointer is a global variable that is set to be thekill-ring.
Even though the kill-ring-yank-pointer is called a ‘pointer’, it is a variable just like the kill ring. However, the name has been chosen to help humans
understand how the variable is used.
Now, to return to an early expression in the body of the function:(if (fboundp 'menu-bar-update-yank-menu)(menu-bar-update-yank-menu string (and replace (car kill-ring))))It starts with an if expression
In this case, the expression tests first to see whether menu-bar-update-yankmenu exists as a function, and if so, calls it. The fboundp function returns true if
the symbol it is testing has a function definition that is not void. If the symbol’sfunction definition were void, we would receive an error message, as we did when wecreated errors intentionally (see Section 1.3 “Generate an Error Message”, page 4).
The then-part contains an expression whose first element is the function and.
The and special form evaluates each of its arguments until one of the arguments
returns a value of nil, in which case the and expression returns nil; however, if
none of the arguments returns a value of nil, the value resulting from evaluating
the last argument is returned. (Since such a value is not nil, it is considered truein Emacs Lisp.) In other words, an and expression returns a true value only if all its
arguments are true. (See Section 5.4 “Second Buffer Related Review”, page 71.)
The expression determines whether the second argument to menu-bar-updateyank-menu is true or not.
menu-bar-update-yank-menu is one of the functions that make it possible touse the “Select and Paste” menu in the Edit item of a menu bar; using a mouse,
you can look at the various pieces of text you have saved and select one piece topaste.
The last expression in the kill-new function adds the newly copied string towhatever facility exists for copying and pasting among different programs runningin a windowing system. In the X Windowing system, for example, the x-selecttext function takes the string and stores it in memory operated by X. You canpaste the string in another program, such as an Xterm.

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The expression looks like this:(if interprogram-cut-function(funcall interprogram-cut-function string (not replace))))If an interprogram-cut-function exists, then Emacs executes funcall, which
in turn calls its first argument as a function and passes the remaining arguments toit. (Incidentally, as far as I can see, this if expression could be replaced by an andexpression similar to the one in the first part of the function.)
We are not going to discuss windowing systems and other programs further, but
merely note that this is a mechanism that enables GNU Emacs to work easily andwell with other programs.
This code for placing text in the kill ring, either concatenated with an existingelement or as a new element, leads us to the code for bringing back text that has
been cut out of the buffer—the yank commands. However, before discussing theyank commands, it is better to learn how lists are implemented in a computer. This
will make clear such mysteries as the use of the term “pointer”. But before that,
we will digress into C.

8.4 Digression into C
The copy-region-as-kill function (see Section 8.3 “copy-region-as-kill”,
page 93) uses the filter-buffer-substring function, which in turn uses thedelete-and-extract-region function. It removes the contents of a region andyou cannot get them back.
Unlike the other code discussed here, the delete-and-extract-region function
is not written in Emacs Lisp; it is written in C and is one of the primitives of the
GNU Emacs system. Since it is very simple, I will digress briefly from Lisp anddescribe it here.
Like many of the other Emacs primitives, delete-and-extract-region is written as an instance of a C macro, a macro being a template for code. The complete
macro looks like this:
DEFUN ("delete-and-extract-region", Fdelete_and_extract_region,
Sdelete_and_extract_region, 2, 2, 0,
doc: /* Delete the text between START and END and return it.
(Lisp_Object start, Lisp_Object end){
validate_region (&start, &end);
if (XFIXNUM (start) == XFIXNUM (end))
return empty_unibyte_string;
return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);}*/)

Without going into the details of the macro writing process, let me point out
that this macro starts with the word DEFUN. The word DEFUN was chosen since thecode serves the same purpose as defun does in Lisp. (The DEFUN C macro is definedin emacs/src/lisp.h.)
The word DEFUN is followed by seven parts inside of parentheses:• The first part is the name given to the function in Lisp, delete-and-extractregion.

Section 8.4: Digression into C

103• The second part is the name of the function in C, Fdelete_and_extract_
region. By convention, it starts with ‘F’. Since C does not use hyphens in
names, underscores are used instead.
• The third part is the name for the C constant structure that records information on this function for internal use. It is the name of the function in C but
begins with an ‘S’ instead of an ‘F’.
• The fourth and fifth parts specify the minimum and maximum number ofarguments the function can have. This function demands exactly 2 arguments.
• The sixth part is nearly like the argument that follows the interactive declaration in a function written in Lisp: a letter followed, perhaps, by a prompt.
The only difference from Lisp is when the macro is called with no arguments.
Then you write a 0 (which is a null string), as in this macro.
If you were to specify arguments, you would place them between quotation
marks. The C macro for goto-char includes "NGoto char: " in this position
to indicate that the function expects a raw prefix, in this case, a numericallocation in a buffer, and provides a prompt.
• The seventh part is a documentation string, just like the one for a function
written in Emacs Lisp. This is written as a C comment. (When you build
Emacs, the program lib-src/make-docfile extracts these comments and uses
them to make the documentation.)
In a C macro, the formal parameters come next, with a statement of what kindof object they are, followed by the body of the macro. For delete-and-extractregion the body consists of the following four lines:
validate_region (&start, &end);
if (XFIXNUM (start) == XFIXNUM (end))
return empty_unibyte_string;
return del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);

The validate_region function checks whether the values passed as the beginning and end of the region are the proper type and are within range. If the beginningand end positions are the same, then return an empty string.
The del_range_1 function actually deletes the text. It is a complex function wewill not look into. It updates the buffer and does other things. However, it is worth
looking at the two arguments passed to del_range_1. These are XFIXNUM (start)
and XFIXNUM (end).
As far as the C language is concerned, start and end are two opaque values
that mark the beginning and end of the region to be deleted. More precisely, andrequiring more expert knowledge to understand, the two values are of type Lisp_
Object, which might be a C pointer, a C integer, or a C struct; C code ordinarily
should not care how Lisp_Object is implemented.
Lisp_Object widths depend on the machine, and are typically 32 or 64 bits. A
few of the bits are used to specify the type of information; the remaining bits areused as content.
‘XFIXNUM’ is a C macro that extracts the relevant integer from the longer collection of bits; the type bits are discarded.

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The command in delete-and-extract-region looks like this:del_range_1 (XFIXNUM (start), XFIXNUM (end), 1, 1);

It deletes the region between the beginning position, start, and the ending position,
end.
From the point of view of the person writing Lisp, Emacs is all very simple; but
hidden underneath is a great deal of complexity to make it all work.

8.5 Initializing a Variable with defvar
The copy-region-as-kill function is written in Emacs Lisp. Two functions within
it, kill-append and kill-new, copy a region in a buffer and save it in a variable
called the kill-ring. This section describes how the kill-ring variable is created
and initialized using the defvar special form.
(Again we note that the term kill-ring is a misnomer. The text that is clippedout of the buffer can be brought back; it is not a ring of corpses, but a ring ofresurrectable text.)
In Emacs Lisp, a variable such as the kill-ring is created and given an initialvalue by using the defvar special form. The name comes from “define variable”.
The defvar special form is similar to setq in that it sets the value of a variable.
It is unlike setq in three ways: first, it marks the variable as “special” so that it
is always dynamically bound, even when lexical-binding is t (see Section 3.6.4“How let Binds Variables”, page 36). Second, it only sets the value of the variable
if the variable does not already have a value. If the variable already has a value,
defvar does not override the existing value. Third, defvar has a documentation
string.
(There is a related macro, defcustom, designed for variables that people customize. It has more features than defvar. (See Section 16.2 “Setting Variables
with defcustom”, page 197.)
You can see the current value of a variable, any variable, by using thedescribe-variable function, which is usually invoked by typing C-h v. If you
type C-h v and then kill-ring (followed by RET) when prompted, you will see
what is in your current kill ring—this may be quite a lot! Conversely, if you havebeen doing nothing this Emacs session except read this document, you may havenothing in it. Also, you will see the documentation for kill-ring:Documentation:
List of killed text sequences.
Since the kill ring is supposed to interact nicely with cut-and-paste
facilities offered by window systems, use of this variable shouldinteract nicely with `interprogram-cut-function' and`interprogram-paste-function'. The functions `kill-new',`kill-append', and `current-kill' are supposed to implement thisinteraction; you may want to use them instead of manipulating the killring directly.

Section 8.6: Review

105

The kill ring is defined by a defvar in the following way:
(defvar kill-ring nil"List of killed text sequences.
...")

In this variable definition, the variable is given an initial value of nil, which makes
sense, since if you have saved nothing, you want nothing back if you give a yankcommand. The documentation string is written just like the documentation stringof a defun. As with the documentation string of the defun, the first line of the
documentation should be a complete sentence, since some commands, like apropos,
print only the first line of documentation. Succeeding lines should not be indented;
otherwise they look odd when you use C-h v (describe-variable).
8.5.1 defvar and an asterisk
In the past, Emacs used the defvar special form both for internal variables thatyou would not expect a user to change and for variables that you do expect a user
to change. Although you can still use defvar for user customizable variables, pleaseuse defcustom instead, since it provides a path into the Customization commands.
(See Section 16.2 “Specifying Variables using defcustom”, page 197.)
When you specified a variable using the defvar special form, you could distinguish a variable that a user might want to change from others by typing an asterisk,‘*’, in the first column of its documentation string. For example:(defvar shell-command-default-error-buffer nil"*Buffer name for `shell-command' ... error output.
... ")

You could (and still can) use the set-variable command to change the value ofshell-command-default-error-buffer temporarily. However, options set using
set-variable are set only for the duration of your editing session. The new values
are not saved between sessions. Each time Emacs starts, it reads the original value,
unless you change the value within your .emacs file, either by setting it manually
or by using customize. See Chapter 16 “Your .emacs File”, page 196.
For me, the major use of the set-variable command is to suggest variables
that I might want to set in my .emacs file. There are now more than 700 suchvariables, far too many to remember readily. Fortunately, you can press TAB after
calling the M-x set-variable command to see the list of variables. (See Section“Examining and Setting Variables” in The GNU Emacs Manual.)

8.6 Review
Here is a brief summary of some recently introduced functions.
carcdr

car returns the first element of a list; cdr returns the second andsubsequent elements of a list.

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For example:(car '(1 2 3 4 5 6 7))⇒ 1(cdr '(1 2 3 4 5 6 7))⇒ (2 3 4 5 6 7)

conscons constructs a list by prepending its first argument to its second
argument.
For example:(cons 1 '(2 3 4))⇒ (1 2 3 4)

funcall

funcall evaluates its first argument as a function. It passes its remaining arguments to its first argument.

nthcdrReturn the result of taking cdr n times on a list. The nth cdr. The“rest of the rest”, as it were.
For example:(nthcdr 3 '(1 2 3 4 5 6 7))⇒ (4 5 6 7)

setcar
setcdrsetcar changes the first element of a list; setcdr changes the secondand subsequent elements of a list.
For example:(setq triple (list 1 2 3))(setcar triple '37)
triple⇒ (37 2 3)
(setcdr triple '("foo" "bar"))
triple⇒ (37 "foo" "bar")progn

Evaluate each argument in sequence and then return the value of thelast.
For example:(progn 1 2 3 4)
⇒ 4

save-restriction
Record whatever narrowing is in effect in the current buffer, if any,
and restore that narrowing after evaluating the arguments.
search-forward
Search for a string, and if the string is found, move point. With a regular expression, use the similar re-search-forward. (See Chapter 12

Section 8.7: Searching Exercises

107“Regular Expression Searches”, page 141, for an explanation of regularexpression patterns and searches.)search-forward and re-search-forward take four arguments:
1. The string or regular expression to search for.
2. Optionally, the limit of the search.
3. Optionally, what to do if the search fails, return nil or an error
message.
4. Optionally, how many times to repeat the search; if negative, thesearch goes backwards.
kill-regiondelete-and-extract-region
copy-region-as-killkill-region cuts the text between point and mark from the bufferand stores that text in the kill ring, so you can get it back by yanking.
copy-region-as-kill copies the text between point and mark into
the kill ring, from which you can get it by yanking. The function does
not cut or remove the text from the buffer.
delete-and-extract-region removes the text between point and mark fromthe buffer and throws it away. You cannot get it back. (This is not an interactive
command.)

8.7 Searching Exercises• Write an interactive function that searches for a string. If the search finds thestring, leave point after it and display a message that says “Found!”. (Do notuse search-forward for the name of this function; if you do, you will overwrite
the existing version of search-forward that comes with Emacs. Use a namesuch as test-search instead.)• Write a function that prints the third element of the kill ring in the echo area,
if any; if the kill ring does not contain a third element, print an appropriate
message.

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Chapter 9: How Lists are Implemented

9 How Lists are Implemented
In Lisp, atoms are recorded in a straightforward fashion; if the implementation is
not straightforward in practice, it is, nonetheless, straightforward in theory. The
atom ‘rose’, for example, is recorded as the four contiguous letters ‘r’, ‘o’, ‘s’, ‘e’.
A list, on the other hand, is kept differently. The mechanism is equally simple, but
it takes a moment to get used to the idea. A list is kept using a series of pairs of
pointers. In the series, the first pointer in each pair points to an atom or to anotherlist, and the second pointer in each pair points to the next pair, or to the symbol
nil, which marks the end of the list.
A pointer itself is quite simply the electronic address of what is pointed to.
Hence, a list is kept as a series of electronic addresses.
For example, the list (rose violet buttercup) has three elements, ‘rose’,‘violet’, and ‘buttercup’. In the computer, the electronic address of ‘rose’ isrecorded in a segment of computer memory called a cons cell (because it’s what thefunction cons actually creates). That cons cell also holds the address of a secondcons cell, whose car is the atom ‘violet’; and that address (the one that tells
where to find ‘violet’) is kept along with the address of a third cons cell whichholds the address for the atom ‘buttercup’.
This sounds more complicated than it is and is easier seen in a diagram:

nil
rose

violetbuttercup

In the diagram, each box represents a word of computer memory that holds a Lisp
object, usually in the form of a memory address. The boxes, i.e., the addresses, arein pairs. Each arrow points to what the address is the address of, either an atom oranother pair of addresses. The first box is the electronic address of ‘rose’ and thearrow points to ‘rose’; the second box is the address of the next pair of boxes, thefirst part of which is the address of ‘violet’ and the second part of which is theaddress of the next pair. The very last box points to the symbol nil, which marks
the end of the list.
When a variable is set to a list with an operation such as setq, it stores theaddress of the first box in the variable. Thus, evaluation of the expression(setq bouquet '(rose violet buttercup))

109
creates a situation like this:bouquetnilrose

violetbuttercup

In this example, the symbol bouquet holds the address of the first pair of boxes.
This same list can be illustrated in a different sort of box notation like this:bouquet
car

cdr

rose

car
violet

cdr

car
cdr
butter- nil
cup(Symbols consist of more than pairs of addresses, but the structure of a symbol
is made up of addresses. Indeed, the symbol bouquet consists of a group of addressboxes, one of which is the address of the printed word ‘bouquet’, a second of whichis the address of a function definition attached to the symbol, if any, a third ofwhich is the address of the first pair of address-boxes for the list (rose violet
buttercup), and so on. Here we are showing that the symbol’s third address-box
points to the first pair of address-boxes for the list.)
If a symbol is set to the cdr of a list, the list itself is not changed; the symbol
simply has an address further down the list. (In the jargon, car and cdr are“non-destructive”.) Thus, evaluation of the following expression(setq flowers (cdr bouquet))

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Chapter 9: How Lists are Implemented

produces this:bouquetflowersnilrose

violetbuttercup

The value of flowers is (violet buttercup), which is to say, the symbol flowers
holds the address of the pair of address-boxes, the first of which holds the addressof violet, and the second of which holds the address of buttercup.
A pair of address-boxes is called a cons cell or dotted pair. See Section “Cons Celland List Types” in The GNU Emacs Lisp Reference Manual, and Section “Dotted
Pair Notation” in The GNU Emacs Lisp Reference Manual, for more information
about cons cells and dotted pairs.
The function cons adds a new pair of addresses to the front of a series of addresses like that shown above. For example, evaluating the expression(setq bouquet (cons 'lily bouquet))

produces:bouquetflowersnilbuttercup
lily

violetrose

Section 9.1: Symbols as a Chest of Drawers

111

However, this does not change the value of the symbol flowers, as you can see by
evaluating the following,(eq (cdr (cdr bouquet)) flowers)

which returns t for true.
Until it is reset, flowers still has the value (violet buttercup); that is, it has
the address of the cons cell whose first address is of violet. Also, this does notalter any of the pre-existing cons cells; they are all still there.
Thus, in Lisp, to get the cdr of a list, you just get the address of the next cons
cell in the series; to get the car of a list, you get the address of the first element of
the list; to cons a new element on a list, you add a new cons cell to the front of thelist. That is all there is to it! The underlying structure of Lisp is brilliantly simple!
And what does the last address in a series of cons cells refer to? It is the addressof the empty list, of nil.
In summary, when a Lisp variable is set to a value, it is provided with the addressof the list to which the variable refers.

9.1 Symbols as a Chest of Drawers
In an earlier section, I suggested that you might imagine a symbol as being a chestof drawers. The function definition is put in one drawer, the value in another, andso on. What is put in the drawer holding the value can be changed without affecting
the contents of the drawer holding the function definition, and vice versa.
Actually, what is put in each drawer is the address of the value or function
definition. It is as if you found an old chest in the attic, and in one of its drawers
you found a map giving you directions to where the buried treasure lies.
(In addition to its name, symbol definition, and variable value, a symbol has adrawer for a property list which can be used to record other information. Property
lists are not discussed here; see Section “Property Lists” in The GNU Emacs Lisp
Reference Manual.)

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Here is a fanciful representation:
Chest of Drawers

Contents of Drawers

directions tosymbol namemap tobouquetdirections tosymbol definition[none]

directions tovariable namemap to(rose violet buttercup)

directions toproperty list

[not described here]

9.2 Exercise
Set flowers to violet and buttercup. Cons two more flowers on to this list andset this new list to more-flowers. Set the car of flowers to a fish. What does
the more-flowers list now contain?

Section 10.2: The kill-ring-yank-pointer Variable

113

10 Yanking Text Back
Whenever you cut text out of a buffer with a kill command in GNU Emacs, you
can bring it back with a yank command. The text that is cut out of the buffer is
put in the kill ring and the yank commands insert the appropriate contents of thekill ring back into a buffer (not necessarily the original buffer).
A simple C-y (yank) command inserts the first item from the kill ring into
the current buffer. If the C-y command is followed immediately by M-y, the firstelement is replaced by the second element. Successive M-y commands replace thesecond element with the third, fourth, or fifth element, and so on. When the lastelement in the kill ring is reached, it is replaced by the first element and the cycle isrepeated. (Thus the kill ring is called a “ring” rather than just a “list”. However,
the actual data structure that holds the text is a list. See Appendix B “Handling
the Kill Ring”, page 225, for the details of how the list is handled as a ring.)

10.1 Kill Ring Overview
The kill ring is a list of textual strings. This is what it looks like:("some text" "a different piece of text" "yet more text")

If this were the contents of my kill ring and I pressed C-y, the string of characterssaying ‘some text’ would be inserted in this buffer where my cursor is located.
The yank command is also used for duplicating text by copying it. The copied
text is not cut from the buffer, but a copy of it is put on the kill ring and is inserted
by yanking it back.
Three functions are used for bringing text back from the kill ring: yank, which isusually bound to C-y; yank-pop, which is usually bound to M-y; and rotate-yankpointer, which is used by the two other functions.
These functions refer to the kill ring through a variable called the kill-ringyank-pointer. Indeed, the insertion code for both the yank and yank-pop functionsis:(insert (car kill-ring-yank-pointer))(Well, no more. In GNU Emacs 22, the function has been replaced by insert-foryank which calls insert-for-yank-1 repetitively for each yank-handler segment.
In turn, insert-for-yank-1 strips text properties from the inserted text according
to yank-excluded-properties. Otherwise, it is just like insert. We will stickwith plain insert since it is easier to understand.)To begin to understand how yank and yank-pop work, it is first necessary tolook at the kill-ring-yank-pointer variable.

10.2 The kill-ring-yank-pointer Variablekill-ring-yank-pointer is a variable, just as kill-ring is a variable. It points
to something by being bound to the value of what it points to, like any other Lisp
variable.

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Chapter 10: Yanking Text BackThus, if the value of the kill ring is:("some text" "a different piece of text" "yet more text")

and the kill-ring-yank-pointer points to the second clause, the value ofkill-ring-yank-pointer is:("a different piece of text" "yet more text")

As explained in the previous chapter (see Chapter 9 “List Implementation”,
page 108), the computer does not keep two different copies of the text being pointed
to by both the kill-ring and the kill-ring-yank-pointer. The words “a different piece of text” and “yet more text” are not duplicated. Instead, the two Lisp
variables point to the same pieces of text. Here is a diagram:
kill-ringkill-ring-yank-pointernil
yet more text
a different piece of text
some text

Both the variable kill-ring and the variable kill-ring-yank-pointer are
pointers. But the kill ring itself is usually described as if it were actually what
it is composed of. The kill-ring is spoken of as if it were the list rather thanthat it points to the list. Conversely, the kill-ring-yank-pointer is spoken of as
pointing to a list.
These two ways of talking about the same thing sound confusing at first but
make sense on reflection. The kill ring is generally thought of as the complete
structure of data that holds the information of what has recently been cut out of
the Emacs buffers. The kill-ring-yank-pointer on the other hand, serves toindicate—that is, to point to—that part of the kill ring of which the first element(the car) will be inserted.

10.3 Exercises with yank and nthcdr• Using C-h v (describe-variable), look at the value of your kill ring. Add
several items to your kill ring; look at its value again. Using M-y (yank-pop),
move all the way around the kill ring. How many items were in your kill ring?
Find the value of kill-ring-max. Was your kill ring full, or could you have
kept more blocks of text within it?
• Using nthcdr and car, construct a series of expressions to return the first,
second, third, and fourth elements of a list.

Section 11.1: while

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11 Loops and Recursion
Emacs Lisp has two primary ways to cause an expression, or a series of expressions,
to be evaluated repeatedly: one uses a while loop, and the other uses recursion.
Repetition can be very valuable. For example, to move forward four sentences,
you need only write a program that will move forward one sentence and then repeat
the process four times. Since a computer does not get bored or tired, such repetitive
action does not have the deleterious effects that excessive or the wrong kinds ofrepetition can have on humans.
People mostly write Emacs Lisp functions using while loops and their kin; but
you can use recursion, which provides a very powerful way to think about and thento solve problems1 .

11.1 while
The while special form tests whether the value returned by evaluating its firstargument is true or false. This is similar to what the Lisp interpreter does with anif; what the interpreter does next, however, is different.
In a while expression, if the value returned by evaluating the first argument isfalse, the Lisp interpreter skips the rest of the expression (the body of the expression) and does not evaluate it. However, if the value is true, the Lisp interpreter
evaluates the body of the expression and then again tests whether the first argument
to while is true or false. If the value returned by evaluating the first argument isagain true, the Lisp interpreter again evaluates the body of the expression.
The template for a while expression looks like this:(while true-or-false-test
body...)

So long as the true-or-false-test of the while expression returns a true value
when it is evaluated, the body is repeatedly evaluated. This process is called a loop
since the Lisp interpreter repeats the same thing again and again, like an airplane
doing a loop. When the result of evaluating the true-or-false-test is false, the Lisp
interpreter does not evaluate the rest of the while expression and exits the loop.
Clearly, if the value returned by evaluating the first argument to while is always
true, the body following will be evaluated again and again . . . and again . . . forever.
Conversely, if the value returned is never true, the expressions in the body will neverbe evaluated. The craft of writing a while loop consists of choosing a mechanism
such that the true-or-false-test returns true just the number of times that you want
the subsequent expressions to be evaluated, and then have the test return false.
The value returned by evaluating a while is the value of the true-or-false-test.
An interesting consequence of this is that a while loop that evaluates without error
1

You can write recursive functions to be frugal or wasteful of mental or computerresources; as it happens, methods that people find easy—that are frugal of mental
resources—sometimes use considerable computer resources. Emacs was designed to
run on machines that we now consider limited and its default settings are conservative.
You may want to increase the value of max-lisp-eval-depth. In my .emacs file, I set
it to 30 times its default value.

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will return nil or false regardless of whether it has looped 1 or 100 times or none atall. A while expression that evaluates successfully never returns a true value! What
this means is that while is always evaluated for its side effects, which is to say, theconsequences of evaluating the expressions within the body of the while loop. This
makes sense. It is not the mere act of looping that is desired, but the consequencesof what happens when the expressions in the loop are repeatedly evaluated.
11.1.1 A while Loop and a List
A common way to control a while loop is to test whether a list has any elements.
If it does, the loop is repeated; but if it does not, the repetition is ended. Since thisis an important technique, we will create a short example to illustrate it.
A simple way to test whether a list has elements is to evaluate the list: if it
has no elements, it is an empty list and will return the empty list, (), which is asynonym for nil or false. On the other hand, a list with elements will return thoseelements when it is evaluated. Since Emacs Lisp considers as true any value that is
not nil, a list that returns elements will test true in a while loop.
For example, you can set the variable empty-list to nil by evaluating thefollowing setq expression:(setq empty-list ())

After evaluating the setq expression, you can evaluate the variable empty-list in
the usual way, by placing the cursor after the symbol and typing C-x C-e; nil willappear in your echo area:
empty-list

On the other hand, if you set a variable to be a list with elements, the list willappear when you evaluate the variable, as you can see by evaluating the following
two expressions:(setq animals '(gazelle giraffe lion tiger))
animals

Thus, to create a while loop that tests whether there are any items in the list
animals, the first part of the loop will be written like this:(while animals...

When the while tests its first argument, the variable animals is evaluated. It
returns a list. So long as the list has elements, the while considers the results ofthe test to be true; but when the list is empty, it considers the results of the test
to be false.
To prevent the while loop from running forever, some mechanism needs to beprovided to empty the list eventually. An oft-used technique is to have one of thesubsequent forms in the while expression set the value of the list to be the cdrof the list. Each time the cdr function is evaluated, the list will be made shorter,
until eventually only the empty list will be left. At this point, the test of the while
loop will return false, and the arguments to the while will no longer be evaluated.
For example, the list of animals bound to the variable animals can be set to be
the cdr of the original list with the following expression:

Section 11.1: while117(setq animals (cdr animals))

If you have evaluated the previous expressions and then evaluate this expression,
you will see (giraffe lion tiger) appear in the echo area. If you evaluate theexpression again, (lion tiger) will appear in the echo area. If you evaluate itagain and yet again, (tiger) appears and then the empty list, shown by nil.
A template for a while loop that uses the cdr function repeatedly to cause thetrue-or-false-test eventually to test false looks like this:(while test-whether-list-is-empty
body...
set-list-to-cdr-of-list)

This test and use of cdr can be put together in a function that goes through alist and prints each element of the list on a line of its own.
11.1.2 An Example: print-elements-of-list
The print-elements-of-list function illustrates a while loop with a list.
The function requires several lines for its output. If you are reading this in a
recent instance of GNU Emacs, you can evaluate the following expression inside ofInfo, as usual.
If you are using an earlier version of Emacs, you need to copy the necessary
expressions to your *scratch* buffer and evaluate them there. This is because theecho area had only one line in the earlier versions.
You can copy the expressions by marking the beginning of the region with C-SPC(set-mark-command), moving the cursor to the end of the region and then copying
the region using M-w (kill-ring-save, which calls copy-region-as-kill and thenprovides visual feedback). In the *scratch* buffer, you can yank the expressions
back by typing C-y (yank).
After you have copied the expressions to the *scratch* buffer, evaluate each expression in turn.
Be sure to evaluate the last expression,(print-elements-of-list animals), by typing C-u C-x C-e, that is, bygiving an argument to eval-last-sexp. This will cause the result of theevaluation to be printed in the *scratch* buffer instead of being printed in
the echo area. (Otherwise you will see something like this in your echo area:^Jgazelle^J^Jgiraffe^J^Jlion^J^Jtiger^Jnil, in which each ‘^J’ stands for a
newline.)You can evaluate these expressions directly in the Info buffer, and the echo area
will grow to show the results.
(setq animals '(gazelle giraffe lion tiger))(defun print-elements-of-list (list)"Print each element of LIST on a line of its own."(while list
(print (car list))(setq list (cdr list))))(print-elements-of-list animals)

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When you evaluate the three expressions in sequence, you will see this:gazellegiraffe
lion
tiger
nil

Each element of the list is printed on a line of its own (that is what the function
print does) and then the value returned by the function is printed. Since the last
expression in the function is the while loop, and since while loops always return
nil, a nil is printed after the last element of the list.
11.1.3 A Loop with an Incrementing Counter
A loop is not useful unless it stops when it ought. Besides controlling a loop with alist, a common way of stopping a loop is to write the first argument as a test that
returns false when the correct number of repetitions are complete. This means thatthe loop must have a counter—an expression that counts how many times the loop
repeats itself.
The test for a loop with an incrementing counter can be an expression such as(< count desired-number) which returns t for true if the value of count is lessthan the desired-number of repetitions and nil for false if the value of count isequal to or is greater than the desired-number. The expression that increments
the count can be a simple setq such as (setq count (1+ count)), where 1+ is
a built-in function in Emacs Lisp that adds 1 to its argument. (The expression(1+ count) has the same result as (+ count 1), but is easier for a human to read.)
The template for a while loop controlled by an incrementing counter looks likethis:
set-count-to-initial-value(while (< count desired-number)body...
(setq count (1+ count))); true-or-false-test; incrementer

Note that you need to set the initial value of count; usually it is set to 1.
Example with incrementing counter
Suppose you are playing on the beach and decide to make a triangle of pebbles,
putting one pebble in the first row, two in the second row, three in the third row
and so on, like this:
•
• •
• • •
• • • •

(About 2500 years ago, Pythagoras and others developed the beginnings of number
theory by considering questions such as this.)Section 11.1: while

119

Suppose you want to know how many pebbles you will need to make a triangle
with 7 rows?
Clearly, what you need to do is add up the numbers from 1 to 7. There are
two ways to do this; start with the smallest number, one, and add up the list in
sequence, 1, 2, 3, 4 and so on; or start with the largest number and add the list
going down: 7, 6, 5, 4 and so on. Because both mechanisms illustrate common waysof writing while loops, we will create two examples, one counting up and the othercounting down. In this first example, we will start with 1 and add 2, 3, 4 and soon.
If you are just adding up a short list of numbers, the easiest way to do it is to
add up all the numbers at once. However, if you do not know ahead of time how
many numbers your list will have, or if you want to be prepared for a very long
list, then you need to design your addition so that what you do is repeat a simple
process many times instead of doing a more complex process once.
For example, instead of adding up all the pebbles all at once, what you can dois add the number of pebbles in the first row, 1, to the number in the second row,2, and then add the total of those two rows to the third row, 3. Then you can add
the number in the fourth row, 4, to the total of the first three rows; and so on.
The critical characteristic of the process is that each repetitive action is simple.
In this case, at each step we add only two numbers, the number of pebbles in therow and the total already found. This process of adding two numbers is repeatedagain and again until the last row has been added to the total of all the precedingrows. In a more complex loop the repetitive action might not be so simple, but it
will be simpler than doing everything all at once.
The parts of the function definition
The preceding analysis gives us the bones of our function definition: first, we willneed a variable that we can call total that will be the total number of pebbles.
This will be the value returned by the function.
Second, we know that the function will require an argument: this argument willbe the total number of rows in the triangle. It can be called number-of-rows.
Finally, we need a variable to use as a counter. We could call this variablecounter, but a better name is row-number. That is because what the counter
does in this function is count rows, and a program should be written to be as
understandable as possible.
When the Lisp interpreter first starts evaluating the expressions in the function,
the value of total should be set to zero, since we have not added anything to it.
Then the function should add the number of pebbles in the first row to the total,
and then add the number of pebbles in the second to the total, and then add thenumber of pebbles in the third row to the total, and so on, until there are no morerows left to add.
Both total and row-number are used only inside the function, so they can be
declared as local variables with let and given initial values. Clearly, the initialvalue for total should be 0. The initial value of row-number should be 1, since westart with the first row. This means that the let statement will look like this:

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Chapter 11: Loops and Recursion(let ((total 0)(row-number 1))
body...)

After the internal variables are declared and bound to their initial values, we canbegin the while loop. The expression that serves as the test should return a valueof t for true so long as the row-number is less than or equal to the number-of-rows.
(If the expression tests true only so long as the row number is less than the numberof rows in the triangle, the last row will never be added to the total; hence the row
number has to be either less than or equal to the number of rows.)
Lisp provides the <= function that returns true if the value of its first argument
is less than or equal to the value of its second argument and false otherwise. So theexpression that the while will evaluate as its test should look like this:(<= row-number number-of-rows)

The total number of pebbles can be found by repeatedly adding the number ofpebbles in a row to the total already found. Since the number of pebbles in the row
is equal to the row number, the total can be found by adding the row number tothe total. (Clearly, in a more complex situation, the number of pebbles in the row
might be related to the row number in a more complicated way; if this were thecase, the row number would be replaced by the appropriate expression.)(setq total (+ total row-number))

What this does is set the new value of total to be equal to the sum of adding thenumber of pebbles in the row to the previous total.
After setting the value of total, the conditions need to be established for the
next repetition of the loop, if there is one. This is done by incrementing the value ofthe row-number variable, which serves as a counter. After the row-number variable
has been incremented, the true-or-false-test at the beginning of the while loop tests
whether its value is still less than or equal to the value of the number-of-rows andif it is, adds the new value of the row-number variable to the total of the previous
repetition of the loop.
The built-in Emacs Lisp function 1+ adds 1 to a number, so the row-number
variable can be incremented with this expression:(setq row-number (1+ row-number))Putting the function definition together
We have created the parts for the function definition; now we need to put them
together.
First, the contents of the while expression:(while (<= row-number number-of-rows)(setq total (+ total row-number))(setq row-number (1+ row-number))); true-or-false-test; incrementer

Along with the let expression varlist, this very nearly completes the body ofthe function definition. However, it requires one final element, the need for which
is somewhat subtle.
The final touch is to place the variable total on a line by itself after the while
expression. Otherwise, the value returned by the whole function is the value ofSection 11.1: while121

the last expression that is evaluated in the body of the let, and this is the value
returned by the while, which is always nil.
This may not be evident at first sight. It almost looks as if the incrementing
expression is the last expression of the whole function. But that expression is partof the body of the while; it is the last element of the list that starts with the symbol
while. Moreover, the whole of the while loop is a list within the body of the let.
In outline, the function will look like this:(defun name-of-function (argument-list)"documentation..."(let (varlist)(while (true-or-false-test)
body-of-while... )... )); Need final expression here.

The result of evaluating the let is what is going to be returned by the defun
since the let is not embedded within any containing list, except for the defun as awhole. However, if the while is the last element of the let expression, the function
will always return nil. This is not what we want! Instead, what we want is thevalue of the variable total. This is returned by simply placing the symbol as thelast element of the list starting with let. It gets evaluated after the preceding
elements of the list are evaluated, which means it gets evaluated after it has been
assigned the correct value for the total.
It may be easier to see this by printing the list starting with let all on one line.
This format makes it evident that the varlist and while expressions are the second
and third elements of the list starting with let, and the total is the last element:(let (varlist) (while (true-or-false-test) body-of-while... ) total)

Putting everything together, the triangle function definition looks like this:; Version with; incrementing counter.
"Add up the number of pebbles in a triangle.
The first row has one pebble, the second row two pebbles,
the third row three pebbles, and so on.
The argument is NUMBER-OF-ROWS."(let ((total 0)(row-number 1))(while (<= row-number number-of-rows)(setq total (+ total row-number))(setq row-number (1+ row-number)))
total))(defun triangle (number-of-rows)

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After you have installed triangle by evaluating the function, you can try itout. Here are two examples:(triangle 4)(triangle 7)

The sum of the first four numbers is 10 and the sum of the first seven numbers is
28.
11.1.4 Loop with a Decrementing Counter
Another common way to write a while loop is to write the test so that it determines
whether a counter is greater than zero. So long as the counter is greater than zero,
the loop is repeated. But when the counter is equal to or less than zero, the loop is
stopped. For this to work, the counter has to start out greater than zero and thenbe made smaller and smaller by a form that is evaluated repeatedly.
The test will be an expression such as (> counter 0) which returns t for true if
the value of counter is greater than zero, and nil for false if the value of counteris equal to or less than zero. The expression that makes the number smaller andsmaller can be a simple setq such as (setq counter (1- counter)), where 1- isa built-in function in Emacs Lisp that subtracts 1 from its argument.

Section 11.1: while

123

The template for a decrementing while loop looks like this:(while (> counter 0)
body...
(setq counter (1- counter))); true-or-false-test; decrementer

Example with decrementing counter
To illustrate a loop with a decrementing counter, we will rewrite the triangle
function so the counter decreases to zero.
This is the reverse of the earlier version of the function. In this case, to find out
how many pebbles are needed to make a triangle with 3 rows, add the number ofpebbles in the third row, 3, to the number in the preceding row, 2, and then add
the total of those two rows to the row that precedes them, which is 1.
Likewise, to find the number of pebbles in a triangle with 7 rows, add the numberof pebbles in the seventh row, 7, to the number in the preceding row, which is 6,
and then add the total of those two rows to the row that precedes them, which
is 5, and so on. As in the previous example, each addition only involves adding
two numbers, the total of the rows already added up and the number of pebbles in
the row that is being added to the total. This process of adding two numbers is
repeated again and again until there are no more pebbles to add.
We know how many pebbles to start with: the number of pebbles in the lastrow is equal to the number of rows. If the triangle has seven rows, the numberof pebbles in the last row is 7. Likewise, we know how many pebbles are in thepreceding row: it is one less than the number in the row.
The parts of the function definition
We start with three variables: the total number of rows in the triangle; the numberof pebbles in a row; and the total number of pebbles, which is what we want tocalculate. These variables can be named number-of-rows, number-of-pebblesin-row, and total, respectively.
Both total and number-of-pebbles-in-row are used only inside the function
and are declared with let. The initial value of total should, of course, be zero.
However, the initial value of number-of-pebbles-in-row should be equal to thenumber of rows in the triangle, since the addition will start with the longest row.

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Chapter 11: Loops and Recursion
This means that the beginning of the let expression will look like this:(let ((total 0)(number-of-pebbles-in-row number-of-rows))
body...)The total number of pebbles can be found by repeatedly adding the number ofpebbles in a row to the total already found, that is, by repeatedly evaluating thefollowing expression:(setq total (+ total number-of-pebbles-in-row))

After the number-of-pebbles-in-row is added to the total, the number-ofpebbles-in-row should be decremented by one, since the next time the loop repeats, the preceding row will be added to the total.
The number of pebbles in a preceding row is one less than the number of pebbles
in a row, so the built-in Emacs Lisp function 1- can be used to compute the numberof pebbles in the preceding row. This can be done with the following expression:(setq number-of-pebbles-in-row(1- number-of-pebbles-in-row))

Finally, we know that the while loop should stop making repeated additions
when there are no pebbles in a row. So the test for the while loop is simply:(while (> number-of-pebbles-in-row 0)

Putting the function definition together
We can put these expressions together to create a function definition that works.
However, on examination, we find that one of the local variables is unneeded!
The function definition looks like this:;;; First subtractive version.
(defun triangle (number-of-rows)"Add up the number of pebbles in a triangle."(let ((total 0)(number-of-pebbles-in-row number-of-rows))(while (> number-of-pebbles-in-row 0)(setq total (+ total number-of-pebbles-in-row))(setq number-of-pebbles-in-row(1- number-of-pebbles-in-row)))
total))

As written, this function works.
However, we do not need number-of-pebbles-in-row.
When the triangle function is evaluated, the symbol number-of-rows will bebound to a number, giving it an initial value. That number can be changed in the
body of the function as if it were a local variable, without any fear that such achange will effect the value of the variable outside of the function. This is a very
useful characteristic of Lisp; it means that the variable number-of-rows can beused anywhere in the function where number-of-pebbles-in-row is used.

Section 11.2: Save your time: dolist and dotimes

125

Here is a second version of the function written a bit more cleanly:(defun triangle (number); Second version.
"Return sum of numbers 1 through NUMBER inclusive."(let ((total 0))(while (> number 0)(setq total (+ total number))(setq number (1- number)))
total))

In brief, a properly written while loop will consist of three parts:
1. A test that will return false after the loop has repeated itself the correct numberof times.
2. An expression the evaluation of which will return the value desired after being
repeatedly evaluated.
3. An expression to change the value passed to the true-or-false-test so that thetest returns false after the loop has repeated itself the right number of times.

11.2 Save your time: dolist and dotimes
In addition to while, both dolist and dotimes provide for looping. Sometimes
these are quicker to write than the equivalent while loop. Both are Lisp macros.
(See Section “Macros” in The GNU Emacs Lisp Reference Manual. )dolist works like a while loop that cdrs down a list: dolist automaticallyshortens the list each time it loops—takes the cdr of the list—and binds the carof each shorter version of the list to the first of its arguments.
dotimes loops a specific number of times: you specify the number.
The dolist Macro
Suppose, for example, you want to reverse a list, so that “first” “second” “third”
becomes “third” “second” “first”.
In practice, you would use the reverse function, like this:(setq animals '(gazelle giraffe lion tiger))(reverse animals)

Here is how you could reverse the list using a while loop:(setq animals '(gazelle giraffe lion tiger))(defun reverse-list-with-while (list)"Using while, reverse the order of LIST."(let (value) ; make sure list starts empty(while list
(setq value (cons (car list) value))(setq list (cdr list)))value))(reverse-list-with-while animals)

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And here is how you could use the dolist macro:(setq animals '(gazelle giraffe lion tiger))(defun reverse-list-with-dolist (list)"Using dolist, reverse the order of LIST."(let (value) ; make sure list starts empty(dolist (element list value)(setq value (cons element value)))))(reverse-list-with-dolist animals)

In Info, you can place your cursor after the closing parenthesis of each expression
and type C-x C-e; in each case, you should see(tiger lion giraffe gazelle)in the echo area.
For this example, the existing reverse function is obviously best. The while
loop is just like our first example (see Section 11.1.1 “A while Loop and a List”,
page 116). The while first checks whether the list has elements; if so, it constructs
a new list by adding the first element of the list to the existing list (which in thefirst iteration of the loop is nil). Since the second element is prepended in front of
the first element, and the third element is prepended in front of the second element,
the list is reversed.
In the expression using a while loop, the (setq list (cdr list)) expression
shortens the list, so the while loop eventually stops. In addition, it provides thecons expression with a new first element by creating a new and shorter list at each
repetition of the loop.
The dolist expression does very much the same as the while expression, exceptthat the dolist macro does some of the work you have to do when writing a while
expression.
Like a while loop, a dolist loops. What is different is that it automaticallyshortens the list each time it loops—it cdrs down the list on its own—and it
automatically binds the car of each shorter version of the list to the first of its
arguments.
In the example, the car of each shorter version of the list is referred to using
the symbol ‘element’, the list itself is called ‘list’, and the value returned is called‘value’. The remainder of the dolist expression is the body.
The dolist expression binds the car of each shorter version of the list toelement and then evaluates the body of the expression; and repeats the loop. The
result is returned in value.
The dotimes Macro
The dotimes macro is similar to dolist, except that it loops a specific number oftimes.
The first argument to dotimes is assigned the numbers 0, 1, 2 and so forth eachtime around the loop. You need to provide the value of the second argument, which
is how many times the macro loops.

Section 11.3: Recursion

127

For example, the following binds the numbers from 0 up to, but not including,the number 3 to the first argument, number, and then constructs a list of the threenumbers. (The first number is 0, the second number is 1, and the third number is2; this makes a total of three numbers in all, starting with zero as the first number.)(let (value); otherwise a value is a void variable(dotimes (number 3)(setq value (cons number value)))value)⇒ (2 1 0)

The way to use dotimes is to operate on some expression number number of times
and then return the result, either as a list or an atom.
Here is an example of a defun that uses dotimes to add up the number ofpebbles in a triangle.
(defun triangle-using-dotimes (number-of-rows)"Using `dotimes', add up the number of pebbles in a triangle."(let ((total 0)) ; otherwise a total is a void variable(dotimes (number number-of-rows)(setq total (+ total (1+ number))))total))(triangle-using-dotimes 4)

11.3 Recursion
A recursive function contains code that tells the Lisp interpreter to call a programthat runs exactly like itself, but with slightly different arguments. The code runs
exactly the same because it has the same name. However, even though the program
has the same name, it is not the same entity. It is different. In the jargon, it is adifferent “instance”.
Eventually, if the program is written correctly, the slightly different arguments
will become sufficiently different from the first arguments that the final instancewill stop.
11.3.1 Building Robots: Extending the Metaphor
It is sometimes helpful to think of a running program as a robot that does a job.
In doing its job, a recursive function calls on a second robot to help it. The second
robot is identical to the first in every way, except that the second robot helps thefirst and has been passed different arguments than the first.
In a recursive function, the second robot may call a third; and the third may
call a fourth, and so on. Each of these is a different entity; but all are clones.
Since each robot has slightly different instructions—the arguments will differ
from one robot to the next—the last robot should know when to stop.
Let’s expand on the metaphor in which a computer program is a robot.
A function definition provides the blueprints for a robot. When you installa function definition, that is, when you evaluate a defun macro, you install thenecessary equipment to build robots. It is as if you were in a factory, setting up

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an assembly line. Robots with the same name are built according to the same
blueprints. So they have the same model number, but a different serial number.
We often say that a recursive function “calls itself”. What we mean is thatthe instructions in a recursive function cause the Lisp interpreter to run a different
function that has the same name and does the same job as the first, but withdifferent arguments.
It is important that the arguments differ from one instance to the next; otherwise, the process will never stop.
11.3.2 The Parts of a Recursive Definition
A recursive function typically contains a conditional expression which has threeparts:
1. A true-or-false-test that determines whether the function is called again, here
called the do-again-test.
2. The name of the function. When this name is called, a new instance of thefunction—a new robot, as it were—is created and told what to do.
3. An expression that returns a different value each time the function is called,here called the next-step-expression. Consequently, the argument (or arguments) passed to the new instance of the function will be different from thatpassed to the previous instance. This causes the conditional expression, thedo-again-test, to test false after the correct number of repetitions.
Recursive functions can be much simpler than any other kind of function. Indeed, when people first start to use them, they often look so mysteriously simple as
to be incomprehensible. Like riding a bicycle, reading a recursive function definition
takes a certain knack which is hard at first but then seems simple.
There are several different common recursive patterns. A very simple pattern
looks like this:(defun name-of-recursive-function (argument-list)"documentation..."(if do-again-test
body...
(name-of-recursive-function
next-step-expression)))Each time a recursive function is evaluated, a new instance of it is created andtold what to do. The arguments tell the instance what to do.
An argument is bound to the value of the next-step-expression. Each instanceruns with a different value of the next-step-expression.
The value in the next-step-expression is used in the do-again-test.
The value returned by the next-step-expression is passed to the new instanceof the function, which evaluates it (or some transmogrification of it) to determine
whether to continue or stop. The next-step-expression is designed so that the doagain-test returns false when the function should no longer be repeated.
The do-again-test is sometimes called the stop condition, since it stops the repetitions when it tests false.

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11.3.3 Recursion with a List
The example of a while loop that printed the elements of a list of numbers can be
written recursively. Here is the code, including an expression to set the value of thevariable animals to a list.
If you are reading this in Info in Emacs, you can evaluate this expression directly in Info. Otherwise, you must copy the example to the *scratch* buffer andevaluate each expression there. Use C-u C-x C-e to evaluate the (print-elementsrecursively animals) expression so that the results are printed in the buffer; otherwise the Lisp interpreter will try to squeeze the results into the one line of theecho area.
Also, place your cursor immediately after the last closing parenthesis of theprint-elements-recursively function, before the comment. Otherwise, the Lisp
interpreter will try to evaluate the comment.
(setq animals '(gazelle giraffe lion tiger))(defun print-elements-recursively (list)"Print each element of LIST on a line of its own.
Uses recursion."(when list
; do-again-test(print (car list)); body(print-elements-recursively; recursive call(cdr list)))); next-step-expression(print-elements-recursively animals)

The print-elements-recursively function first tests whether there is any
content in the list; if there is, the function prints the first element of the list, thecar of the list. Then the function invokes itself, but gives itself as its argument,
not the whole list, but the second and subsequent elements of the list, the cdr ofthe list.
Put another way, if the list is not empty, the function invokes another instanceof code that is similar to the initial code, but is a different thread of execution, withdifferent arguments than the first instance.
Put in yet another way, if the list is not empty, the first robot assembles a second
robot and tells it what to do; the second robot is a different individual from thefirst, but is the same model.
When the second evaluation occurs, the when expression is evaluated and if true,
prints the first element of the list it receives as its argument (which is the second
element of the original list). Then the function calls itself with the cdr of the list
it is invoked with, which (the second time around) is the cdr of the cdr of theoriginal list.
Note that although we say that the function “calls itself”, what we mean is that
the Lisp interpreter assembles and instructs a new instance of the program. The
new instance is a clone of the first, but is a separate individual.
Each time the function invokes itself, it does so on a shorter version of theoriginal list. It creates a new instance that works on a shorter list.

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Eventually, the function invokes itself on an empty list. It creates a new instance
whose argument is nil. The conditional expression tests the value of list. Since
the value of list is nil, the when expression tests false so the then-part is notevaluated. The function as a whole then returns nil.
When you evaluate the expression (print-elements-recursively animals) inthe *scratch* buffer, you see this result:gazellegiraffe
lion
tiger
nil

11.3.4 Recursion in Place of a Counter
The triangle function described in a previous section can also be written recursively. It looks like this:(defun triangle-recursively (number)"Return the sum of the numbers 1 through NUMBER inclusive.
Uses recursion."(if (= number 1); do-again-test
1; then-part(+ number; else-part(triangle-recursively; recursive call
(1- number))))); next-step-expression(triangle-recursively 7)

You can install this function by evaluating it and then try it by evaluating(triangle-recursively 7). (Remember to put your cursor immediately after thelast parenthesis of the function definition, before the comment.) The function evaluates to 28.
To understand how this function works, let’s consider what happens in thevarious cases when the function is passed 1, 2, 3, or 4 as the value of its argument.
First, what happens if the value of the argument is 1?
The function has an if expression after the documentation string. It tests
whether the value of number is equal to 1; if so, Emacs evaluates the then-part ofthe if expression, which returns the number 1 as the value of the function. (Atriangle with one row has one pebble in it.)Suppose, however, that the value of the argument is 2. In this case, Emacs
evaluates the else-part of the if expression.

Section 11.3: Recursion
The else-part consists
triangle-recursively and
this:

131
of an addition,thea decrementing action;

recursive call toand it looks like(+ number (triangle-recursively (1- number)))

When Emacs evaluates this expression, the innermost expression is evaluatedfirst; then the other parts in sequence. Here are the steps in detail:
Step 1

Evaluate the innermost expression.
The innermost expression is (1- number) so Emacs decrements thevalue of number from 2 to 1.

Step 2

Evaluate the triangle-recursively function.
The Lisp interpreter creates an individual instance of
triangle-recursively.
It does not matter that this function is contained within itself. Emacs passes the result Step 1 as
the argument used by this instance of the triangle-recursivelyfunctionIn this case, Emacs evaluates triangle-recursively with an argument of 1. This means that this evaluation of triangle-recursively
returns 1.

Step 3

Evaluate the value of number.
The variable number is the second element of the list that starts with+; its value is 2.

Step 4

Evaluate the + expression.
The + expression receives two arguments, the first from the evaluation of number (Step 3) and the second from the evaluation oftriangle-recursively (Step 2).
The result of the addition is the sum of 2 plus 1, and the number 3 isreturned, which is correct. A triangle with two rows has three pebbles
in it.

An argument of 3 or 4
Suppose that triangle-recursively is called with an argument of 3.
Step 1

Evaluate the do-again-test.
The if expression is evaluated first. This is the do-again test andreturns false, so the else-part of the if expression is evaluated. (Notethat in this example, the do-again-test causes the function to call itself
when it tests false, not when it tests true.)

Step 2

Evaluate the innermost expression of the else-part.
The innermost expression of the else-part is evaluated, which decrements 3 to 2. This is the next-step-expression.

Step 3

Evaluate the triangle-recursively function.
The number 2 is passed to the triangle-recursively function.

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We already know what happens when Emacs evaluates
triangle-recursively with an argument of 2. After going throughthe sequence of actions described earlier, it returns a value of 3. Sothat is what will happen here.

Step 4

Evaluate the addition.
3 will be passed as an argument to the addition and will be added to
the number with which the function was called, which is 3.

The value returned by the function as a whole will be 6.
Now that we know what will happen when triangle-recursively is called withan argument of 3, it is evident what will happen if it is called with an argument of
4:
In the recursive call, the evaluation of(triangle-recursively (1- 4))

will return the value of evaluating(triangle-recursively 3)

which is 6 and this value will be added to 4 by the addition in the thirdline.
The value returned by the function as a whole will be 10.
Each time triangle-recursively is evaluated, it evaluates a version of itself—
a different instance of itself—with a smaller argument, until the argument is smallenough so that it does not evaluate itself.
Note that this particular design for a recursive function requires that operations
be deferred.
Before (triangle-recursively 7) can calculate its answer, it must call(triangle-recursively 6); and before (triangle-recursively 6) can calculate
its answer, it must call (triangle-recursively 5); and so on. That is to say,the calculation that (triangle-recursively 7) makes must be deferred until(triangle-recursively 6) makes its calculation; and (triangle-recursively6) must defer until (triangle-recursively 5) completes; and so on.
If each of these instances of triangle-recursively are thought of as different
robots, the first robot must wait for the second to complete its job, which must wait
until the third completes, and so on.
There is a way around this kind of waiting, which we will discuss in Section 11.3.7“Recursion without Deferments”, page 136.
11.3.5 Recursion Example Using cond
The version of triangle-recursively described earlier is written with the if
special form. It can also be written using another special form called cond. The
name of the special form cond is an abbreviation of the word ‘conditional’.
Although the cond special form is not used as often in the Emacs Lisp sources
as if, it is used often enough to justify explaining it.

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133

The template for a cond expression looks like this:(condbody...)

where the body is a series of lists.
Written out more fully, the template looks like this:(cond(first-true-or-false-test first-consequent)(second-true-or-false-test second-consequent)(third-true-or-false-test third-consequent)...)

When the Lisp interpreter evaluates the cond expression, it evaluates the firstelement (the car or true-or-false-test) of the first expression in a series of expressions
within the body of the cond.
If the true-or-false-test returns nil the rest of that expression, the consequent,is skipped and the true-or-false-test of the next expression is evaluated. When an
expression is found whose true-or-false-test returns a value that is not nil, theconsequent of that expression is evaluated. The consequent can be one or more
expressions. If the consequent consists of more than one expression, the expressions
are evaluated in sequence and the value of the last one is returned. If the expression
does not have a consequent, the value of the true-or-false-test is returned.
If none of the true-or-false-tests test true, the cond expression returns nil.
Written using cond, the triangle function looks like this:(defun triangle-using-cond (number)(cond ((<= number 0) 0)((= number 1) 1)((> number 1)(+ number (triangle-using-cond (1- number))))))In this example, the cond returns 0 if the number is less than or equal to 0, it
returns 1 if the number is 1 and it evaluates (+ number (triangle-using-cond(1- number))) if the number is greater than 1.
11.3.6 Recursive Patterns
Here are three common recursive patterns. Each involves a list. Recursion does notneed to involve lists, but Lisp is designed for lists and this provides a sense of its
primal capabilities.
Recursive Pattern: every
In the every recursive pattern, an action is performed on every element of a list.

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The basic pattern is:• If a list be empty, return nil.
• Else, act on the beginning of the list (the car of the list)− through a recursive call by the function on the rest (the cdr) of the list,− and, optionally, combine the acted-on element, using cons, with the results of acting on the rest.
Here is an example:(defun square-each (numbers-list)"Square each of a NUMBERS LIST, recursively."(if (not numbers-list); do-again-test
nil
(cons
(* (car numbers-list) (car numbers-list))(square-each (cdr numbers-list))))) ; next-step-expression(square-each '(1 2 3))⇒ (1 4 9)If numbers-list is empty, do nothing. But if it has content, construct a list combining the square of the first number in the list with the result of the recursive
call.
(The example follows the pattern exactly: nil is returned if the numbers’ list
is empty. In practice, you would write the conditional so it carries out the action
when the numbers’ list is not empty.)
The print-elements-recursively function (see Section 11.3.3 “Recursion
with a List”, page 129) is another example of an every pattern, except in this case,rather than bring the results together using cons, we print each element of output.
The print-elements-recursively function looks like this:(setq animals '(gazelle giraffe lion tiger))(defun print-elements-recursively (list)"Print each element of LIST on a line of its own.
Uses recursion."(when list
; do-again-test(print (car list)); body(print-elements-recursively; recursive call(cdr list)))); next-step-expression(print-elements-recursively animals)

Section 11.3: Recursion

135

The pattern for print-elements-recursively is:• When the list is empty, do nothing.
• But when the list has at least one element,− act on the beginning of the list (the car of the list),− and make a recursive call on the rest (the cdr) of the list.
Recursive Pattern: accumulate
Another recursive pattern is called the accumulate pattern. In the accumulate
recursive pattern, an action is performed on every element of a list and the resultof that action is accumulated with the results of performing the action on the otherelements.
This is very like the every pattern using cons, except that cons is not used,
but some other combiner.
The pattern is:• If a list be empty, return zero or some other constant.
• Else, act on the beginning of the list (the car of the list),− and combine that acted-on element, using + or some other combining
function, with− a recursive call by the function on the rest (the cdr) of the list.
Here is an example:(defun add-elements (numbers-list)"Add the elements of NUMBERS-LIST together."(if (not numbers-list)0(+ (car numbers-list) (add-elements (cdr numbers-list)))))(add-elements '(1 2 3 4))⇒ 10

See Section 14.9.2 “Making a List of Files”, page 179, for an example of theaccumulate pattern.
Recursive Pattern: keep
A third recursive pattern is called the keep pattern. In the keep recursive pattern,each element of a list is tested; the element is acted on and the results are kept only
if the element meets a criterion.
Again, this is very like the every pattern, except the element is skipped unless
it meets a criterion.

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The pattern has three parts:• If a list be empty, return nil.
• Else, if the beginning of the list (the car of the list) passes a test
− act on that element and combine it, using cons with− a recursive call by the function on the rest (the cdr) of the list.
• Otherwise, if the beginning of the list (the car of the list) fails the test
− skip on that element,− and, recursively call the function on the rest (the cdr) of the list.
Here is an example that uses cond:(defun keep-three-letter-words (word-list)"Keep three letter words in WORD-LIST."(cond;; First do-again-test: stop-condition((not word-list) nil);; Second do-again-test: when to act((eq 3 (length (symbol-name (car word-list))));; combine acted-on element with recursive call on shorter list
(cons (car word-list) (keep-three-letter-words (cdr word-list))));; Third do-again-test: when to skip element;;;
recursively call shorter list with next-step expression(t (keep-three-letter-words (cdr word-list)))))(keep-three-letter-words '(one two three four five six))⇒ (one two six)

It goes without saying that you need not use nil as the test for when to stop;and you can, of course, combine these patterns.
11.3.7 Recursion without Deferments
Let’s consider again what happens with the triangle-recursively function. We
will find that the intermediate calculations are deferred until all can be done.
Here is the function definition:(defun triangle-recursively (number)"Return the sum of the numbers 1 through NUMBER inclusive.
Uses recursion."(if (= number 1); do-again-test
1; then-part(+ number; else-part(triangle-recursively; recursive call
(1- number))))); next-step-expression

What happens when we call this function with an argument of 7?
The first instance of the triangle-recursively function adds the number 7
to the value returned by a second instance of triangle-recursively, an instancethat has been passed an argument of 6. That is to say, the first calculation is:(+ 7 (triangle-recursively 6))

Section 11.3: Recursion

137

The first instance of triangle-recursively—you may want to think of it asa little robot—cannot complete its job. It must hand off the calculation for(triangle-recursively 6) to a second instance of the program, to a second robot.
This second individual is completely different from the first one; it is, in the jargon,
a “different instantiation”. Or, put another way, it is a different robot. It is thesame model as the first; it calculates triangle numbers recursively; but it has adifferent serial number.
And what does (triangle-recursively 6) return? It returns the number 6
added to the value returned by evaluating triangle-recursively with an argument of 5. Using the robot metaphor, it asks yet another robot to help it.
Now the total is:(+ 7 6 (triangle-recursively 5))

And what happens next?
(+ 7 6 5 (triangle-recursively 4))

Each time triangle-recursively is called, except for the last time, it createsanother instance of the program—another robot—and asks it to make a calculation.
Eventually, the full addition is set up and performed:(+ 7 6 5 4 3 2 1)

This design for the function defers the calculation of the first step until thesecond can be done, and defers that until the third can be done, and so on. Eachdeferment means the computer must remember what is being waited on. This is
not a problem when there are only a few steps, as in this example. But it can be a
problem when there are more steps.
11.3.8 No Deferment Solution
The solution to the problem of deferred operations is to write in a manner that
does not defer operations2 . This requires writing to a different pattern, often one
that involves writing two function definitions, an initialization function and a helper
function.
The initialization function sets up the job; the helper function does the work.

2

The phrase tail recursive is used to describe such a process, one that uses constantspace.

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Here are the two function definitions for adding up numbers. They are so simple,
I find them hard to understand.
(defun triangle-initialization (number)"Return the sum of the numbers 1 through NUMBER inclusive.
This is the initialization component of a two function
duo that uses recursion."(triangle-recursive-helper 0 0 number))(defun triangle-recursive-helper (sum counter number)"Return SUM, using COUNTER, through NUMBER inclusive.
This is the helper component of a two function duo
that uses recursion."(if (> counter number)sum(triangle-recursive-helper (+ sum counter) ; sum(1+ counter); counter
number))); numberInstall both function definitions
triangle-initialization with 2 rows:byevaluating

them,

then

call

(triangle-initialization 2)⇒ 3

The initialization function calls the first instance of the helper function withthree arguments: zero, zero, and a number which is the number of rows in thetriangle.
The first two arguments passed to the helper function are initialization values. These values are changed when triangle-recursive-helper invokes new
instances.3
Let’s see what happens when we have a triangle that has one row. (This triangle
will have one pebble in it!)triangle-initialization will call its helper with the arguments 0 0 1. Thatfunction will run the conditional test whether (> counter number):(> 0 1)and find that the result is false, so it will invoke the else-part of the if clause:(triangle-recursive-helper(+ sum counter) ; sum plus counter ⇒ sum(1+ counter); increment counter ⇒ counter
number); number stays the same3

The jargon is mildly confusing: triangle-recursive-helper uses a process that is
iterative in a procedure that is recursive. The process is called iterative because thecomputer need only record the three values, sum, counter, and number; the procedure isrecursive because the function calls itself. On the other hand, both the process and theprocedure used by triangle-recursively are called recursive. The word “recursive”
has different meanings in the two contexts.

Section 11.4: Looping Exercise

139

which will first compute:(triangle-recursive-helper (+ 0 0)
(1+ 0)
1)which is:; sum
; counter; number(triangle-recursive-helper 0 1 1)

Again, (> counter number) will be false, so again, the Lisp interpreter will evaluate triangle-recursive-helper, creating a new instance with new arguments.
This new instance will be;(triangle-recursive-helper(+ sum counter) ; sum plus counter ⇒ sum(1+ counter); increment counter ⇒ counter
number); number stays the same
which is:(triangle-recursive-helper 1 2 1)

In this case, the (> counter number) test will be true! So the instance will
return the value of the sum, which will be 1, as expected.
Now, let’s pass triangle-initialization an argument of 2, to find out how
many pebbles there are in a triangle with two rows.
That function calls (triangle-recursive-helper 0 0 2).
In stages, the instances called will be:sum counter number(triangle-recursive-helper 0
12)(triangle-recursive-helper 1

2

2)(triangle-recursive-helper 3

3

2)

When the last instance is called, the (> counter number) test will be true, so
the instance will return the value of sum, which will be 3.
This kind of pattern helps when you are writing functions that can use many
resources in a computer.

11.4 Looping Exercise• Write a function similar to triangle in which each row has a value which isthe square of the row number. Use a while loop.
• Write a function similar to triangle that multiplies instead of adds the values.
• Rewrite these two functions recursively. Rewrite these functions using cond.
• Write a function for Texinfo mode that creates an index entry at the beginningof a paragraph for every ‘@dfn’ within the paragraph. (In a Texinfo file, ‘@dfn’
marks a definition. This book is written in Texinfo.)
Many of the functions you will need are described in two of the previous
chapters, Chapter 8 “Cutting and Storing Text”, page 85, and Chapter 10“Yanking Text Back”, page 113. If you use forward-paragraph to put the140

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index entry at the beginning of the paragraph, you will have to use C-h f(describe-function) to find out how to make the command go backwards.
For more information, see “Indicating Definitions, Commands, etc.” in Texinfo,
The GNU Documentation Format.

Section 12.1: The Regular Expression for sentence-end

141

12 Regular Expression Searches
Regular expression searches are used extensively in GNU Emacs. The two functions,forward-sentence and forward-paragraph, illustrate these searches well. Theyuse regular expressions to find where to move point. The phrase “regular expression”
is often written as “regexp”.
Regular expression searches are described in Section “Regular Expression
Search” in The GNU Emacs Manual, as well as in Section “Regular Expressions” in
The GNU Emacs Lisp Reference Manual. In writing this chapter, I am presumingthat you have at least a mild acquaintance with them. The major point to remember is that regular expressions permit you to search for patterns as well as for literal
strings of characters. For example, the code in forward-sentence searches for thepattern of possible characters that could mark the end of a sentence, and moves
point to that spot.
Before looking at the code for the forward-sentence function, it is worth considering what the pattern that marks the end of a sentence must be. The pattern is
discussed in the next section; following that is a description of the regular expression
search function, re-search-forward. The forward-sentence function is describedin the section following. Finally, the forward-paragraph function is described in
the last section of this chapter. forward-paragraph is a complex function that
introduces several new features.

12.1 The Regular Expression for sentence-end
The symbol sentence-end is bound to the pattern that marks the end of a sentence.
What should this regular expression be?
Clearly, a sentence may be ended by a period, a question mark, or an exclamation
mark. Indeed, in English, only clauses that end with one of those three charactersshould be considered the end of a sentence. This means that the pattern should
include the character set:[.?!]

However, we do not want forward-sentence merely to jump to a period, aquestion mark, or an exclamation mark, because such a character might be usedin the middle of a sentence. A period, for example, is used after abbreviations. So
other information is needed.
According to convention, you type two spaces after every sentence, but only one
space after a period, a question mark, or an exclamation mark in the body of asentence. So a period, a question mark, or an exclamation mark followed by two
spaces is a good indicator of an end of sentence. However, in a file, the two spaces
may instead be a tab or the end of a line. This means that the regular expression
should include these three items as alternatives.

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This group of alternatives will look like this:\\($\\| \\| \\)^^^TAB SPC

Here, ‘$’ indicates the end of the line, and I have pointed out where the tab andtwo spaces are inserted in the expression. Both are inserted by putting the actual
characters into the expression.
Two backslashes, ‘\\’, are required before the parentheses and vertical bars: thefirst backslash quotes the following backslash in Emacs; and the second indicatesthat the following character, the parenthesis or the vertical bar, is special.
Also, a sentence may be followed by one or more carriage returns, like this:[]*Like tabs and spaces, a carriage return is inserted into a regular expression by
inserting it literally. The asterisk indicates that the RET is repeated zero or moretimes.
But a sentence end does not consist only of a period, a question mark or an
exclamation mark followed by appropriate space: a closing quotation mark or aclosing brace of some kind may precede the space. Indeed more than one such markor brace may precede the space. These require a expression that looks like this:[]\"')}]*In this expression, the first ‘]’ is the first character in the expression; the second
character is ‘"’, which is preceded by a ‘\’ to tell Emacs the ‘"’ is not special. The
last three characters are ‘'’, ‘)’, and ‘}’.
All this suggests what the regular expression pattern for matching the end of
a sentence should be; and, indeed, if we evaluate sentence-end we find that it
returns the following value:sentence-end⇒ "[.?!][]\"')}]*\\($\\|
]*"

\\|

\\)[

(Well, not in GNU Emacs 22; that is because of an effort to make the process simpler and to handle more glyphs and languages. When the value of sentence-end isnil, then use the value defined by the function sentence-end. (Here is a use of thedifference between a value and a function in Emacs Lisp.) The function returns avalue constructed from the variables sentence-end-base, sentence-end-doublespace, sentence-end-without-period, and sentence-end-without-space. The
critical variable is sentence-end-base; its global value is similar to the one described above but it also contains two additional quotation marks. These havediffering degrees of curliness. The sentence-end-without-period variable, when
true, tells Emacs that a sentence may end without a period, such as text in Thai.)

12.2 The re-search-forward Function
The re-search-forward function is very like the search-forward function. (See
Section 8.1.3 “The search-forward Function”, page 87.)

Section 12.3: forward-sentence143

re-search-forward searches for a regular expression. If the search is successful,it leaves point immediately after the last character in the target. If the search isbackwards, it leaves point just before the first character in the target. You may tellre-search-forward to return t for true. (Moving point is therefore a side effect.)
Like search-forward, the re-search-forward function takes four arguments:
1. The first argument is the regular expression that the function searches for.
The regular expression will be a string between quotation marks.
2. The optional second argument limits how far the function will search; it is abound, which is specified as a position in the buffer.
3. The optional third argument specifies how the function responds to failure:
nil as the third argument causes the function to signal an error (and print a
message) when the search fails; any other value causes it to return nil if thesearch fails and t if the search succeeds.
4. The optional fourth argument is the repeat count. A negative repeat countcauses re-search-forward to search backwards.
The template for re-search-forward looks like this:(re-search-forward "regular-expression"
limit-of-search
what-to-do-if-search-fails
repeat-count)

The second, third, and fourth arguments are optional. However, if you want topass a value to either or both of the last two arguments, you must also pass a value
to all the preceding arguments. Otherwise, the Lisp interpreter will mistake whichargument you are passing the value to.
In the forward-sentence function, the regular expression will be the value ofthe variable sentence-end. In simple form, that is:"[.?!][]\"')}]*\\($\\|
]*"

\\|

\\)[

The limit of the search will be the end of the paragraph (since a sentence cannot
go beyond a paragraph). If the search fails, the function will return nil; and therepeat count will be provided by the argument to the forward-sentence function.

12.3 forward-sentence
The command to move the cursor forward a sentence is a straightforward illustrationof how to use regular expression searches in Emacs Lisp. Indeed, the function looks
longer and more complicated than it is; this is because the function is designed togo backwards as well as forwards; and, optionally, over more than one sentence.
The function is usually bound to the key command M-e.

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Here is the code for forward-sentence:(defun forward-sentence (&optional arg)
"Move forward to next end of sentence. With argument, repeat.
With negative argument, move backward repeatedly to start of sentence.
The variable `sentence-end' is a regular expression that matches ends ofsentences. Also, every paragraph boundary terminates sentences as well."(interactive "p")(or arg (setq arg 1))(let ((opoint (point))(sentence-end (sentence-end)))(while (< arg 0)(let ((pos (point))(par-beg (save-excursion (start-of-paragraph-text) (point))))(if (and (re-search-backward sentence-end par-beg t)(or (< (match-end 0) pos)(re-search-backward sentence-end par-beg t)))(goto-char (match-end 0))(goto-char par-beg)))(setq arg (1+ arg)))(while (> arg 0)(let ((par-end (save-excursion (end-of-paragraph-text) (point))))(if (re-search-forward sentence-end par-end t)(skip-chars-backward " \t\n")(goto-char par-end)))(setq arg (1- arg)))(constrain-to-field nil opoint t)))

The function looks long at first sight and it is best to look at its skeleton first,
and then its muscle. The way to see the skeleton is to look at the expressions thatstart in the left-most columns:(defun forward-sentence (&optional arg)
"documentation..."(interactive "p")(or arg (setq arg 1))(let ((opoint (point)) (sentence-end (sentence-end)))(while (< arg 0)(let ((pos (point))(par-beg (save-excursion (start-of-paragraph-text) (point))))rest-of-body-of-while-loop-when-going-backwards(while (> arg 0)(let ((par-end (save-excursion (end-of-paragraph-text) (point))))rest-of-body-of-while-loop-when-going-forwards
handle-forms-and-equivalentThis looks much simpler! The function definition consists of documentation, aninteractive expression, an or expression, a let expression, and while loops.
Let’s look at each of these parts in turn.
We note that the documentation is thorough and understandable.
The function has an interactive "p" declaration. This means that the processed prefix argument, if any, is passed to the function as its argument. (This will

Section 12.3: forward-sentence145

be a number.) If the function is not passed an argument (it is optional) then theargument arg will be bound to 1.
When forward-sentence is called non-interactively without an argument, arg
is bound to nil. The or expression handles this. What it does is either leave thevalue of arg as it is, but only if arg is bound to a value; or it sets the value of arg
to 1, in the case when arg is bound to nil.
Next is a let. That specifies the values of two local variables, opoint andsentence-end. The local value of point, from before the search, is used in
the constrain-to-field function which handles forms and equivalents. The
sentence-end variable is set by the sentence-end function.
The while loops
Two while loops follow. The first while has a true-or-false-test that tests true if
the prefix argument for forward-sentence is a negative number. This is for goingbackwards. The body of this loop is similar to the body of the second while clause,
but it is not exactly the same. We will skip this while loop and concentrate on thesecond while loop.
The second while loop is for moving point forward. Its skeleton looks like this:(while (> arg 0); true-or-false-test(let varlist(if (true-or-false-test)
then-partelse-part(setq arg (1- arg)))); while loop decrementer

The while loop is of the decrementing kind. (See Section 11.1.4 “A Loop witha Decrementing Counter”, page 122.) It has a true-or-false-test that tests true solong as the counter (in this case, the variable arg) is greater than zero; and it has
a decrementer that subtracts 1 from the value of the counter every time the loop
repeats.
If no prefix argument is given to forward-sentence, which is the most common
way the command is used, this while loop will run once, since the value of arg willbe 1.
The body of the while loop consists of a let expression, which creates andbinds a local variable, and has, as its body, an if expression.

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The body of the while loop looks like this:(let ((par-end(save-excursion (end-of-paragraph-text) (point))))(if (re-search-forward sentence-end par-end t)(skip-chars-backward " \t\n")(goto-char par-end)))

The let expression creates and binds the local variable par-end. As we shall see,this local variable is designed to provide a bound or limit to the regular expression
search. If the search fails to find a proper sentence ending in the paragraph, it willstop on reaching the end of the paragraph.
But first, let us examine how par-end is bound to the value of the end of theparagraph. What happens is that the let sets the value of par-end to the value
returned when the Lisp interpreter evaluates the expression(save-excursion (end-of-paragraph-text) (point))

In this expression, (end-of-paragraph-text) moves point to the end of the paragraph, (point) returns the value of point, and then save-excursion restores point
to its original position. Thus, the let binds par-end to the value returned by thesave-excursion expression, which is the position of the end of the paragraph. (The
end-of-paragraph-text function uses forward-paragraph, which we will discussshortly.)
Emacs next evaluates the body of the let, which is an if expression that looks
like this:(if (re-search-forward sentence-end par-end t) ; if-part(skip-chars-backward " \t\n"); then-part(goto-char par-end))); else-part

The if tests whether its first argument is true and if so, evaluates its then-part;otherwise, the Emacs Lisp interpreter evaluates the else-part. The true-or-false-testof the if expression is the regular expression search.
It may seem odd to have what looks like the real work of the forward-sentencefunction buried here, but this is a common way this kind of operation is carried out
in Lisp.
The regular expression search
The re-search-forward function searches for the end of the sentence, that is,for the pattern defined by the sentence-end regular expression. If the pattern isfound—if the end of the sentence is found—then the re-search-forward function
does two things:
1. The re-search-forward function carries out a side effect, which is to movepoint to the end of the occurrence found.
2. The re-search-forward function returns a value of true. This is the value
received by the if, and means that the search was successful.
The side effect, the movement of point, is completed before the if function is handed
the value returned by the successful conclusion of the search.
When the if function receives the value of true from a successful call to
re-search-forward, the if evaluates the then-part, which is the expression

Section 12.4: forward-paragraph: a Goldmine of Functions

147(skip-chars-backward " \t\n"). This expression moves backwards over any
blank spaces, tabs or carriage returns until a printed character is found and thenleaves point after the character. Since point has already been moved to the end of
the pattern that marks the end of the sentence, this action leaves point right after
the closing printed character of the sentence, which is usually a period.
On the other hand, if the re-search-forward function fails to find a patternmarking the end of the sentence, the function returns false. The false then causes
the if to evaluate its third argument, which is (goto-char par-end): it moves
point to the end of the paragraph.
(And if the text is in a form or equivalent, and point may not move fully, thenthe constrain-to-field function comes into play.)
Regular expression searches are exceptionally useful and the pattern illustrated
by re-search-forward, in which the search is the test of an if expression, is handy.
You will see or write code incorporating this pattern often.

12.4 forward-paragraph: a Goldmine of Functions
The forward-paragraph function moves point forward to the end of the paragraph.
It is usually bound to M-} and makes use of a number of functions that are importantin themselves, including let*, match-beginning, and looking-at.
The function definition for forward-paragraph is considerably longer than thefunction definition for forward-sentence because it works with a paragraph, eachline of which may begin with a fill prefix.
A fill prefix consists of a string of characters that are repeated at the beginningof each line. For example, in Lisp code, it is a convention to start each line ofa paragraph-long comment with ‘;;; ’. In Text mode, four blank spaces makeup another common fill prefix, creating an indented paragraph. (See Section “FillPrefix” in The GNU Emacs Manual, for more information about fill prefixes.)
The existence of a fill prefix means that in addition to being able to find the endof a paragraph whose lines begin on the left-most column, the forward-paragraphfunction must be able to find the end of a paragraph when all or many of the lines
in the buffer begin with the fill prefix.
Moreover, it is sometimes practical to ignore a fill prefix that exists, especiallywhen blank lines separate paragraphs. This is an added complication.
Rather than print all of the forward-paragraph function, we will only print
parts of it. Read without preparation, the function can be daunting!
In outline, the function looks like this:(defun forward-paragraph (&optional arg)
"documentation..."(interactive "p")(or arg (setq arg 1))(let*varlist(while (and (< arg 0) (not (bobp)))...
(while (and (> arg 0) (not (eobp)))...

; backward-moving-code; forward-moving-code

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The first parts of the function are routine: the function’s argument list consistsof one optional argument. Documentation follows.
The lower case ‘p’ in the interactive declaration means that the processed
prefix argument, if any, is passed to the function. This will be a number, and is
the repeat count of how many paragraphs point will move. The or expression in
the next line handles the common case when no argument is passed to the function,
which occurs if the function is called from other code rather than interactively. Thiscase was described earlier. (See Section 12.3 “forward-sentence”, page 143.) Now
we reach the end of the familiar part of this function.
The let* expression
The next line of the forward-paragraph function begins a let* expression (see“let* introduced”, page 56), in which Emacs binds a total of seven variables: opoint, fill-prefix-regexp, parstart, parsep, sp-parstart, start, andfound-start. The first part of the let* expression looks like below:(let* ((opoint (point))(fill-prefix-regexp(and fill-prefix (not (equal fill-prefix ""))
(not paragraph-ignore-fill-prefix)(regexp-quote fill-prefix)));; Remove ^ from paragraph-start and paragraph-sep if they are there.
;; These regexps shouldn't be anchored, because we look for them;; starting at the left-margin. This allows paragraph commands to;; work normally with indented text.
;; This hack will not find problem cases like "whatever\\|^something".
(parstart (if (and (not (equal "" paragraph-start))(equal ?^ (aref paragraph-start 0)))(substring paragraph-start 1)
paragraph-start))(parsep (if (and (not (equal "" paragraph-separate))(equal ?^ (aref paragraph-separate 0)))(substring paragraph-separate 1)
paragraph-separate))(parsep(if fill-prefix-regexp(concat parsep "\\|"
fill-prefix-regexp "[ \t]*$")
parsep));; This is used for searching.
(sp-parstart (concat "^[ \t]*\\(?:" parstart "\\|" parsep "\\)"))
start found-start)...)

The variable parsep appears twice, first, to remove instances of ‘^’, and second,
to handle fill prefixes.
The variable opoint is just the value of point. As you can guess, it is used ina constrain-to-field expression, just as in forward-sentence.
The variable fill-prefix-regexp is set to the value returned by evaluating thefollowing list:

Section 12.4: forward-paragraph: a Goldmine of Functions

149(and fill-prefix(not (equal fill-prefix ""))
(not paragraph-ignore-fill-prefix)(regexp-quote fill-prefix))This is an expression whose first element is the and special form.
As we learned earlier (see “The kill-new function”, page 97), the and special
form evaluates each of its arguments until one of the arguments returns a value ofnil, in which case the and expression returns nil; however, if none of the arguments
returns a value of nil, the value resulting from evaluating the last argument isreturned. (Since such a value is not nil, it is considered true in Lisp.) In otherwords, an and expression returns a true value only if all its arguments are true.
In this case, the variable fill-prefix-regexp is bound to a non-nil value only
if the following four expressions produce a true (i.e., a non-nil) value when they
are evaluated; otherwise, fill-prefix-regexp is bound to nil.
fill-prefix
When this variable is evaluated, the value of the fill prefix, if any, isreturned. If there is no fill prefix, this variable returns nil.
(not (equal fill-prefix "")This expression checks whether an existing fill prefix is an empty
string, that is, a string with no characters in it. An empty string
is not a useful fill prefix.
(not paragraph-ignore-fill-prefix)
This expression returns nil if the variable paragraph-ignore-fillprefix has been turned on by being set to a true value such as t.
(regexp-quote fill-prefix)
This is the last argument to the and special form. If all the arguments
to the and are true, the value resulting from evaluating this expression
will be returned by the and expression and bound to the variable
fill-prefix-regexp,
The result of evaluating this and expression successfully is that fill-prefixregexp will be bound to the value of fill-prefix as modified by the regexp-quote
function. What regexp-quote does is read a string and return a regular expression that will exactly match the string and match nothing else. This means thatfill-prefix-regexp will be set to a value that will exactly match the fill prefix if
the fill prefix exists. Otherwise, the variable will be set to nil.
The next two local variables in the let* expression are designed to remove
instances of ‘^’ from parstart and parsep, the local variables which indicate theparagraph start and the paragraph separator. The next expression sets parsep
again. That is to handle fill prefixes.
This is the setting that requires the definition call let* rather than let. The
true-or-false-test for the if depends on whether the variable fill-prefix-regexp
evaluates to nil or some other value.

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If fill-prefix-regexp does not have a value, Emacs evaluates the else-partof the if expression and binds parsep to its local value. (parsep is a regularexpression that matches what separates paragraphs.)
But if fill-prefix-regexp does have a value, Emacs evaluates the then-partof the if expression and binds parsep to a regular expression that includes thefill-prefix-regexp as part of the pattern.
Specifically, parsep is set to the original value of the paragraph separate regular expression concatenated with an alternative expression that consists of thefill-prefix-regexp followed by optional whitespace to the end of the line. The
whitespace is defined by "[ \t]*$".) The ‘\\|’ defines this portion of the regexp
as an alternative to parsep.
According to a comment in the code, the next local variable, sp-parstart, isused for searching, and then the final two, start and found-start, are set to nil.
Now we get into the body of the let*. The first part of the body of the let*deals with the case when the function is given a negative argument and is therefore
moving backwards. We will skip this section.
The forward motion while loop
The second part of the body of the let* deals with forward motion. It is a while
loop that repeats itself so long as the value of arg is greater than zero. In the mostcommon use of the function, the value of the argument is 1, so the body of thewhile loop is evaluated exactly once, and the cursor moves forward one paragraph.
This part handles three situations: when point is between paragraphs, when
there is a fill prefix and when there is no fill prefix.
The while loop looks like this:;; going forwards and not at the end of the buffer(while (and (> arg 0) (not (eobp)));; between paragraphs;; Move forward over separator lines...
(while (and (not (eobp))(progn (move-to-left-margin) (not (eobp)))(looking-at parsep))(forward-line 1));; This decrements the loop
(unless (eobp) (setq arg (1- arg)));; ... and one more line.
(forward-line 1)(if fill-prefix-regexp;; There is a fill prefix; it overrides parstart;;; we go forward line by line
(while (and (not (eobp))(progn (move-to-left-margin) (not (eobp)))(not (looking-at parsep))(looking-at fill-prefix-regexp))(forward-line 1))Section 12.4: forward-paragraph: a Goldmine of Functions

151;; There is no fill prefix;;; we go forward character by character(while (and (re-search-forward sp-parstart nil 1)(progn (setq start (match-beginning 0))(goto-char start)(not (eobp)))(progn (move-to-left-margin)(not (looking-at parsep)))(or (not (looking-at parstart))(and use-hard-newlines(not (get-text-property (1- start) 'hard)))))(forward-char 1));; and if there is no fill prefix and if we are not at the end,;;
go to whatever was found in the regular expression search;;for sp-parstart(if (< (point) (point-max))(goto-char start))))We can see that this is a decrementing counter while loop, using the expression (setq arg (1- arg)) as the decrementer. That expression is not far from thewhile, but is hidden in another Lisp macro, an unless macro. Unless we are at theend of the buffer—that is what the eobp function determines; it is an abbreviationof ‘End Of Buffer P’—we decrease the value of arg by one.
(If we are at the end of the buffer, we cannot go forward any more and thenext loop of the while expression will test false since the test is an and with (not(eobp)). The not function means exactly as you expect; it is another name fornull, a function that returns true when its argument is false.)
Interestingly, the loop count is not decremented until we leave the space between
paragraphs, unless we come to the end of buffer or stop seeing the local value of theparagraph separator.
That second while also has a (move-to-left-margin) expression. The function
is self-explanatory. It is inside a progn expression and not the last element of its
body, so it is only invoked for its side effect, which is to move point to the left
margin of the current line.
The looking-at function is also self-explanatory; it returns true if the text after
point matches the regular expression given as its argument.
The rest of the body of the loop looks difficult at first, but makes sense as youcome to understand it.
First consider what happens if there is a fill prefix:(if fill-prefix-regexp;; There is a fill prefix; it overrides parstart;;; we go forward line by line
(while (and (not (eobp))(progn (move-to-left-margin) (not (eobp)))(not (looking-at parsep))(looking-at fill-prefix-regexp))(forward-line 1))

This expression moves point forward line by line so long as four conditions are true:

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1. Point is not at the end of the buffer.
2. We can move to the left margin of the text and are not at the end of the buffer.
3. The text following point does not separate paragraphs.
4. The pattern following point is the fill prefix regular expression.
The last condition may be puzzling, until you remember that point was moved
to the beginning of the line early in the forward-paragraph function. This meansthat if the text has a fill prefix, the looking-at function will see it.
Consider what happens when there is no fill prefix.
(while (and (re-search-forward sp-parstart nil 1)(progn (setq start (match-beginning 0))(goto-char start)(not (eobp)))(progn (move-to-left-margin)(not (looking-at parsep)))(or (not (looking-at parstart))(and use-hard-newlines(not (get-text-property (1- start) 'hard)))))(forward-char 1))

This while loop has us searching forward for sp-parstart, which is the combinationof possible whitespace with the local value of the start of a paragraph or of aparagraph separator. (The latter two are within an expression starting \(?: sothat they are not referenced by the match-beginning function.)
The two expressions,(setq start (match-beginning 0))(goto-char start)

mean go to the start of the text matched by the regular expression search.
The (match-beginning 0) expression is new. It returns a number specifying
the location of the start of the text that was matched by the last search.
The match-beginning function is used here because of a characteristic of aforward search: a successful forward search, regardless of whether it is a plain
search or a regular expression search, moves point to the end of the text that isfound. In this case, a successful search moves point to the end of the pattern for
sp-parstart.
However, we want to put point at the end of the current paragraph, not somewhere else. Indeed, since the search possibly includes the paragraph separator,
point may end up at the beginning of the next one unless we use an expression that
includes match-beginning.
When given an argument of 0, match-beginning returns the position that is thestart of the text matched by the most recent search. In this case, the most recent
search looks for sp-parstart. The (match-beginning 0) expression returns the
beginning position of that pattern, rather than the end position of that pattern.
(Incidentally, when passed a positive number as an argument, thematch-beginning function returns the location of point at that parenthesized
expression in the last search unless that parenthesized expression begins with\(?:. I don’t know why \(?: appears here since the argument is 0.)

Section 12.5: Review

153

The last expression when there is no fill prefix is(if (< (point) (point-max))(goto-char start))))(Note that this code snippet is copied verbatim from the original code, so the two
extra ending parentheses are matching the previous if and while.)
This says that if there is no fill prefix and if we are not at the end, point should
move to the beginning of whatever was found by the regular expression search for
sp-parstart.
The full definition for the forward-paragraph function not only includes codefor going forwards, but also code for going backwards.
If you are reading this inside of GNU Emacs and you want to see the wholefunction, you can type C-h f (describe-function) and the name of the function.
This gives you the function documentation and the name of the library containing
the function’s source. Place point over the name of the library and press the RET
key; you will be taken directly to the source. (Be sure to install your sources!
Without them, you are like a person who tries to drive a car with his eyes shut!)

12.5 Review
Here is a brief summary of some recently introduced functions.
while

Repeatedly evaluate the body of the expression so long as the firstelement of the body tests true. Then return nil. (The expression isevaluated only for its side effects.)
For example:(let ((foo 2))(while (> foo 0)(insert (format "foo is %d.\n" foo))(setq foo (1- foo))))⇒foo is 2.
foo is 1.
nil(The insert function inserts its arguments at point; the format function returns a string formatted from its arguments the way message
formats its arguments; \n produces a new line.)
re-search-forward
Search for a pattern, and if the pattern is found, move point to rest
just after it.
Takes four arguments, like search-forward:
1. A regular expression that specifies the pattern to search for. (Remember to put quotation marks around this argument!)
2. Optionally, the limit of the search.
3. Optionally, what to do if the search fails, return nil or an error
message.

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4. Optionally, how many times to repeat the search; if negative, thesearch goes backwards.

let*Bind some variables locally to particular values, and then evaluate
the remaining arguments, returning the value of the last one. Whilebinding the local variables, use the local values of variables boundearlier, if any.
For example:(let* ((foo 7)(bar (* 3 foo)))(message "`bar' is %d." bar))⇒ ‘bar’ is 21.

match-beginning
Return the position of the start of the text found by the last regularexpression search.
looking-at
Return t for true if the text after point matches the argument, whichshould be a regular expression.
eobp

Return t for true if point is at the end of the accessible part of abuffer. The end of the accessible part is the end of the buffer if thebuffer is not narrowed; it is the end of the narrowed part if the buffer
is narrowed.

12.6 Exercises with re-search-forward• Write a function to search for a regular expression that matches two or more
blank lines in sequence.
• Write a function to search for duplicated words, such as “the the”. See Section“Syntax of Regular Expressions” in The GNU Emacs Manual, for information
on how to write a regexp (a regular expression) to match a string that iscomposed of two identical halves. You can devise several regexps; some are
better than others. The function I use is described in an appendix, along withseveral regexps. See Appendix A “the-the Duplicated Words Function”,
page 223.

Section 13.1: The count-words-example Function

155

13 Counting via Repetition and Regexps
Repetition and regular expression searches are powerful tools that you often use
when you write code in Emacs Lisp. This chapter illustrates the use of regularexpression searches through the construction of word count commands using whileloops and recursion.
The standard Emacs distribution contains functions for counting the number oflines and words within a region.
Certain types of writing ask you to count words. Thus, if you write an essay,
you may be limited to 800 words; if you write a novel, you may discipline yourself towrite 1000 words a day. It seems odd, but for a long time, Emacs lacked a word countcommand. Perhaps people used Emacs mostly for code or types of documentationthat did not require word counts; or perhaps they restricted themselves to theoperating system word count command, wc. Alternatively, people may have followed
the publishers’ convention and computed a word count by dividing the number ofcharacters in a document by five.
There are many ways to implement a command to count words. Here are someexamples, which you may wish to compare with the standard Emacs command,count-words-region.

13.1 The count-words-example Function
A word count command could count words in a line, paragraph, region, or buffer.
What should the command cover? You could design the command to count the
number of words in a complete buffer. However, the Emacs tradition encourages
flexibility—you may want to count words in just a section, rather than all of a buffer.
So it makes more sense to design the command to count the number of words in aregion. Once you have a command to count words in a region, you can, if you wish,
count words in a whole buffer by marking it with C-x h (mark-whole-buffer).
Clearly, counting words is a repetitive act: starting from the beginning of theregion, you count the first word, then the second word, then the third word, and so
on, until you reach the end of the region. This means that word counting is ideallysuited to recursion or to a while loop.
First, we will implement the word count command with a while loop, then withrecursion. The command will, of course, be interactive.
The template for an interactive function definition is, as always:(defun name-of-function (argument-list)"documentation..."(interactive-expression...)body...)

What we need to do is fill in the slots.
The name of the function should be self-explanatory and easy to remember. count-words-region is the obvious choice. Since that name is used forthe standard Emacs command to count words, we will name our implementation
count-words-example.

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The function counts words within a region. This means that the argument list
must contain symbols that are bound to the two positions, the beginning and endof the region. These two positions can be called ‘beginning’ and ‘end’ respectively.
The first line of the documentation should be a single sentence, since that is allthat is printed as documentation by a command such as apropos. The interactive
expression will be of the form ‘(interactive "r")’, since that will cause Emacs topass the beginning and end of the region to the function’s argument list. All this isroutine.
The body of the function needs to be written to do three tasks: first, to set up
conditions under which the while loop can count words, second, to run the while
loop, and third, to send a message to the user.
When a user calls count-words-example, point may be at the beginning or theend of the region. However, the counting process must start at the beginning of
the region. This means we will want to put point there if it is not already there.
Executing (goto-char beginning) ensures this. Of course, we will want to return
point to its expected position when the function finishes its work. For this reason,
the body must be enclosed in a save-excursion expression.
The central part of the body of the function consists of a while loop in which
one expression jumps point forward word by word, and another expression counts
those jumps. The true-or-false-test of the while loop should test true so long as
point should jump forward, and false when point is at the end of the region.
We could use (forward-word 1) as the expression for moving point forward
word by word, but it is easier to see what Emacs identifies as a “word” if we use aregular expression search.
A regular expression search that finds the pattern for which it is searching leaves
point after the last character matched. This means that a succession of successful
word searches will move point forward word by word.
As a practical matter, we want the regular expression search to jump over whitespace and punctuation between words as well as over the words themselves. A regexp that refuses to jump over interword whitespace would never jump more than
one word! This means that the regexp should include the whitespace and punctuation that follows a word, if any, as well as the word itself. (A word may end a buffer
and not have any following whitespace or punctuation, so that part of the regexp
must be optional.)
Thus, what we want for the regexp is a pattern defining one or more word
constituent characters followed, optionally, by one or more characters that are notword constituents. The regular expression for this is:\w+\W*The buffer’s syntax table determines which characters are and are not word constituents. For more information about syntax, see Section “Syntax Tables” in The
GNU Emacs Lisp Reference Manual.

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157

The search expression looks like this:(re-search-forward "\\w+\\W*")(Note that paired backslashes precede the ‘w’ and ‘W’. A single backslash has special
meaning to the Emacs Lisp interpreter. It indicates that the following character isinterpreted differently than usual. For example, the two characters, ‘\n’, stand for‘newline’, rather than for a backslash followed by ‘n’. Two backslashes in a row
stand for an ordinary, unspecial backslash, so Emacs Lisp interpreter ends of seeing
a single backslash followed by a letter. So it discovers the letter is special.)
We need a counter to count how many words there are; this variable must firstbe set to 0 and then incremented each time Emacs goes around the while loop.
The incrementing expression is simply:(setq count (1+ count))

Finally, we want to tell the user how many words there are in the region. The
message function is intended for presenting this kind of information to the user.
The message has to be phrased so that it reads properly regardless of how many
words there are in the region: we don’t want to say that “there are 1 words in theregion”. The conflict between singular and plural is ungrammatical. We can solve
this problem by using a conditional expression that evaluates different messages
depending on the number of words in the region. There are three possibilities: nowords in the region, one word in the region, and more than one word. This meansthat the cond special form is appropriate.

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All this leads to the following function definition:;;; First version; has bugs!
(defun count-words-example (beginning end)
"Print number of words in the region.
Words are defined as at least one word-constituent
character followed by at least one character thatis not a word-constituent. The buffer's syntax
table determines which characters these are."(interactive "r")(message "Counting words in region ... ");;; 1. Set up appropriate conditions.
(save-excursion(goto-char beginning)(let ((count 0));;; 2. Run the while loop.
(while (< (point) end)
(re-search-forward "\\w+\\W*")(setq count (1+ count)));;; 3. Send a message to the user.
(cond ((zerop count)(message"The region does NOT have any words."))((= 1 count)(message"The region has 1 word."))(t(message"The region has %d words." count))))))

As written, the function works, but not in all circumstances.
13.1.1 The Whitespace Bug in count-words-example
The count-words-example command described in the preceding section has two
bugs, or rather, one bug with two manifestations. First, if you mark a region
containing only whitespace in the middle of some text, the count-words-example
command tells you that the region contains one word! Second, if you mark a region
containing only whitespace at the end of the buffer or the accessible portion of anarrowed buffer, the command displays an error message that looks like this:Search failed: "\\w+\\W*"

If you are reading this in Info in GNU Emacs, you can test for these bugs
yourself.
First, evaluate the function in the usual manner to install it.

Section 13.1: The count-words-example Function

159

If you wish, you can also install this key binding by evaluating it:(global-set-key "\C-c=" 'count-words-example)

To conduct the first test, set mark and point to the beginning and end of thefollowing line and then type C-c = (or M-x count-words-example if you have notbound C-c =):
one

two

three

Emacs will tell you, correctly, that the region has three words.
Repeat the test, but place mark at the beginning of the line and place point
just before the word ‘one’. Again type the command C-c = (or M-x count-wordsexample). Emacs should tell you that the region has no words, since it is composed
only of the whitespace at the beginning of the line. But instead Emacs tells youthat the region has one word!
For the third test, copy the sample line to the end of the *scratch* buffer
and then type several spaces at the end of the line. Place mark right after theword ‘three’ and point at the end of line. (The end of the line will be the end of
the buffer.) Type C-c = (or M-x count-words-example) as you did before. Again,
Emacs should tell you that the region has no words, since it is composed only of thewhitespace at the end of the line. Instead, Emacs displays an error message saying‘Search failed’.
The two bugs stem from the same problem.
Consider the first manifestation of the bug, in which the command tells youthat the whitespace at the beginning of the line contains one word. What happens
is this: The M-x count-words-example command moves point to the beginning of
the region. The while tests whether the value of point is smaller than the valueof end, which it is. Consequently, the regular expression search looks for and finds
the first word. It leaves point after the word. count is set to one. The while loop
repeats; but this time the value of point is larger than the value of end, the loop
is exited; and the function displays a message saying the number of words in theregion is one. In brief, the regular expression search looks for and finds the word
even though it is outside the marked region.
In the second manifestation of the bug, the region is whitespace at the end of thebuffer. Emacs says ‘Search failed’. What happens is that the true-or-false-test in
the while loop tests true, so the search expression is executed. But since there are
no more words in the buffer, the search fails.
In both manifestations of the bug, the search extends or attempts to extend
outside of the region.
The solution is to limit the search to the region—this is a fairly simple action,
but as you may have come to expect, it is not quite as simple as you might think.
As we have seen, the re-search-forward function takes a search pattern as
its first argument. But in addition to this first, mandatory argument, it accepts
three optional arguments. The optional second argument bounds the search. The
optional third argument, if t, causes the function to return nil rather than signal
an error if the search fails. The optional fourth argument is a repeat count. (InEmacs, you can see a function’s documentation by typing C-h f, the name of thefunction, and then RET.)

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In the count-words-example definition, the value of the end of the region isheld by the variable end which is passed as an argument to the function. Thus, we
can add end as an argument to the regular expression search expression:(re-search-forward "\\w+\\W*" end)

However, if you make only this change to the count-words-example definition
and then test the new version of the definition on a stretch of whitespace, you will
receive an error message saying ‘Search failed’.
What happens is this: the search is limited to the region, and fails as you expect
because there are no word-constituent characters in the region. Since it fails, we
receive an error message. But we do not want to receive an error message in thiscase; we want to receive the message “The region does NOT have any words.”
The solution to this problem is to provide re-search-forward with a thirdargument of t, which causes the function to return nil rather than signal an error
if the search fails.
However, if you make this change and try it, you will see the message “Counting
words in region ... ” and . . . you will keep on seeing that message . . . , until you
type C-g (keyboard-quit).
Here is what happens: the search is limited to the region, as before, and it
fails because there are no word-constituent characters in the region, as expected.
Consequently, the re-search-forward expression returns nil. It does nothingelse. In particular, it does not move point, which it does as a side effect if it finds
the search target. After the re-search-forward expression returns nil, the next
expression in the while loop is evaluated. This expression increments the count.
Then the loop repeats. The true-or-false-test tests true because the value of pointis still less than the value of end, since the re-search-forward expression did not
move point. . . . and the cycle repeats . . .
The count-words-example definition requires yet another modification, to cause
the true-or-false-test of the while loop to test false if the search fails. Put anotherway, there are two conditions that must be satisfied in the true-or-false-test before
the word count variable is incremented: point must still be within the region andthe search expression must have found a word to count.
Since both the first condition and the second condition must be true together,
the two expressions, the region test and the search expression, can be joined withan and special form and embedded in the while loop as the true-or-false-test, likethis:(and (< (point) end) (re-search-forward "\\w+\\W*" end t))(For information about and, see “The kill-new function”, page 97.)
The re-search-forward expression returns t if the search succeeds and as aside effect moves point. Consequently, as words are found, point is moved throughthe region. When the search expression fails to find another word, or when point
reaches the end of the region, the true-or-false-test tests false, the while loop exits,
and the count-words-example function displays one or other of its messages.
After incorporating these final changes, the count-words-example works without bugs (or at least, without bugs that I have found!). Here is what it looks
like:Section 13.2: Count Words Recursively

161;;; Final version: while(defun count-words-example (beginning end)
"Print number of words in the region."(interactive "r")(message "Counting words in region ... ");;; 1. Set up appropriate conditions.
(save-excursion(let ((count 0))(goto-char beginning);;; 2. Run the while loop.
(while (and (< (point) end)
(re-search-forward "\\w+\\W*" end t))(setq count (1+ count)));;; 3. Send a message to the user.
(cond ((zerop count)(message"The region does NOT have any words."))((= 1 count)(message"The region has 1 word."))(t(message"The region has %d words." count))))))

13.2 Count Words Recursively
You can write the function for counting words recursively as well as with a while
loop. Let’s see how this is done.
First, we need to recognize that the count-words-example function has three
jobs: it sets up the appropriate conditions for counting to occur; it counts the words
in the region; and it sends a message to the user telling how many words there are.
If we write a single recursive function to do everything, we will receive a message
for every recursive call. If the region contains 13 words, we will receive thirteen
messages, one right after the other. We don’t want this! Instead, we must write
two functions to do the job, one of which (the recursive function) will be used insideof the other. One function will set up the conditions and display the message; theother will return the word count.
Let us start with the function that causes the message to be displayed. We cancontinue to call this count-words-example.
This is the function that the user will call. It will be interactive. Indeed, it
will be similar to our previous versions of this function, except that it will callrecursive-count-words to determine how many words are in the region.

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We can readily construct a template for this function, based on our previous
versions:;; Recursive version; uses regular expression search(defun count-words-example (beginning end)
"documentation..."(interactive-expression...);;; 1. Set up appropriate conditions.
(explanatory message)(set-up functions...
;;; 2. Count the words.
recursive call;;; 3. Send a message to the user.
message providing word count))

The definition looks straightforward, except that somehow the count returnedby the recursive call must be passed to the message displaying the word count. A
little thought suggests that this can be done by making use of a let expression:
we can bind a variable in the varlist of a let expression to the number of words inthe region, as returned by the recursive call; and then the cond expression, usingbinding, can display the value to the user.
Often, one thinks of the binding within a let expression as somehow secondary
to the primary work of a function. But in this case, what you might consider the
primary job of the function, counting words, is done within the let expression.

Section 13.2: Count Words Recursively

163

Using let, the function definition looks like this:(defun count-words-example (beginning end)
"Print number of words in the region."(interactive "r");;; 1. Set up appropriate conditions.
(message "Counting words in region ... ")(save-excursion(goto-char beginning);;; 2. Count the words.
(let ((count (recursive-count-words end)));;; 3. Send a message to the user.
(cond ((zerop count)(message"The region does NOT have any words."))((= 1 count)(message"The region has 1 word."))(t(message"The region has %d words." count))))))

Next, we need to write the recursive counting function.
A recursive function has at least three parts: the do-again-test, the next-stepexpression, and the recursive call.
The do-again-test determines whether the function will or will not be calledagain. Since we are counting words in a region and can use a function that moves
point forward for every word, the do-again-test can check whether point is still
within the region. The do-again-test should find the value of point and determine
whether point is before, at, or after the value of the end of the region. We can use
the point function to locate point. Clearly, we must pass the value of the end of
the region to the recursive counting function as an argument.
In addition, the do-again-test should also test whether the search finds a word.
If it does not, the function should not call itself again.
The next-step-expression changes a value so that when the recursive function
is supposed to stop calling itself, it stops. More precisely, the next-step-expression
changes a value so that at the right time, the do-again-test stops the recursivefunction from calling itself again. In this case, the next-step-expression can be theexpression that moves point forward, word by word.
The third part of a recursive function is the recursive call.
Somewhere, we also need a part that does the work of the function, a part that
does the counting. A vital part!

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But already, we have an outline of the recursive counting function:(defun recursive-count-words (region-end)"documentation..."do-again-test
next-step-expression
recursive call)

Now we need to fill in the slots. Let’s start with the simplest cases first: if point
is at or beyond the end of the region, there cannot be any words in the region, so
the function should return zero. Likewise, if the search fails, there are no words tocount, so the function should return zero.
On the other hand, if point is within the region and the search succeeds, thefunction should call itself again.
Thus, the do-again-test should look like this:(and (< (point) region-end)(re-search-forward "\\w+\\W*" region-end t))

Note that the search expression is part of the do-again-test—the function returns
t if its search succeeds and nil if it fails. (See Section 13.1.1 “The Whitespace
Bug in count-words-example”, page 158, for an explanation of how re-searchforward works.)
The do-again-test is the true-or-false test of an if clause. Clearly, if the doagain-test succeeds, the then-part of the if clause should call the function again;
but if it fails, the else-part should return zero since either point is outside the region
or the search failed because there were no words to find.
But before considering the recursive call, we need to consider the next-stepexpression. What is it? Interestingly, it is the search part of the do-again-test.
In addition to returning t or nil for the do-again-test, re-search-forwardmoves point forward as a side effect of a successful search. This is the action thatchanges the value of point so that the recursive function stops calling itself whenpoint completes its movement through the region. Consequently, the re-searchforward expression is the next-step-expression.
In outline, then, the body of the recursive-count-words function looks likethis:
(if do-again-test-and-next-step-combined;; thenrecursive-call-returning-count;; elsereturn-zero)

How to incorporate the mechanism that counts?
If you are not used to writing recursive functions, a question like this can betroublesome. But it can and should be approached systematically.
We know that the counting mechanism should be associated in some way withthe recursive call. Indeed, since the next-step-expression moves point forward by one
word, and since a recursive call is made for each word, the counting mechanism must
be an expression that adds one to the value returned by a call to recursive-countwords.

Section 13.2: Count Words Recursively

165

Consider several cases:• If there are two words in the region, the function should return a value resulting
from adding one to the value returned when it counts the first word, plus thenumber returned when it counts the remaining words in the region, which inthis case is one.
• If there is one word in the region, the function should return a value resulting
from adding one to the value returned when it counts that word, plus thenumber returned when it counts the remaining words in the region, which inthis case is zero.
• If there are no words in the region, the function should return zero.
From the sketch we can see that the else-part of the if returns zero for the caseof no words. This means that the then-part of the if must return a value resulting
from adding one to the value returned from a count of the remaining words.
The expression will look like this, where 1+ is a function that adds one to its
argument.
(1+ (recursive-count-words region-end))

The whole recursive-count-words function will then look like this:(defun recursive-count-words (region-end)"documentation...";;; 1. do-again-test(if (and (< (point) region-end)(re-search-forward "\\w+\\W*" region-end t));;; 2. then-part: the recursive call
(1+ (recursive-count-words region-end));;; 3. else-part0))

Let’s examine how this works:
If there are no words in the region, the else part of the if expression is evaluatedand consequently the function returns zero.
If there is one word in the region, the value of point is less than the value ofregion-end and the search succeeds. In this case, the true-or-false-test of the if
expression tests true, and the then-part of the if expression is evaluated. The
counting expression is evaluated. This expression returns a value (which will be thevalue returned by the whole function) that is the sum of one added to the value
returned by a recursive call.
Meanwhile, the next-step-expression has caused point to jump over the first (andin this case only) word in the region. This means that when (recursive-countwords region-end) is evaluated a second time, as a result of the recursive call, thevalue of point will be equal to or greater than the value of region end. So this time,recursive-count-words will return zero. The zero will be added to one, and theoriginal evaluation of recursive-count-words will return one plus zero, which isone, which is the correct amount.

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Clearly, if there are two words in the region, the first call to recursive-countwords returns one added to the value returned by calling recursive-count-words
on a region containing the remaining word—that is, it adds one to one, producing
two, which is the correct amount.
Similarly, if there are three words in the region, the first call torecursive-count-words returns one added to the value returned by callingrecursive-count-words on a region containing the remaining two words—and soon and so on.
With full documentation the two functions look like this:The recursive function:(defun recursive-count-words (region-end)"Number of words between point and REGION-END.";;; 1. do-again-test(if (and (< (point) region-end)(re-search-forward "\\w+\\W*" region-end t));;; 2. then-part: the recursive call
(1+ (recursive-count-words region-end));;; 3. else-part0))

The wrapper:;;; Recursive version(defun count-words-example (beginning end)
"Print number of words in the region.
Words are defined as at least one word-constituent
character followed by at least one character that is
not a word-constituent. The buffer's syntax table
determines which characters these are."(interactive "r")(message "Counting words in region ... ")(save-excursion(goto-char beginning)(let ((count (recursive-count-words end)))(cond ((zerop count)(message"The region does NOT have any words."))((= 1 count)(message "The region has 1 word."))(t(message"The region has %d words." count))))))

13.3 Exercise: Counting Punctuation
Using a while loop, write a function to count the number of punctuation marks ina region—period, comma, semicolon, colon, exclamation mark, and question mark.
Do the same using recursion.

Section 14.1: What to Count?

167

14 Counting Words in a defun
Our next project is to count the number of words in a function definition. Clearly,
this can be done using some variant of count-words-example. See Chapter 13“Counting via Repetition and Regexps”, page 155. If we are just going to countthe words in one definition, it is easy enough to mark the definition with the C-M-h(mark-defun) command, and then call count-words-example.
However, I am more ambitious: I want to count the words and symbols inevery definition in the Emacs sources and then print a graph that shows how many
functions there are of each length: how many contain 40 to 49 words or symbols,
how many contain 50 to 59 words or symbols, and so on. I have often been curious
how long a typical function is, and this will tell.
Described in one phrase, the histogram project is daunting; but divided into
numerous small steps, each of which we can take one at a time, the project becomesless fearsome. Let us consider what the steps must be:• First, write a function to count the words in one definition. This includes the
problem of handling symbols as well as words.
• Second, write a function to list the number of words in each function in a file.
This function can use the count-words-in-defun function.
• Third, write a function to list the number of words in each function in each of
several files. This entails automatically finding the various files, switching tothem, and counting the words in the definitions within them.
• Fourth, write a function to convert the list of numbers that we created in step
three to a form that will be suitable for printing as a graph.
• Fifth, write a function to print the results as a graph.
This is quite a project! But if we take each step slowly, it will not be difficult.

14.1 What to Count?
When we first start thinking about how to count the words in a function definition,
the first question is (or ought to be) what are we going to count? When we speak of“words” with respect to a Lisp function definition, we are actually speaking, in largepart, of symbols. For example, the following multiply-by-seven function contains
the five symbols defun, multiply-by-seven, number, *, and 7. In addition, in the
documentation string, it contains the four words ‘Multiply’, ‘NUMBER’, ‘by’, and‘seven’. The symbol ‘number’ is repeated, so the definition contains a total of ten
words and symbols.
(defun multiply-by-seven (number)"Multiply NUMBER by seven."(* 7 number))

However, if we mark the multiply-by-seven definition with C-M-h (mark-defun),and then call count-words-example on it, we will find that count-words-example
claims the definition has eleven words, not ten! Something is wrong!
The problem is twofold: count-words-example does not count the ‘*’ as a word,
and it counts the single symbol, multiply-by-seven, as containing three words.

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The hyphens are treated as if they were interword spaces rather than intraword
connectors: ‘multiply-by-seven’ is counted as if it were written ‘multiply by
seven’.
The cause of this confusion is the regular expression search within the
count-words-example definition that moves point forward word by word. In the
canonical version of count-words-example, the regexp is:"\\w+\\W*"

This regular expression is a pattern defining one or more word constituent characterspossibly followed by one or more characters that are not word constituents. Whatis meant by “word constituent characters” brings us to the issue of syntax, which
is worth a section of its own.

14.2 What Constitutes a Word or Symbol?
Emacs treats different characters as belonging to different syntax categories. For
example, the regular expression, ‘\\w+’, is a pattern specifying one or more word
constituent characters. Word constituent characters are members of one syntax
category. Other syntax categories include the class of punctuation characters, suchas the period and the comma, and the class of whitespace characters, such as theblank space and the tab character. (For more information, see Section “Syntax
Tables” in The GNU Emacs Lisp Reference Manual.)
Syntax tables specify which characters belong to which categories. Usually, ahyphen is not specified as a word constituent character. Instead, it is specified
as being in the class of characters that are part of symbol names but not words.
This means that the count-words-example function treats it in the same way
it treats an interword white space, which is why count-words-example counts‘multiply-by-seven’ as three words.
There are two ways to cause Emacs to count ‘multiply-by-seven’ as one symbol: modify the syntax table or modify the regular expression.
We could redefine a hyphen as a word constituent character by modifying thesyntax table that Emacs keeps for each mode. This action would serve our purpose,
except that a hyphen is merely the most common character within symbols that is
not typically a word constituent character; there are others, too.
Alternatively, we can redefine the regexp used in the count-words-example
definition so as to include symbols. This procedure has the merit of clarity, but thetask is a little tricky.

Section 14.3: The count-words-in-defun Function

169

The first part is simple enough: the pattern must match at least one character
that is a word or symbol constituent. Thus:"\\(\\w\\|\\s_\\)+"The ‘\\(’ is the first part of the grouping construct that includes the ‘\\w’ andthe ‘\\s_’ as alternatives, separated by the ‘\\|’. The ‘\\w’ matches any wordconstituent character and the ‘\\s_’ matches any character that is part of a symbolname but not a word-constituent character. The ‘+’ following the group indicatesthat the word or symbol constituent characters must be matched at least once.
However, the second part of the regexp is more difficult to design. What wewant is to follow the first part with optionally one or more characters that are notconstituents of a word or symbol. At first, I thought I could define this with thefollowing:"\\(\\W\\|\\S_\\)*"The upper case ‘W’ and ‘S’ match characters that are not word or symbol constituents. Unfortunately, this expression matches any character that is either not aword constituent or not a symbol constituent. This matches any character!
I then noticed that every word or symbol in my test region was followed by whitespace (blank space, tab, or newline). So I tried placing a pattern to match one ormore blank spaces after the pattern for one or more word or symbol constituents.
This failed, too. Words and symbols are often separated by whitespace, but in
actual code parentheses may follow symbols and punctuation may follow words. So
finally, I designed a pattern in which the word or symbol constituents are followed
optionally by characters that are not white space and then followed optionally bywhite space.
Here is the full regular expression:"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"14.3 The count-words-in-defun Function
We have seen that there are several ways to write a count-words-region function.
To write a count-words-in-defun, we need merely adapt one of these versions.
The version that uses a while loop is easy to understand, so I am going to adapt
that. Because count-words-in-defun will be part of a more complex program, it
need not be interactive and it need not display a message but just return the count.
These considerations simplify the definition a little.
On the other hand, count-words-in-defun will be used within a buffer that
contains function definitions. Consequently, it is reasonable to ask that the function
determine whether it is called when point is within a function definition, and if it
is, to return the count for that definition. This adds complexity to the definition,
but saves us from needing to pass arguments to the function.

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These considerations lead us to prepare the following template:(defun count-words-in-defun ()"documentation..."(set up...
(while loop...)return count)

As usual, our job is to fill in the slots.
First, the set up.
We are presuming that this function will be called within a buffer containing
function definitions. Point will either be within a function definition or not. For
count-words-in-defun to work, point must move to the beginning of the definition,a counter must start at zero, and the counting loop must stop when point reaches
the end of the definition.
The beginning-of-defun function searches backwards for an opening delimitersuch as a ‘(’ at the beginning of a line, and moves point to that position, or else to
the limit of the search. In practice, this means that beginning-of-defun moves
point to the beginning of an enclosing or preceding function definition, or else to
the beginning of the buffer. We can use beginning-of-defun to place point where
we wish to start.
The while loop requires a counter to keep track of the words or symbols being
counted. A let expression can be used to create a local variable for this purpose,and bind it to an initial value of zero.
The end-of-defun function works like beginning-of-defun except that it
moves point to the end of the definition. end-of-defun can be used as part ofan expression that determines the position of the end of the definition.
The set up for count-words-in-defun takes shape rapidly: first we move point
to the beginning of the definition, then we create a local variable to hold the count,and finally, we record the position of the end of the definition so the while loop
will know when to stop looping.

Section 14.3: The count-words-in-defun Function

171

The code looks like this:(beginning-of-defun)(let ((count 0)(end (save-excursion (end-of-defun) (point))))

The code is simple. The only slight complication is likely to concern end: it isbound to the position of the end of the definition by a save-excursion expressionthat returns the value of point after end-of-defun temporarily moves it to the endof the definition.
The second part of the count-words-in-defun, after the set up, is the while
loop.
The loop must contain an expression that jumps point forward word by wordand symbol by symbol, and another expression that counts the jumps. The true-orfalse-test for the while loop should test true so long as point should jump forward,
and false when point is at the end of the definition. We have already redefined theregular expression for this, so the loop is straightforward:(while (and (< (point) end)
(re-search-forward"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*" end t))(setq count (1+ count)))

The third part of the function definition returns the count of words and symbols.
This part is the last expression within the body of the let expression, and can be,very simply, the local variable count, which when evaluated returns the count.
Put together, the count-words-in-defun definition looks like this:(defun count-words-in-defun ()"Return the number of words and symbols in a defun."(beginning-of-defun)(let ((count 0)(end (save-excursion (end-of-defun) (point))))(while(and (< (point) end)
(re-search-forward"\\(\\w\\|\\s_\\)+[^ \t\n]*[ \t\n]*"end t))(setq count (1+ count)))
count))How to test this? The function is not interactive, but it is easy to put a wrapper
around the function to make it interactive; we can use almost the same code as forthe recursive version of count-words-example:;;; Interactive version.
(defun count-words-defun ()"Number of words and symbols in a function definition."(interactive)(message"Counting words and symbols in function definition ... ")(let ((count (count-words-in-defun)))(cond((zerop count)(message"The definition does NOT have any words or symbols."))

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Chapter 14: Counting Words in a defun((= 1 count)(message"The definition has 1 word or symbol."))(t(message"The definition has %d words or symbols." count)))))

Let’s reuse C-c = as a convenient key binding:(global-set-key "\C-c=" 'count-words-defun)

Now we can try out count-words-defun: install both count-words-in-defunand count-words-defun, and set the key binding. Then copy the following to
an Emacs Lisp buffer (like, for instance, *scratch*), place the cursor within thedefinition, and use the C-c = command.
(defun multiply-by-seven (number)"Multiply NUMBER by seven."(* 7 number))⇒ 10

Success! The definition has 10 words and symbols.
The next problem is to count the numbers of words and symbols in several
definitions within a single file.

14.4 Count Several defuns Within a FileA file such as simple.el may have a hundred or more function definitions within
it. Our long term goal is to collect statistics on many files, but as a first step, ourimmediate goal is to collect statistics on one file.
The information will be a series of numbers, each number being the length of afunction definition. We can store the numbers in a list.
We know that we will want to incorporate the information regarding one filewith information about many other files; this means that the function for counting
definition lengths within one file need only return the list of lengths. It need not
and should not display any messages.
The word count commands contain one expression to jump point forward wordby word and another expression to count the jumps. The function to return thelengths of definitions can be designed to work the same way, with one expression
to jump point forward definition by definition and another expression to construct
the lengths’ list.
This statement of the problem makes it elementary to write the function definition. Clearly, we will start the count at the beginning of the file, so the firstcommand will be (goto-char (point-min)). Next, we start the while loop; andthe true-or-false test of the loop can be a regular expression search for the next function definition—so long as the search succeeds, point is moved forward and thenthe body of the loop is evaluated. The body needs an expression that constructs
the lengths’ list. cons, the list construction command, can be used to create thelist. That is almost all there is to it.

Section 14.5: Find a File

173

Here is what this fragment of code looks like:(goto-char (point-min))(while (re-search-forward "^(defun" nil t)(setq lengths-list(cons (count-words-in-defun) lengths-list)))What we have left out is the mechanism for finding the file that contains thefunction definitions.
In previous examples, we either used this, the Info file, or we switched back andforth to some other buffer, such as the *scratch* buffer.
Finding a file is a new process that we have not yet discussed.

14.5 Find a FileTo find a file in Emacs, you use the C-x C-f (find-file) command. This commandis almost, but not quite right for the lengths problem.
Let’s look at the source for find-file:(defun find-file (filename)"Edit file FILENAME.
Switch to a buffer visiting file FILENAME,
creating one if none already exists."(interactive "FFind file: ")(switch-to-buffer (find-file-noselect filename)))(The most recent version of the find-file function definition permits you to specify
optional wildcards to visit multiple files; that makes the definition more complex
and we will not discuss it here, since it is not relevant. You can see its source using
either M-. (xref-find-definitions) or C-h f (describe-function).)The definition I am showing possesses short but complete documentation andan interactive specification that prompts you for a file name when you use the command interactively. The body of the definition contains two functions, find-filenoselect and switch-to-buffer.
According to its documentation as shown by C-h f (the describe-functioncommand), the find-file-noselect function reads the named file into a buffer
and returns the buffer. (Its most recent version includes an optional wildcards
argument, too, as well as another to read a file literally and another to suppress
warning messages. These optional arguments are irrelevant.)
However, the find-file-noselect function does not select the buffer in which
it puts the file. Emacs does not switch its attention (or yours if you are using
find-file-noselect) to the selected buffer. That is what switch-to-buffer
does: it switches the buffer to which Emacs attention is directed; and it switches
the buffer displayed in the window to the new buffer. We have discussed buffer
switching elsewhere. (See Section 2.3 “Switching Buffers”, page 23.)
In this histogram project, we do not need to display each file on the screen as the
program determines the length of each definition within it. Instead of employing
switch-to-buffer, we can work with set-buffer, which redirects the attention of
the computer program to a different buffer but does not redisplay it on the screen.
So instead of calling on find-file to do the job, we must write our own expression.

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The task is easy: use find-file-noselect and set-buffer.

14.6 lengths-list-file in Detail
The core of the lengths-list-file function is a while loop containing a function
to move point forward defun by defun, and a function to count the number ofwords and symbols in each defun. This core must be surrounded by functions that
do various other tasks, including finding the file, and ensuring that point starts out
at the beginning of the file. The function definition looks like this:(defun lengths-list-file (filename)"Return list of definitions' lengths within FILE.
The returned list is a list of numbers.
Each number is the number of words orsymbols in one function definition."(message "Working on `%s' ... " filename)(save-excursion(let ((buffer (find-file-noselect filename))(lengths-list))(set-buffer buffer)(setq buffer-read-only t)(widen)(goto-char (point-min))(while (re-search-forward "^(defun" nil t)(setq lengths-list(cons (count-words-in-defun) lengths-list)))(kill-buffer buffer)
lengths-list)))The function is passed one argument, the name of the file on which it will work.
It has four lines of documentation, but no interactive specification. Since people
worry that a computer is broken if they don’t see anything going on, the first lineof the body is a message.
The next line contains a save-excursion that returns Emacs’s attention to thecurrent buffer when the function completes. This is useful in case you embed thisfunction in another function that presumes point is restored to the original buffer.
In the varlist of the let expression, Emacs finds the file and binds the local
variable buffer to the buffer containing the file. At the same time, Emacs creates
lengths-list as a local variable.
Next, Emacs switches its attention to the buffer.
In the following line, Emacs makes the buffer read-only. Ideally, this line is
not necessary. None of the functions for counting words and symbols in a function
definition should change the buffer. Besides, the buffer is not going to be saved,
even if it were changed. This line is entirely the consequence of great, perhaps
excessive, caution. The reason for the caution is that this function and those it
calls work on the sources for Emacs and it is inconvenient if they are inadvertently
modified. It goes without saying that I did not realize a need for this line until anexperiment went awry and started to modify my Emacs source files . . .
Next comes a call to widen the buffer if it is narrowed. This function is usuallynot needed—Emacs creates a fresh buffer if none already exists; but if a bufferSection 14.7: Count Words in defuns in Different Files

175

visiting the file already exists Emacs returns that one. In this case, the buffer may
be narrowed and must be widened. If we wanted to be fully user-friendly, we would
arrange to save the restriction and the location of point, but we won’t.
The (goto-char (point-min)) expression moves point to the beginning of thebuffer.
Then comes a while loop in which the work of the function is carried out. In
the loop, Emacs determines the length of each definition and constructs a lengths’
list containing the information.
Emacs kills the buffer after working through it. This is to save space inside ofEmacs. My version of GNU Emacs 19 contained over 300 source files of interest;
GNU Emacs 22 contains over a thousand source files. Another function will apply
lengths-list-file to each of the files.
Finally, the last expression within the let expression is the lengths-list variable; its value is returned as the value of the whole function.
You can try this function by installing it in the usual fashion. Then place yourcursor after the following expression and type C-x C-e (eval-last-sexp).
(lengths-list-file"/usr/local/share/emacs/22.1/lisp/emacs-lisp/debug.el")

You may need to change the pathname of the file; the one here is for GNU Emacs
version 22.1. To change the expression, copy it to the *scratch* buffer and edit it.
Also, to see the full length of the list, rather than a truncated version, you may
have to evaluate the following:(custom-set-variables '(eval-expression-print-length nil))(See Section 16.2 “Specifying Variables using defcustom”, page 197. Then evaluate
the lengths-list-file expression.)
The lengths’ list for debug.el takes less than a second to produce and looks likethis in GNU Emacs 22:(83 113 105 144 289 22 30 97 48 89 25 52 52 88 28 29 77 49 43 290 232 587)

(Using my old machine, the version 19 lengths’ list for debug.el took sevenseconds to produce and looked like this:(75 41 80 62 20 45 44 68 45 12 34 235)

The newer version of debug.el contains more defuns than the earlier one; and my
new machine is much faster than the old one.)
Note that the length of the last definition in the file is first in the list.

14.7 Count Words in defuns in Different Files
In the previous section, we created a function that returns a list of the lengths ofeach definition in a file. Now, we want to define a function to return a master listof the lengths of the definitions in a list of files.
Working on each of a list of files is a repetitious act, so we can use either a while
loop or recursion.
The design using a while loop is routine. The argument passed to the function
is a list of files. As we saw earlier (see Section 11.1.1 “Loop Example”, page 116),
you can write a while loop so that the body of the loop is evaluated if such a list

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contains elements, but to exit the loop if the list is empty. For this design to work,
the body of the loop must contain an expression that shortens the list each time
the body is evaluated, so that eventually the list is empty. The usual technique isto set the value of the list to the value of the cdr of the list each time the body isevaluated.
The template looks like this:(while test-whether-list-is-empty
body...
set-list-to-cdr-of-list)

Also, we remember that a while loop returns nil (the result of evaluating thetrue-or-false-test), not the result of any evaluation within its body. (The evaluations
within the body of the loop are done for their side effects.) However, the expressionthat sets the lengths’ list is part of the body—and that is the value that we want
returned by the function as a whole. To do this, we enclose the while loop within
a let expression, and arrange that the last element of the let expression contains
the value of the lengths’ list. (See “Loop Example with an Incrementing Counter”,
page 118.)
These considerations lead us directly to the function itself:;;; Use while loop.
(defun lengths-list-many-files (list-of-files)"Return list of lengths of defuns in LIST-OF-FILES."(let (lengths-list);;; true-or-false-test(while list-of-files(setq lengths-list(appendlengths-list;;; Generate a lengths’ list.
(lengths-list-file(expand-file-name (car list-of-files)))));;; Make files’ list shorter.
(setq list-of-files (cdr list-of-files)));;; Return final value of lengths’ list.
lengths-list))

expand-file-name is a built-in function that converts a file name to the absolute, long, path name form. The function employs the name of the directory inwhich the function is called.

Section 14.8: Recursively Count Words in Different Files

177

Thus, if expand-file-name is called on debug.el when Emacs is visiting the/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/ directory,
debug.elbecomes/usr/local/share/emacs/22.1.1/lisp/emacs-lisp/debug.el

The only other new element of this function definition is the as yet unstudied
function append, which merits a short section for itself.
14.7.1 The append Function
The append function attaches one list to another. Thus,(append '(1 2 3 4) '(5 6 7 8))

produces the list
(1 2 3 4 5 6 7 8)This is exactly how we want to attach two lengths’ lists produced bylengths-list-file to each other. The results contrast with cons,(cons '(1 2 3 4) '(5 6 7 8))

which constructs a new list in which the first argument to cons becomes the firstelement of the new list:((1 2 3 4) 5 6 7 8)

14.8 Recursively Count Words in Different Files
Besides a while loop, you can work on each of a list of files with recursion. A
recursive version of lengths-list-many-files is short and simple.
The recursive function has the usual parts: the do-again-test, the next-stepexpression, and the recursive call. The do-again-test determines whether the function should call itself again, which it will do if the list-of-files contains any
remaining elements; the next-step-expression resets the list-of-files to the cdr
of itself, so eventually the list will be empty; and the recursive call calls itself on
the shorter list. The complete function is shorter than this description!
(defun recursive-lengths-list-many-files (list-of-files)"Return list of lengths of each defun in LIST-OF-FILES."(if list-of-files; do-again-test(append(lengths-list-file(expand-file-name (car list-of-files)))(recursive-lengths-list-many-files(cdr list-of-files)))))In a sentence, the function returns the lengths’ list for the first of the list-of-files
appended to the result of calling itself on the rest of the list-of-files.
Here is a test of recursive-lengths-list-many-files, along with the resultsof running lengths-list-file on each of the files individually.
Install recursive-lengths-list-many-files and lengths-list-file, if necessary, and then evaluate the following expressions. You may need to change thefiles’ pathnames; those here work when this Info file and the Emacs sources are

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located in their customary places. To change the expressions, copy them to the*scratch* buffer, edit them, and then evaluate them.
The results are shown after the ‘⇒’. (These results are for files from Emacs
version 22.1.1; files from other versions of Emacs may produce different results.)(cd "/usr/local/share/emacs/22.1.1/")(lengths-list-file "./lisp/macros.el")⇒ (283 263 480 90)(lengths-list-file "./lisp/mail/mailalias.el")⇒ (38 32 29 95 178 180 321 218 324)
(lengths-list-file "./lisp/hex-util.el")⇒ (82 71)
(recursive-lengths-list-many-files'("./lisp/macros.el""./lisp/mail/mailalias.el""./lisp/hex-util.el"))⇒ (283 263 480 90 38 32 29 95 178 180 321 218 324 82 71)

The recursive-lengths-list-many-files function produces the output wewant.
The next step is to prepare the data in the list for display in a graph.

14.9 Prepare the Data for Display in a Graph
The recursive-lengths-list-many-files function returns a list of numbers.
Each number records the length of a function definition. What we need to do
now is transform this data into a list of numbers suitable for generating a graph.
The new list will tell how many functions definitions contain less than 10 words
and symbols, how many contain between 10 and 19 words and symbols, how many
contain between 20 and 29 words and symbols, and so on.
In brief, we need to go through the lengths’ list produced by therecursive-lengths-list-many-files function and count the number of defuns
within each range of lengths, and produce a list of those numbers.
Based on what we have done before, we can readily foresee that it should not be
too hard to write a function that cdrs down the lengths’ list, looks at each element,
determines which length range it is in, and increments a counter for that range.
However, before beginning to write such a function, we should consider theadvantages of sorting the lengths’ list first, so the numbers are ordered from smallest
to largest. First, sorting will make it easier to count the numbers in each range,
since two adjacent numbers will either be in the same length range or in adjacent
ranges. Second, by inspecting a sorted list, we can discover the highest and lowest
number, and thereby determine the largest and smallest length range that we willneed.

Section 14.9: Prepare the Data for Display in a Graph179

14.9.1 Sorting Lists
Emacs contains a function to sort lists, called (as you might guess) sort. The sort
function takes two arguments, the list to be sorted, and a predicate that determines
whether the first of two list elements is less than the second.
As we saw earlier (see Section 1.8.4 “Using the Wrong Type Object as an Argument”, page 13), a predicate is a function that determines whether some property istrue or false. The sort function will reorder a list according to whatever property
the predicate uses; this means that sort can be used to sort non-numeric lists by
non-numeric criteria—it can, for example, alphabetize a list.
The < function is used when sorting a numeric list. For example,(sort '(4 8 21 17 33 7 21 7) '<)produces this:(4 7 7 8 17 21 21 33)(Note that in this example, both the arguments are quoted so that the symbols are
not evaluated before being passed to sort as arguments.)
Sorting the list returned by the recursive-lengths-list-many-files function
is straightforward; it uses the < function:(sort(recursive-lengths-list-many-files'("./lisp/macros.el""./lisp/mailalias.el""./lisp/hex-util.el"))'<)which produces:(29 32 38 71 82 90 95 178 180 218 263 283 321 324 480)

(Note that in this example, the first argument to sort is not quoted, since theexpression must be evaluated so as to produce the list that is passed to sort.)
14.9.2 Making a List of Files
The recursive-lengths-list-many-files function requires a list of files as its
argument. For our test examples, we constructed such a list by hand; but theEmacs Lisp source directory is too large for us to do for that. Instead, we will write
a function to do the job for us. In this function, we will use both a while loop anda recursive call.
We did not have to write a function like this for older versions of GNU Emacs,
since they placed all the ‘.el’ files in one directory. Instead, we were able to use
the directory-files function, which lists the names of files that match a specified
pattern within a single directory.
However, recent versions of Emacs place Emacs Lisp files in sub-directories of
the top level lisp directory. This re-arrangement eases navigation. For example,
all the mail related files are in a lisp sub-directory called mail. But at the sametime, this arrangement forces us to create a file listing function that descends into
the sub-directories.
We can create this function, called files-in-below-directory, using familiar
functions such as car, nthcdr, and substring in conjunction with an existing

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function called directory-files-and-attributes. This latter function not only
lists all the filenames in a directory, including the names of sub-directories, but also
their attributes.
To restate our goal: to create a function that will enable us to feed filenames torecursive-lengths-list-many-files as a list that looks like this (but with moreelements):("./lisp/macros.el""./lisp/mail/rmail.el""./lisp/hex-util.el")

The directory-files-and-attributes function returns a list of lists. Each of
the lists within the main list consists of 13 elements. The first element is a stringthat contains the name of the file—which, in GNU/Linux, may be a directory file,that is to say, a file with the special attributes of a directory. The second element of
the list is t for a directory, a string for symbolic link (the string is the name linkedto), or nil.
For example, the first ‘.el’ file in the lisp/ directory is abbrev.el. Its nameis /usr/local/share/emacs/22.1.1/lisp/abbrev.el and it is not a directory ora symbolic link.
This is how directory-files-and-attributes lists that file and its attributes:("abbrev.el"
nil
1
1000
100
(20615 27034 579989 697000)(17905 55681 0 0)(20615 26327 734791 805000)1
13188"-rw-r--r--"t2971624773)

1

If current-time-list isnil the three timestamps are (1351051674579989697. 1000000000), (1173477761000000000 . 1000000000), and(1351050967734791805 . 1000000000), respectively.

Section 14.9: Prepare the Data for Display in a Graph

181

On the other hand, mail/ is a directory within the lisp/ directory. The beginning of its listing looks like this:("mail"t...
)(To learn about the different attributes, look at the documentation of
file-attributes. Bear in mind that the file-attributes function does not list
the filename, so its first element is directory-files-and-attributes’s second
element.)
We will want our new function, files-in-below-directory, to list the ‘.el’files in the directory it is told to check, and in any directories below that directory.
This gives us a hint on how to construct files-in-below-directory: within adirectory, the function should add ‘.el’ filenames to a list; and if, within a directory,
the function comes upon a sub-directory, it should go into that sub-directory andrepeat its actions.
However, we should note that every directory contains a name that refers toitself, called . (“dot”), and a name that refers to its parent directory, called ..
(“dot dot”). (In /, the root directory, .. refers to itself, since / has no parent.)
Clearly, we do not want our files-in-below-directory function to enter those
directories, since they always lead us, directly or indirectly, to the current directory.
Consequently, our files-in-below-directory function must do several tasks:• Check to see whether it is looking at a filename that ends in ‘.el’; and if so,
add its name to a list.
• Check to see whether it is looking at a filename that is the name of a directory;
and if so,− Check to see whether it is looking at . or ..; and if so skip it.
− Or else, go into that directory and repeat the process.
Let’s write a function definition to do these tasks. We will use a while loop to
move from one filename to another within a directory, checking what needs to bedone; and we will use a recursive call to repeat the actions on each sub-directory.
The recursive pattern is Accumulate (see “Accumulate”, page 135), using append
as the combiner.
Here is the function:(defun files-in-below-directory (directory)"List the .el files in DIRECTORY and in its sub-directories.";; Although the function will be used non-interactively,;; it will be easier to test if we make it interactive.
;; The directory will have a name such as;; "/usr/local/share/emacs/22.1.1/lisp/"(interactive "DDirectory name: ")(let (el-files-list(current-directory-list(directory-files-and-attributes directory t)));; while we are in the current directory(while current-directory-list

182

Chapter 14: Counting Words in a defun(cond;; check to see whether filename ends in '.el';; and if so, add its name to a list.
((equal ".el" (substring (car (car current-directory-list)) -3))
(setq el-files-list(cons (car (car current-directory-list)) el-files-list)));; check whether filename is that of a directory((eq t (car (cdr (car current-directory-list))));; decide whether to skip or recurse(if(equal "."(substring (car (car current-directory-list)) -1));; then do nothing since filename is that of;;current directory or parent, "." or ".."();; else descend into the directory and repeat the process(setq el-files-list(append(files-in-below-directory(car (car current-directory-list)))el-files-list)))));; move to the next filename in the list; this also;; shortens the list so the while loop eventually comes to an end(setq current-directory-list (cdr current-directory-list)));; return the filenames
el-files-list))

The files-in-below-directory directory-files function takes one argument, the name of a directory.
Thus, on my system,(length(files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/"))tells me that in and below my Lisp sources directory are 1031 ‘.el’ files.
files-in-below-directory returns a list in reverse alphabetical order. An
expression to sort the list in alphabetical order looks like this:(sort(files-in-below-directory "/usr/local/share/emacs/22.1.1/lisp/")'string-lessp)

14.9.3 Counting function definitions
Our immediate goal is to generate a list that tells us how many function definitions
contain fewer than 10 words and symbols, how many contain between 10 and 19
words and symbols, how many contain between 20 and 29 words and symbols, andso on.
With a sorted list of numbers, this is easy: count how many elements of the list
are smaller than 10, then, after moving past the numbers just counted, count how
many are smaller than 20, then, after moving past the numbers just counted, counthow many are smaller than 30, and so on. Each of the numbers, 10, 20, 30, 40,
and the like, is one larger than the top of that range. We can call the list of such
numbers the top-of-ranges list.

Section 14.9: Prepare the Data for Display in a Graph183

If we wished, we could generate this list automatically, but it is simpler to write
a list manually. Here it is:(defvar top-of-ranges'(10 20 30 40 50
60 70 80 90 100
110 120 130 140 150
160 170 180 190 200
210 220 230 240 250
260 270 280 290 300)
"List specifying ranges for `defuns-per-range'.")To change the ranges, we edit this list.
Next, we need to write the function that creates the list of the number of definitions within each range. Clearly, this function must take the sorted-lengths andthe top-of-ranges lists as arguments.
The defuns-per-range function must do two things again and again: it must
count the number of definitions within a range specified by the current top-of-range
value; and it must shift to the next higher value in the top-of-ranges list after
counting the number of definitions in the current range. Since each of these actions
is repetitive, we can use while loops for the job. One loop counts the number ofdefinitions in the range defined by the current top-of-range value, and the otherloop selects each of the top-of-range values in turn.
Several entries of the sorted-lengths list are counted for each range; this meansthat the loop for the sorted-lengths list will be inside the loop for the top-ofranges list, like a small gear inside a big gear.
The inner loop counts the number of definitions within the range. It is a simple
counting loop of the type we have seen before. (See Section 11.1.3 “A loop with anincrementing counter”, page 118.) The true-or-false test of the loop tests whether
the value from the sorted-lengths list is smaller than the current value of the topof the range. If it is, the function increments the counter and tests the next value
from the sorted-lengths list.
The inner loop looks like this:(while length-element-smaller-than-top-of-range(setq number-within-range (1+ number-within-range))(setq sorted-lengths (cdr sorted-lengths)))

The outer loop must start with the lowest value of the top-of-ranges list, andthen be set to each of the succeeding higher values in turn. This can be done witha loop like this:(while top-of-ranges
body-of-loop...
(setq top-of-ranges (cdr top-of-ranges)))

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Put together, the two loops look like this:(while top-of-ranges;; Count the number of elements within the current range.
(while length-element-smaller-than-top-of-range(setq number-within-range (1+ number-within-range))(setq sorted-lengths (cdr sorted-lengths)));; Move to next range.
(setq top-of-ranges (cdr top-of-ranges)))

In addition, in each circuit of the outer loop, Emacs should record the numberof definitions within that range (the value of number-within-range) in a list. We
can use cons for this purpose. (See Section 7.2 “cons”, page 78.)
The cons function works fine, except that the list it constructs will containthe number of definitions for the highest range at its beginning and the numberof definitions for the lowest range at its end. This is because cons attaches newelements of the list to the beginning of the list, and since the two loops are working
their way through the lengths’ list from the lower end first, the defuns-per-rangelist will end up largest number first. But we will want to print our graph withsmallest values first and the larger later. The solution is to reverse the order ofthe defuns-per-range-list. We can do this using the nreverse function, whichreverses the order of a list.
For example,(nreverse '(1 2 3 4))

produces:(4 3 2 1)

Note that the nreverse function is destructive—that is, it changes the list towhich it is applied; this contrasts with the car and cdr functions, which are nondestructive. In this case, we do not want the original defuns-per-range-list, so
it does not matter that it is destroyed. (The reverse function provides a reversed
copy of a list, leaving the original list as is.)Section 14.9: Prepare the Data for Display in a Graph185

Put all together, the defuns-per-range looks like this:(defun defuns-per-range (sorted-lengths top-of-ranges)"SORTED-LENGTHS defuns in each TOP-OF-RANGES range."(let ((top-of-range (car top-of-ranges))(number-within-range 0)
defuns-per-range-list);; Outer loop.
(while top-of-ranges;; Inner loop.
(while (and;; Need number for numeric test.
(car sorted-lengths)(< (car sorted-lengths) top-of-range));; Count number of definitions within current range.
(setq number-within-range (1+ number-within-range))(setq sorted-lengths (cdr sorted-lengths)));; Exit inner loop but remain within outer loop.
(setq defuns-per-range-list(cons number-within-range defuns-per-range-list))(setq number-within-range 0); Reset count to zero.
;; Move to next range.
(setq top-of-ranges (cdr top-of-ranges));; Specify next top of range value.
(setq top-of-range (car top-of-ranges)));; Exit outer loop and count the number of defuns larger than;; the largest top-of-range value.
(setq defuns-per-range-list(cons(length sorted-lengths)
defuns-per-range-list));; Return a list of the number of definitions within each range,;; smallest to largest.
(nreverse defuns-per-range-list)))The function is straightforward except for one subtle feature. The true-or-false testof the inner loop looks like this:(and (car sorted-lengths)(< (car sorted-lengths) top-of-range))

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instead of like this:(< (car sorted-lengths) top-of-range)

The purpose of the test is to determine whether the first item in thesorted-lengths list is less than the value of the top of the range.
The simple version of the test works fine unless the sorted-lengths list has
a nil value. In that case, the (car sorted-lengths) expression function returns
nil. The < function cannot compare a number to nil, which is an empty list,so Emacs signals an error and stops the function from attempting to continue to
execute.
The sorted-lengths list always becomes nil when the counter reaches the endof the list. This means that any attempt to use the defuns-per-range function
with the simple version of the test will fail.
We solve the problem by using the (car sorted-lengths) expression in conjunction with the and expression. The (car sorted-lengths) expression returns a
non-nil value so long as the list has at least one number within it, but returns nil
if the list is empty. The and expression first evaluates the (car sorted-lengths)
expression, and if it is nil, returns false without evaluating the < expression. But if
the (car sorted-lengths) expression returns a non-nil value, the and expression
evaluates the < expression, and returns that value as the value of the and expression.
This way, we avoid an error.
(For information about and, see “The kill-new function”, page 97.)
Here is a short test of the defuns-per-range function. First, evaluate theexpression that binds (a shortened) top-of-ranges list to the list of values, thenevaluate the expression for binding the sorted-lengths list, and then evaluate thedefuns-per-range function.
;; (Shorter list than we will use later.)(setq top-of-ranges'(110 120 130 140 150
160 170 180 190 200))(setq sorted-lengths'(85 86 110 116 122 129 154 176 179 200 265 300 300))(defuns-per-range sorted-lengths top-of-ranges)

The list returned looks like this:(2 2 2 0 0 1 0 2 0 0 4)

Indeed, there are two elements of the sorted-lengths list smaller than 110, twoelements between 110 and 119, two elements between 120 and 129, and so on. There
are four elements with a value of 200 or larger.

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15 Readying a Graph
Our goal is to construct a graph showing the numbers of function definitions ofvarious lengths in the Emacs lisp sources.
As a practical matter, if you were creating a graph, you would probably use a
program such as gnuplot to do the job. (gnuplot is nicely integrated into GNU
Emacs.) In this case, however, we create one from scratch, and in the process wewill re-acquaint ourselves with some of what we learned before and learn more.
In this chapter, we will first write a simple graph printing function. This first
definition will be a prototype, a rapidly written function that enables us to reconnoiter this unknown graph-making territory. We will discover dragons, or find thatthey are myth. After scouting the terrain, we will feel more confident and enhance
the function to label the axes automatically.
Since Emacs is designed to be flexible and work with all kinds of terminals,
including character-only terminals, the graph will need to be made from one of thetypewriter symbols. An asterisk will do; as we enhance the graph-printing function,
we can make the choice of symbol a user option.
We can call this function graph-body-print; it will take a numbers-list as its
only argument. At this stage, we will not label the graph, but only print its body.
The graph-body-print function inserts a vertical column of asterisks for eachelement in the numbers-list. The height of each line is determined by the valueof that element of the numbers-list.
Inserting columns is a repetitive act; that means that this function can be written
either with a while loop or recursively.
Our first challenge is to discover how to print a column of asterisks. Usually, in
Emacs, we print characters onto a screen horizontally, line by line, by typing. We
have two routes we can follow: write our own column-insertion function or discoverwhether one exists in Emacs.
To see whether there is one in Emacs, we can use the M-x apropos command.
This command is like the C-h a (command-apropos) command, except that thelatter finds only those functions that are commands. The M-x apropos commandlists all symbols that match a regular expression, including functions that are notinteractive.
What we want to look for is some command that prints or inserts columns. Verylikely, the name of the function will contain either the word “print” or the word
“insert” or the word “column”. Therefore, we can simply type M-x apropos RET
print\|insert\|column RET and look at the result. On my system, this command
once took quite some time, and then produced a list of 79 functions and variables.
Now it does not take much time at all and produces a list of 211 functions andvariables. Scanning down the list, the only function that looks as if it might do thejob is insert-rectangle.

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Chapter 15: Readying a Graph
Indeed, this is the function we want; its documentation says:
insert-rectangle:
Insert text of RECTANGLE with upper left corner at point.
RECTANGLE's first line is inserted at point,
its second line is inserted at a point vertically under point, etc.
RECTANGLE should be a list of strings.
After this command, the mark is at the upper left cornerand point is at the lower right corner.

We can run a quick test, to make sure it does what we expect of it.
Here is the result of placing the cursor after the insert-rectangle expression and typing C-u C-x C-e (eval-last-sexp). The function inserts the strings‘"first"’, ‘"second"’, and ‘"third"’ at and below point. Also the function returns
nil.
(insert-rectangle '("first" "second" "third"))firstsecondthirdnil

Of course, we won’t be inserting the text of the insert-rectangle expression itself
into the buffer in which we are making the graph, but will call the function fromour program. We shall, however, have to make sure that point is in the buffer at
the place where the insert-rectangle function will insert its column of strings.
If you are reading this in Info, you can see how this works by switching to anotherbuffer, such as the *scratch* buffer, placing point somewhere in the buffer, typing
M-:, typing the insert-rectangle expression into the minibuffer at the prompt,
and then typing RET. This causes Emacs to evaluate the expression in the minibuffer,
but to use as the value of point the position of point in the *scratch* buffer. (M-: isthe key binding for eval-expression. Also, nil does not appear in the *scratch*buffer since the expression is evaluated in the minibuffer.)
We find when we do this that point ends up at the end of the last inserted
line—that is to say, this function moves point as a side-effect. If we were to repeat
the command, with point at this position, the next insertion would be below andto the right of the previous insertion. We don’t want this! If we are going to makea bar graph, the columns need to be beside each other.
So we discover that each cycle of the column-inserting while loop must reposition point to the place we want it, and that place will be at the top, not thebottom, of the column. Moreover, we remember that when we print a graph, we
do not expect all the columns to be the same height. This means that the top ofeach column may be at a different height from the previous one. We cannot simply
reposition point to the same line each time, but moved over to the right—or perhaps
we can. . .
We are planning to make the columns of the bar graph out of asterisks. The
number of asterisks in the column is the number specified by the current element of
the numbers-list. We need to construct a list of asterisks of the right length foreach call to insert-rectangle. If this list consists solely of the requisite numberof asterisks, then we will have to position point the right number of lines above thebase for the graph to print correctly. This could be difficult.

189
Alternatively, if we can figure out some way to pass insert-rectangle a list of
the same length each time, then we can place point on the same line each time, but
move it over one column to the right for each new column. If we do this, however,some of the entries in the list passed to insert-rectangle must be blanks rather
than asterisks. For example, if the maximum height of the graph is 5, but the heightof the column is 3, then insert-rectangle requires an argument that looks likethis:(" " " " "*" "*" "*")

This last proposal is not so difficult, so long as we can determine the column
height. There are two ways for us to specify the column height: we can arbitrarily
state what it will be, which would work fine for graphs of that height; or we cansearch through the list of numbers and use the maximum height of the list as the
maximum height of the graph. If the latter operation were difficult, then the former
procedure would be easiest, but there is a function built into Emacs that determines
the maximum of its arguments. We can use that function. The function is called
max and it returns the largest of all its arguments, which must be numbers. Thus,
for example,(max

3 4 6 5 7 3)

returns 7. (A corresponding function called min returns the smallest of all its
arguments.)
However, we cannot simply call max on the numbers-list; the max function
expects numbers as its argument, not a list of numbers. Thus, the following expression,(max'(3 4 6 5 7 3))

produces the following error message;
Wrong type of argument:

number-or-marker-p, (3 4 6 5 7 3)

We need a function that passes a list of arguments to a function. This function
is apply. This function applies its first argument (a function) to its remainingarguments, the last of which may be a list.
For example,(apply 'max 3 4 7 3 '(4 8 5))

returns 8.
(Incidentally, I don’t know how you would learn of this function without a booksuch as this. It is possible to discover other functions, like search-forward orinsert-rectangle, by guessing at a part of their names and then using apropos.
Even though its base in metaphor is clear—apply its first argument to the rest—I
doubt a novice would come up with that particular word when using apropos orother aid. Of course, I could be wrong; after all, the function was first named bysomeone who had to invent it.)
The second and subsequent arguments to apply are optional, so we can use
apply to call a function and pass the elements of a list to it, like this, which also
returns 8:(apply 'max '(4 8 5))This latter way is how we will use apply. The recursive-lengths-list-manyfiles function returns a numbers’ list to which we can apply max (we could also

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apply max to the sorted numbers’ list; it does not matter whether the list is sorted
or not.)Hence, the operation for finding the maximum height of the graph is this:(setq max-graph-height (apply 'max numbers-list))

Now we can return to the question of how to create a list of strings for a columnof the graph. Told the maximum height of the graph and the number of asterisks
that should appear in the column, the function should return a list of strings forthe insert-rectangle command to insert.
Each column is made up of asterisks or blanks. Since the function is passed thevalue of the height of the column and the number of asterisks in the column, thenumber of blanks can be found by subtracting the number of asterisks from theheight of the column. Given the number of blanks and the number of asterisks, twowhile loops can be used to construct the list:;;; First version.
(defun column-of-graph (max-graph-height actual-height)"Return list of strings that is one column of a graph."(let ((insert-list nil)
(number-of-top-blanks(- max-graph-height actual-height)));; Fill in asterisks.
(while (> actual-height 0)(setq insert-list (cons "*" insert-list))(setq actual-height (1- actual-height)));; Fill in blanks.
(while (> number-of-top-blanks 0)(setq insert-list (cons " " insert-list))(setq number-of-top-blanks(1- number-of-top-blanks)));; Return whole list.
insert-list))

If you install this function and then evaluate the following expression you will
see that it returns the list as desired:(column-of-graph 5 3)

returns(" " " " "*" "*" "*")

As written, column-of-graph contains a major flaw: the symbols used for theblank and for the marked entries in the column are hard-coded as a space andasterisk. This is fine for a prototype, but you, or another user, may wish to use othersymbols. For example, in testing the graph function, you may want to use a period
in place of the space, to make sure the point is being repositioned properly eachtime the insert-rectangle function is called; or you might want to substitute a ‘+’sign or other symbol for the asterisk. You might even want to make a graph-columnthat is more than one display column wide. The program should be more flexible.
The way to do that is to replace the blank and the asterisk with two variables that
we can call graph-blank and graph-symbol and define those variables separately.

191
Also, the documentation is not well written. These considerations lead us to thesecond version of the function:(defvar graph-symbol "*""String used as symbol in graph, usually an asterisk.")(defvar graph-blank " ""String used as blank in graph, usually a blank space.
graph-blank must be the same number of columns wide
as graph-symbol.")(For an explanation of defvar, see Section 8.5 “Initializing a Variable with defvar”,
page 104.);;; Second version.
(defun column-of-graph (max-graph-height actual-height)"Return MAX-GRAPH-HEIGHT strings; ACTUAL-HEIGHT are graph-symbols.
The graph-symbols are contiguous entries at the endof the list.
The list will be inserted as one column of a graph.
The strings are either graph-blank or graph-symbol."(let ((insert-list nil)
(number-of-top-blanks(- max-graph-height actual-height)));; Fill in graph-symbols.
(while (> actual-height 0)(setq insert-list (cons graph-symbol insert-list))(setq actual-height (1- actual-height)));; Fill in graph-blanks.
(while (> number-of-top-blanks 0)(setq insert-list (cons graph-blank insert-list))(setq number-of-top-blanks(1- number-of-top-blanks)));; Return whole list.
insert-list))

If we wished, we could rewrite column-of-graph a third time to provide optionally for a line graph as well as for a bar graph. This would not be hard to do. One
way to think of a line graph is that it is no more than a bar graph in which the partof each bar that is below the top is blank. To construct a column for a line graph,the function first constructs a list of blanks that is one shorter than the value, thenit uses cons to attach a graph symbol to the list; then it uses cons again to attach
the top blanks to the list.
It is easy to see how to write such a function, but since we don’t need it, we
will not do it. But the job could be done, and if it were done, it would be donewith column-of-graph. Even more important, it is worth noting that few changes
would have to be made anywhere else. The enhancement, if we ever wish to makeit, is simple.

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Now, finally, we come to our first actual graph printing function. This prints
the body of a graph, not the labels for the vertical and horizontal axes, so we cancall this graph-body-print.

15.1 The graph-body-print Function
After our preparation in the preceding section, the graph-body-print function
is straightforward. The function will print column after column of asterisks andblanks, using the elements of a numbers’ list to specify the number of asterisks ineach column. This is a repetitive act, which means we can use a decrementing while
loop or recursive function for the job. In this section, we will write the definition
using a while loop.
The column-of-graph function requires the height of the graph as an argument,
so we should determine and record that as a local variable.
This leads us to the following template for the while loop version of this function:(defun graph-body-print (numbers-list)"documentation..."(let ((height ...
...))(while numbers-list
insert-columns-and-reposition-point(setq numbers-list (cdr numbers-list)))))

We need to fill in the slots of the template.
Clearly, we can use the (apply 'max numbers-list) expression to determine
the height of the graph.
The while loop will cycle through the numbers-list one element at a time. As
it is shortened by the (setq numbers-list (cdr numbers-list)) expression, thecar of each instance of the list is the value of the argument for column-of-graph.
At each cycle of the while loop, the insert-rectangle function inserts thelist returned by column-of-graph. Since the insert-rectangle function moves
point to the lower right of the inserted rectangle, we need to save the location ofpoint at the time the rectangle is inserted, move back to that position after therectangle is inserted, and then move horizontally to the next place from whichinsert-rectangle is called.
If the inserted columns are one character wide, as they will be if single blanks
and asterisks are used, the repositioning command is simply (forward-char 1);
however, the width of a column may be greater than one. This means that therepositioning command should be written (forward-char symbol-width). The
symbol-width itself is the length of a graph-blank and can be found using the expression (length graph-blank). The best place to bind the symbol-width variable
to the value of the width of graph column is in the varlist of the let expression.

Section 15.1: The graph-body-print Function

193

These considerations lead to the following function definition:(defun graph-body-print (numbers-list)"Print a bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values."(let ((height (apply 'max numbers-list))(symbol-width (length graph-blank))
from-position)(while numbers-list(setq from-position (point))(insert-rectangle(column-of-graph height (car numbers-list)))(goto-char from-position)(forward-char symbol-width);; Draw graph column by column.
(sit-for 0)(setq numbers-list (cdr numbers-list)));; Place point for X axis labels.
(forward-line height)(insert "\n")))The one unexpected expression in this function is the (sit-for 0) expression in thewhile loop. This expression makes the graph printing operation more interesting towatch than it would be otherwise. The expression causes Emacs to sit or do nothing
for a zero length of time and then redraw the screen. Placed here, it causes Emacs
to redraw the screen column by column. Without it, Emacs would not redraw thescreen until the function exits.
We can test graph-body-print with a short list of numbers.
1. Install graph-symbol, graph-blank, column-of-graph, which are in
Chapter 15 “Readying a Graph”, page 187, and graph-body-print.
2. Copy the following expression:(graph-body-print '(1 2 3 4 6 4 3 5 7 6 5 2 3))

3. Switch to the *scratch* buffer and place the cursor where you want the graphto start.
4. Type M-: (eval-expression).
5. Yank the graph-body-print expression into the minibuffer with C-y (yank).
6. Press RET to evaluate the graph-body-print expression.
Emacs will print a graph like this:***** ******* ************* **************************

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Chapter 15: Readying a Graph15.2 The recursive-graph-body-print Function
The graph-body-print function may also be written recursively. The recursive
solution is divided into two parts: an outside wrapper that uses a let expression
to determine the values of several variables that need only be found once, such as
the maximum height of the graph, and an inside function that is called recursively
to print the graph.
The wrapper is uncomplicated:(defun recursive-graph-body-print (numbers-list)"Print a bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values."(let ((height (apply 'max numbers-list))(symbol-width (length graph-blank))
from-position)(recursive-graph-body-print-internal
numbers-list
heightsymbol-width)))The recursive function is a little more difficult. It has four parts: the do-againtest, the printing code, the recursive call, and the next-step-expression. The doagain-test is a when expression that determines whether the numbers-list contains
any remaining elements; if it does, the function prints one column of the graph using
the printing code and calls itself again. The function calls itself again according to
the value produced by the next-step-expression which causes the call to act on ashorter version of the numbers-list.
(defun recursive-graph-body-print-internal(numbers-list height symbol-width)"Print a bar graph.
Used within recursive-graph-body-print function."(when numbers-list(setq from-position (point))(insert-rectangle(column-of-graph height (car numbers-list)))(goto-char from-position)(forward-char symbol-width)(sit-for 0); Draw graph column by column.
(recursive-graph-body-print-internal(cdr numbers-list) height symbol-width)))Section 15.4: Exercise

195

After installation, this expression can be tested; here is a sample:(recursive-graph-body-print '(3 2 5 6 7 5 3 4 6 4 3 2 1))

Here is what recursive-graph-body-print produces:******** ***** **** **********************************

Either of these two functions, graph-body-print or recursive-graph-bodyprint, create the body of a graph.

15.3 Need for Printed Axes
A graph needs printed axes, so you can orient yourself. For a do-once project, it
may be reasonable to draw the axes by hand using Emacs’s Picture mode; but agraph drawing function may be used more than once.
For this reason, I have written enhancements to the basic print-graph-bodyfunction that automatically print labels for the horizontal and vertical axes. Since
the label printing functions do not contain much new material, I have placed theirdescription in an appendix. See Appendix C “A Graph with Labeled Axes”,
page 233.

15.4 Exercise
Write a line graph version of the graph printing functions.

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Chapter 16: Your .emacs File

16 Your .emacs File
“You don’t have to like Emacs to like it”—this seemingly paradoxical statement is
the secret of GNU Emacs. The plain, out-of-the-box Emacs is a generic tool. Most
people who use it customize it to suit themselves.
GNU Emacs is mostly written in Emacs Lisp; this means that by writing expressions in Emacs Lisp you can change or extend Emacs.
There are those who appreciate Emacs’s default configuration. After all, Emacs
starts you in C mode when you edit a C file, starts you in Fortran mode when you
edit a Fortran file, and starts you in Fundamental mode when you edit an unadorned
file. This all makes sense, if you do not know who is going to use Emacs. Who
knows what a person hopes to do with an unadorned file? Fundamental mode is
the right default for such a file, just as C mode is the right default for editing C
code. (Enough programming languages have syntaxes that enable them to share or
nearly share features, so C mode is now provided by CC mode, the C Collection.)
But when you do know who is going to use Emacs—you, yourself—then it makes
sense to customize Emacs.
For example, I seldom want Fundamental mode when I edit an otherwise undistinguished file; I want Text mode. This is why I customize Emacs: so it suitsme.
You can customize and extend Emacs by writing or adapting a ~/.emacs file.
This is your personal initialization file; its contents, written in Emacs Lisp, tellEmacs what to do.1
A ~/.emacs file contains Emacs Lisp code. You can write this code yourself; oryou can use Emacs’s customize feature to write the code for you. You can combine
your own expressions and auto-written Customize expressions in your .emacs file.
(I myself prefer to write my own expressions, except for those, particularly fonts,
that I find easier to manipulate using the customize command. I combine the two
methods.)
Most of this chapter is about writing expressions yourself. It describes a simple.emacs file; for more information, see Section “The Init File” in The GNU Emacs
Manual, and Section “The Init File” in The GNU Emacs Lisp Reference Manual.

16.1 Site-wide Initialization Files
In addition to your personal initialization file, Emacs automatically loads various
site-wide initialization files, if they exist. These have the same form as your .emacs
file, but are loaded by everyone.
Two site-wide initialization files, site-load.el and site-init.el, are loaded
into Emacs and then dumped if a dumped version of Emacs is created, as is mostcommon. (Dumped copies of Emacs load more quickly. However, once a file is1

You may also add .el to ~/.emacs and call it a ~/.emacs.el file. In the past, you wereforbidden to type the extra keystrokes that the name ~/.emacs.el requires, but now
you may. The new format is consistent with the Emacs Lisp file naming conventions;the old format saves typing.

Section 16.2: Specifying Variables using defcustom

197

loaded and dumped, a change to it does not lead to a change in Emacs unless you
load it yourself or re-dump Emacs. See Section “Building Emacs” in The GNU
Emacs Lisp Reference Manual, and the INSTALL file.)
Three other site-wide initialization files are loaded automatically each time you
start Emacs, if they exist. These are site-start.el, which is loaded before your.emacs file, and default.el, and the terminal type file, which are both loaded
after your .emacs file.
Settings and definitions in your .emacs file will overwrite conflicting settingsand definitions in a site-start.el file, if it exists; but the settings and definitions
in a default.el or terminal type file will overwrite those in your .emacs file. (Youcan prevent interference from a terminal type file by setting term-file-prefix tonil. See Section 16.11 “A Simple Extension”, page 207.)
The INSTALL file that comes in the distribution contains descriptions of thesite-init.el and site-load.el files.
The loadup.el, startup.el, and loaddefs.el files control loading. These files
are in the lisp directory of the Emacs distribution and are worth perusing.
The loaddefs.el file contains a good many suggestions as to what to put into
your own .emacs file, or into a site-wide initialization file.

16.2 Specifying Variables using defcustom
You can specify variables using defcustom so that you and others can then use
Emacs’s customize feature to set their values. (You cannot use customize towrite function definitions; but you can write defuns in your .emacs file. Indeed,
you can write any Lisp expression in your .emacs file.)
The customize feature depends on the defcustom macro. Although you can
use defvar or setq for variables that users set, the defcustom macro is designed
for the job.
You can use your knowledge of defvar for writing the first three arguments for
defcustom. The first argument to defcustom is the name of the variable. The
second argument is the variable’s initial value, if any; and this value is set only if
the value has not already been set. The third argument is the documentation.
The fourth and subsequent arguments to defcustom specify types and options;these are not featured in defvar. (These arguments are optional.)
Each of these arguments consists of a keyword followed by a value. Each keyword
starts with the colon character ‘:’.

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For example, the customizable user option variable text-mode-hook looks likethis:(defcustom text-mode-hook nil"Normal hook run when entering Text mode and many related modes.":type 'hook:options '(turn-on-auto-fill flyspell-mode):group 'wp)

The name of the variable is text-mode-hook; it has no default value; and its documentation string tells you what it does.
The :type keyword tells Emacs the kind of data to which text-mode-hookshould be set and how to display the value in a Customization buffer.
The :options keyword specifies a suggested list of values for the variable. Usually, :options applies to a hook. The list is only a suggestion; it is not exclusive;a person who sets the variable may set it to other values; the list shown following
the :options keyword is intended to offer convenient choices to a user.
Finally, the :group keyword tells the Emacs Customization command in whichgroup the variable is located. This tells where to find it.
The defcustom macro recognizes more than a dozen keywords. For more information, see Section “Writing Customization Definitions” in The GNU Emacs Lisp
Reference Manual.
Consider text-mode-hook as an example.
There are two ways to customize this variable. You can use the customizationcommand or write the appropriate expressions yourself.
Using the customization command, you can type:M-x customize

and find that the group for editing files of text is called “Text”. Enter that group.
Text Mode Hook is the first member. You can click on its various options, such as
turn-on-auto-fill, to set the values. After you click on the button to
Save for Future Sessions

Emacs will write an expression into your .emacs file. It will look like this:(custom-set-variables;; custom-set-variables was added by Custom.
;; If you edit it by hand, you could mess it up, so be careful.
;; Your init file should contain only one such instance.
;; If there is more than one, they won't work right.
'(text-mode-hook '(turn-on-auto-fill text-mode-hook-identify)))(The text-mode-hook-identify function tells toggle-text-mode-auto-fillwhich buffers are in Text mode. It comes on automatically.)
The custom-set-variables function works somewhat differently than a setq.
While I have never learned the differences, I modify the custom-set-variables
expressions in my .emacs file by hand: I make the changes in what appears to me
to be a reasonable manner and have not had any problems. Others prefer to use
the Customization command and let Emacs do the work for them.
Another custom-set-... function is custom-set-faces. This function sets thevarious font faces. Over time, I have set a considerable number of faces. Some of theSection 16.3: Beginning a .emacs File

199

time, I re-set them using customize; other times, I simply edit the custom-setfaces expression in my .emacs file itself.
The second way to customize your text-mode-hook is to set it yourself in your.emacs file using code that has nothing to do with the custom-set-... functions.
When you do this, and later use customize, you will see a message that says
CHANGED outside Customize; operating on it here may be unreliable.

This message is only a warning. If you click on the button to
Save for Future Sessions

Emacs will write a custom-set-... expression near the end of your .emacs filethat will be evaluated after your hand-written expression. It will, therefore, overrule
your hand-written expression. No harm will be done. When you do this, however,be careful to remember which expression is active; if you forget, you may confuse
yourself.
So long as you remember where the values are set, you will have no trouble. In
any event, the values are always set in your initialization file, which is usually called.emacs.
I myself use customize for hardly anything. Mostly, I write expressions myself.
Incidentally, to be more complete concerning defines: defsubst defines an inline
function. The syntax is just like that of defun. defconst defines a symbol as aconstant. The intent is that neither programs nor users should ever change a value
set by defconst. (You can change it; the value set is a variable; but please do not.)16.3 Beginning a .emacs File
When you start Emacs, it loads your .emacs file unless you tell it not to by specifying‘-q’ on the command line. (The emacs -q command gives you a plain, out-of-thebox Emacs.)
A .emacs file contains Lisp expressions. Often, these are no more than expressions to set values; sometimes they are function definitions.
See Section “The Init File ~/.emacs” in The GNU Emacs Manual, for a short
description of initialization files.
This chapter goes over some of the same ground, but is a walk among extracts
from a complete, long-used .emacs file—my own.
The first part of the file consists of comments: reminders to myself. By now, ofcourse, I remember these things, but when I started, I did not.
;;;; Bob's .emacs file
; Robert J. Chassell; 26 September 1985

Look at that date! I started this file a long time ago. I have been adding to it eversince.
; Each section in this file is introduced by a
; line beginning with four semicolons; and each
; entry is introduced by a line beginning with; three semicolons.

This describes the usual conventions for comments in Emacs Lisp. Everything on
a line that follows a semicolon is a comment. Two, three, and four semicolons are

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Chapter 16: Your .emacs Fileused as subsection and section markers. (See Section “Comments” in The GNU
Emacs Lisp Reference Manual, for more about comments.);;;; The Help Key; Control-h is the help key;
; after typing control-h, type a letter to; indicate the subject about which you want help.
; For an explanation of the help facility,; type control-h two times in a row.

Just remember: type C-h two times for help.
; To find out about any mode, type control-h m; while in that mode. For example, to find out; about mail mode, enter mail mode and then type
; control-h m.

“Mode help”, as I call this, is very helpful. Usually, it tells you all you need toknow.
Of course, you don’t need to include comments like these in your .emacs file. I
included them in mine because I kept forgetting about Mode help or the conventions
for comments—but I was able to remember to look here to remind myself.

16.4 Text and Auto Fill Mode
Now we come to the part that turns on Text mode and Auto Fill mode2 .
;;; Text mode and Auto Fill mode;; The next two lines put Emacs into Text mode;; and Auto Fill mode, and are for writers who;; want to start writing prose rather than code.
(setq-default major-mode 'text-mode)(add-hook 'text-mode-hook 'turn-on-auto-fill)

Here is the first part of this .emacs file that does something besides remind aforgetful human!
The first of the two lines in parentheses tells Emacs to turn on Text mode when
you find a file, unless that file should go into some other mode, such as C mode.
When Emacs reads a file, it looks at the extension to the file name, if any. (Theextension is the part that comes after a ‘.’.) If the file ends with a ‘.c’ or ‘.h’extension then Emacs turns on C mode. Also, Emacs looks at first nonblank lineof the file; if the line says ‘-*- C -*-’, Emacs turns on C mode. Emacs possesses alist of extensions and specifications that it uses automatically. In addition, Emacs
looks near the last page for a per-buffer, local variables list, if any.
See sections “How Major Modes are Chosen” and “Local Variables in Files” in
The GNU Emacs Manual.
Now, back to the .emacs file.

2

This section suggests settings that are more suitable for writers. For programmers,
the default mode will be set to the corresponding prog-mode automatically based on
the type of the file. And it’s perfectly fine if you want to keep the fundamental modeas the default mode.

Section 16.4: Text and Auto Fill Mode

201

Here is the line again; how does it work?
(setq-default major-mode 'text-mode)This line is a short, but complete Emacs Lisp expression.
We are already familiar with setq. We use a similar macro setq-default to set
the following variable, major-mode3 , to the subsequent value, which is text-mode.
The single-quote before text-mode tells Emacs to deal directly with the text-mode
symbol, not with whatever it might stand for. See Section 1.9 “Setting the Valueof a Variable”, page 16, for a reminder of how setq works. The main point is that
there is no difference between the procedure you use to set a value in your .emacs
file and the procedure you use anywhere else in Emacs.
Here is the next line:(add-hook 'text-mode-hook 'turn-on-auto-fill)

In this line, the add-hook command adds turn-on-auto-fill to the variable.
turn-on-auto-fill is the name of a program, that, you guessed it!, turns on
Auto Fill mode.
Every time Emacs turns on Text mode, Emacs runs the commands hooked onto
Text mode. So every time Emacs turns on Text mode, Emacs also turns on Auto
Fill mode.
In brief, the first line causes Emacs to enter Text mode when you edit a file,unless the file name extension, a first non-blank line, or local variables to tell Emacs
otherwise.
Text mode among other actions, sets the syntax table to work conveniently for
writers. In Text mode, Emacs considers an apostrophe as part of a word like aletter; but Emacs does not consider a period or a space as part of a word. Thus,
M-f moves you over ‘it's’. On the other hand, in C mode, M-f stops just after the‘t’ of ‘it's’.
The second line causes Emacs to turn on Auto Fill mode when it turns on Text
mode. In Auto Fill mode, Emacs automatically breaks a line that is too wide andbrings the excessively wide part of the line down to the next line. Emacs breaks
lines between words, not within them.
When Auto Fill mode is turned off, lines continue to the right as you type them.
Depending on how you set the value of truncate-lines, the words you type either
disappear off the right side of the screen, or else are shown, in a rather ugly andunreadable manner, as a continuation line on the screen.

3

We use setq-default here because text-mode is buffer-local. If we use setq, it will
only apply to the current buffer, whereas using setq-default will also apply to newly
created buffers. This is not recommended for programmers.

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Chapter 16: Your .emacs FileIn addition, in this part of my .emacs file, I tell the Emacs fill commands toinsert two spaces after a colon:(setq colon-double-space t)

16.5 Mail Aliases
Here is a setq that turns on mail aliases, along with more reminders.
;;; Message mode
; To enter message mode, type 'C-x m'; To enter RMAIL (for reading mail),; type 'M-x rmail'(setq mail-aliases t)

This setq sets the value of the variable mail-aliases to t. Since t means true,
the line says, in effect, “Yes, use mail aliases.”
Mail aliases are convenient short names for long email addresses or for lists ofemail addresses. The file where you keep your aliases is ~/.mailrc. You write analias like this:alias geo george@foobar.wiz.edu

When you write a message to George, address it to ‘geo’; the mailer will automatically expand ‘geo’ to the full address.

16.6 Indent Tabs Mode
By default, Emacs inserts tabs in place of multiple spaces when it formats a region. (For example, you might indent many lines of text all at once with theindent-region command.) Tabs look fine on a terminal or with ordinary printing,but they produce badly indented output when you use TEX or Texinfo since TEX
ignores tabs.
The following turns off Indent Tabs mode:;;; Prevent Extraneous Tabs
(setq-default indent-tabs-mode nil)

Note that this line uses setq-default rather than the setq that we have seen
before; setq-default sets values only in buffers that do not have their own local
values for the variable.
See sections “Tabs vs. Spaces” and “Local Variables in Files” in The GNU Emacs
Manual.

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16.7 Some Key Bindings
Now for some personal key bindings:;;; Compare windows(global-set-key "\C-cw" 'compare-windows)

compare-windows is a nifty command that compares the text in your current
window with text in the next window. It makes the comparison by starting at pointin each window, moving over text in each window as far as they match. I use thiscommand all the time.
This also shows how to set a key globally, for all modes.
The command is global-set-key. It is followed by the key binding. In a .emacs
file, the keybinding is written as shown: \C-c stands for Control-C, which means topress the control key and the c key at the same time. The w means to press the w
key. The key binding is surrounded by double quotation marks. In documentation,
you would write this as C-c w. (If you were binding a META key, such as M-c, rather
than a CTRL key, you would write \M-c in your .emacs file. See Section “Rebinding
Keys in Your Init File” in The GNU Emacs Manual, for details.)
The command invoked by the keys is compare-windows.
Note thatcompare-windows is preceded by a single-quote; otherwise, Emacs would first try
to evaluate the symbol to determine its value.
These three things, the double quotation marks, the backslash before the ‘C’, andthe single-quote are necessary parts of key binding that I tend to forget. Fortunately,
I have come to remember that I should look at my existing .emacs file, and adapt
what is there.
As for the key binding itself: C-c w. This combines the prefix key, C-c, with asingle character, in this case, w. This set of keys, C-c followed by a single character,is strictly reserved for individuals’ own use. (I call these own keys, since these arefor my own use.) You should always be able to create such a key binding for yourown use without stomping on someone else’s key binding. If you ever write an
extension to Emacs, please avoid taking any of these keys for public use. Create a
key like C-c C-w instead. Otherwise, we will run out of own keys.
Here is another key binding, with a comment:;;; Key binding for 'occur'; I use occur a lot, so let's bind it to a key:
(global-set-key "\C-co" 'occur)

The occur command shows all the lines in the current buffer that contain amatch for a regular expression. When the region is active, occur restricts matches
to such region. Otherwise it uses the entire buffer. Matching lines are shown in abuffer called *Occur*. That buffer serves as a menu to jump to occurrences.

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Here is how to unbind a key, so it does not work:;;; Unbind 'C-x f'(global-unset-key "\C-xf")

There is a reason for this unbinding: I found I inadvertently typed C-x f when
I meant to type C-x C-f. Rather than find a file, as I intended, I accidentally set
the width for filled text, almost always to a width I did not want. Since I hardly
ever reset my default width, I simply unbound the key.
The following rebinds an existing key:;;; Rebind 'C-x C-b' for 'buffer-menu'(global-set-key "\C-x\C-b" 'buffer-menu)

By default, C-x C-b runs the list-buffers command. This command lists
your buffers in another window. Since I almost always want to do something inthat window, I prefer the buffer-menu command, which not only lists the buffers,
but moves point into that window.

16.8 Keymaps
Emacs uses keymaps to record which keys call which commands. When you use
global-set-key to set the key binding for a single command in all parts of Emacs,
you are specifying the key binding in current-global-map.
Specific modes, such as C mode or Text mode, have their own keymaps; themode-specific keymaps override the global map that is shared by all buffers.
The global-set-key function binds, or rebinds, the global keymap. For example, the following binds the key C-x C-b to the function buffer-menu:(global-set-key "\C-x\C-b" 'buffer-menu)

Mode-specific keymaps are bound using the define-key function, which takes a
specific keymap as an argument, as well as the key and the command. For example,
my .emacs file contains the following expression to bind the texinfo-insert@group command to C-c C-c g:(define-key texinfo-mode-map "\C-c\C-cg" 'texinfo-insert-@group)

The texinfo-insert-@group function itself is a little extension to Texinfo modethat inserts ‘@group’ into a Texinfo file. I use this command all the time andprefer to type the three strokes C-c C-c g rather than the six strokes @ g r o u p.
(‘@group’ and its matching ‘@end group’ are commands that keep all enclosed text
together on one page; many multi-line examples in this book are surrounded by‘@group ... @end group’.)Section 16.9: Loading Files

205

Here is the texinfo-insert-@group function definition:(defun texinfo-insert-@group ()"Insert the string @group in a Texinfo buffer."(interactive)(beginning-of-line)(insert "@group\n"))(Of course, I could have used Abbrev mode to save typing, rather than write
a function to insert a word; but I prefer key strokes consistent with other Texinfo
mode key bindings.)
You will see numerous define-key expressions in loaddefs.el as well as in thevarious mode libraries, such as cc-mode.el and lisp-mode.el.
See Section “Customizing Key Bindings” in The GNU Emacs Manual, andSection “Keymaps” in The GNU Emacs Lisp Reference Manual, for more information about keymaps.

16.9 Loading Files
Many people in the GNU Emacs community have written extensions to Emacs. As
time goes by, these extensions are often included in new releases. For example, theCalendar and Diary packages are now part of the standard GNU Emacs, as is Calc.
You can use a load command to evaluate a complete file and thereby install all
the functions and variables in the file into Emacs. For example:(load "~/emacs/slowsplit")

This evaluates, i.e., loads, the slowsplit.el file or if it exists, the faster, byte
compiled slowsplit.elc file from the emacs sub-directory of your home directory.
The file contains the function split-window-quietly, which John Robinson wrote
in 1989.
The split-window-quietly function splits a window with the minimum ofredisplay. I installed it in 1989 because it worked well with the slow 1200 baud
terminals I was then using. Nowadays, I only occasionally come across such a slow
connection, but I continue to use the function because I like the way it leaves thebottom half of a buffer in the lower of the new windows and the top half in theupper window.
To replace the key binding for the default split-window-vertically, you mustalso unset that key and bind the keys to split-window-quietly, like this:(global-unset-key "\C-x2")(global-set-key "\C-x2" 'split-window-quietly)

If you load many extensions, as I do, then instead of specifying the exact locationof the extension file, as shown above, you can specify that directory as part ofEmacs’s load-path. Then, when Emacs loads a file, it will search that directory as
well as its default list of directories. (The default list is specified in paths.h when
Emacs is built.)

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The following command adds your ~/emacs directory to the existing load path:;;; Emacs Load Path
(setq load-path (cons "~/emacs" load-path))

Incidentally, load-library is an interactive interface to the load function. The
complete function looks like this:(defun load-library (library)"Load the Emacs Lisp library named LIBRARY.
This is an interface to the function `load'. LIBRARY is searched
for in `load-path', both with and without `load-suffixes' (aswell as `load-file-rep-suffixes').
See Info node `(emacs)Lisp Libraries' for more details.
See `load-file' for a different interface to `load'."(interactive(list (completing-read "Load library: "(apply-partially 'locate-file-completion-tableload-path(get-load-suffixes)))))(load library))

The name of the function, load-library, comes from the use of “library” as aconventional synonym for “file”. The source for the load-library command is in
the files.el library.
Another interactive command that does a slightly different job is load-file.
See Section “Libraries of Lisp Code for Emacs” in The GNU Emacs Manual, for
information on the distinction between load-library and this command.

16.10 Autoloading
Instead of installing a function by loading the file that contains it, or by evaluating
the function definition, you can make the function available but not actually installit until it is first called. This is called autoloading.
When you execute an autoloaded function, Emacs automatically evaluates thefile that contains the definition, and then calls the function.
Emacs starts quicker with autoloaded functions, since their libraries are notloaded right away; but you need to wait a moment when you first use such afunction, while its containing file is evaluated.
Rarely used functions are frequently autoloaded. The loaddefs.el library contains thousands of autoloaded functions, from 5x5 to zone. Of course, you may
come to use a rare function frequently. When you do, you should load that function’s file with a load expression in your .emacs file.
In my .emacs file, I load 14 libraries that contain functions that would otherwise
be autoloaded. (Actually, it would have been better to include these files in mydumped Emacs, but I forgot. See Section “Building Emacs” in The GNU Emacs
Lisp Reference Manual, and the INSTALL file for more about dumping.)
You may also want to include autoloaded expressions in your .emacs file.
autoload is a built-in function that takes up to five arguments, the final threeof which are optional. The first argument is the name of the function to be au-

Section 16.11: A Simple Extension: line-to-top-of-window

207

toloaded; the second is the name of the file to be loaded. The third argument isdocumentation for the function, and the fourth tells whether the function can becalled interactively. The fifth argument tells what type of object—autoload canhandle a keymap or macro as well as a function (the default is a function).
Here is a typical example:(autoload 'html-helper-mode"html-helper-mode" "Edit HTML documents" t)(html-helper-mode is an older alternative to html-mode, which is a standard partof the distribution.)
This expression autoloads the html-helper-mode function. It takes it fromthe html-helper-mode.el file (or from the byte compiled version html-helpermode.elc, if that exists.) The file must be located in a directory specified byload-path. The documentation says that this is a mode to help you edit documents
written in the HyperText Markup Language. You can call this mode interactively
by typing M-x html-helper-mode. (You need to duplicate the function’s regular
documentation in the autoload expression because the regular function is not yet
loaded, so its documentation is not available.)See Section “Autoload” in The GNU Emacs Lisp Reference Manual, for more
information.

16.11 A Simple Extension: line-to-top-of-window
Here is a simple extension to Emacs that moves the line that point is on to the topof the window. I use this all the time, to make text easier to read.
You can put the following code into a separate file and then load it from your.emacs file, or you can include it within your .emacs file.
Here is the definition:;;; Line to top of window;;;; replace three keystroke sequence C-u 0 C-l(defun line-to-top-of-window ()"Move the line that point is on to top of window."(interactive)(recenter 0))

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Chapter 16: Your .emacs FileNow for the key binding.
Function keys as well as mouse button events and non-ascii characters are
written within square brackets, without quotation marks.
I bind line-to-top-of-window to my F6 function key like this:(global-set-key [f6] 'line-to-top-of-window)

For more information, see Section “Rebinding Keys in Your Init File” in The
GNU Emacs Manual.
If you run two versions of GNU Emacs, such as versions 27 and 28, and use one.emacs file, you can select which code to evaluate with the following conditional:(cond((= 27 emacs-major-version);; evaluate version 27 code
( ... ))((= 28 emacs-major-version);; evaluate version 28 code
( ... )))For example, recent versions blink their cursors by default. I hate such blinking,as well as other features, so I placed the following in my .emacs file4 :(when (>= emacs-major-version 21)(blink-cursor-mode 0);; Insert newline when you press 'C-n' (next-line);; at the end of the buffer(setq next-line-add-newlines t);; Turn on image viewing(auto-image-file-mode t);; Turn on menu bar (this bar has text);; (Use numeric argument to turn on)(menu-bar-mode 1);; Turn off tool bar (this bar has icons);; (Use numeric argument to turn on)(tool-bar-mode nil);; Turn off tooltip mode for tool bar;; (This mode causes icon explanations to pop up);; (Use numeric argument to turn on)(tooltip-mode nil);; If tooltips turned on, make tips appear promptly(setq tooltip-delay 0.1) ; default is 0.7 second)

4

When I start instances of Emacs that do not load my .emacs file or any site file, I also
turn off blinking:emacs -q --no-site-file -eval '(blink-cursor-mode nil)'

Or nowadays, using an even more sophisticated set of options,emacs -Q -D

Section 16.12: X11 Colors

209

16.12 X11 Colors
You can specify colors when you use Emacs with the MIT X Windowing system.
I dislike the default colors and specify my own.
Here are the expressions in my .emacs file that set values:;; Set cursor color(set-cursor-color "white");; Set mouse color(set-mouse-color "white");; Set foreground and background(set-foreground-color "white")(set-background-color "darkblue");;; Set highlighting colors for isearch and drag(set-face-foreground 'highlight "white")(set-face-background 'highlight "blue")(set-face-foreground 'region "cyan")(set-face-background 'region "blue")(set-face-foreground 'secondary-selection "skyblue")(set-face-background 'secondary-selection "darkblue");; Set calendar highlighting colors(with-eval-after-load 'calendar(set-face-foreground 'diary"skyblue")(set-face-background 'holiday "slate blue")(set-face-foreground 'holiday "white"))The various shades of blue soothe my eye and prevent me from seeing the screen
flicker.
Alternatively, I could have set my specifications in various X initialization files.
For example, I could set the foreground, background, cursor, and pointer (i.e.,mouse) colors in my ~/.Xresources file like this:Emacs*foreground:
white
Emacs*background:darkblue
Emacs*cursorColor: white
Emacs*pointerColor: white

In any event, since it is not part of Emacs, I set the root color of my X window
in my ~/.xinitrc file, like this5 :xsetroot -solid Navy -fg white &5

I also run more modern window managers, such as Enlightenment, Gnome, or KDE;
in those cases, I often specify an image rather than a plain color.

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16.13 Miscellaneous Settings for a .emacs File
Here are a few miscellaneous settings:− Set the shape and color of the mouse cursor:; Cursor shapes are defined in; '/usr/include/X11/cursorfont.h';; for example, the 'target' cursor is number 128;
; the 'top_left_arrow' cursor is number 132.
(let ((mpointer (x-get-resource "*mpointer""*emacs*mpointer")));; If you have not set your mouse pointer;;then set it, otherwise leave as is:(if (eq mpointer nil)(setq mpointer "132")) ; top_left_arrow(setq x-pointer-shape (string-to-number mpointer))(set-mouse-color "white"))− Or you can set the values of a variety of features in an alist, like this:(setq-default
default-frame-alist'((cursor-color . "white")(mouse-color . "white")(foreground-color . "white")(background-color . "DodgerBlue4");; (cursor-type . bar)(cursor-type . box)
(tool-bar-lines . 0)
(menu-bar-lines . 1)
(width . 80)(height . 58)(font .
"-Misc-Fixed-Medium-R-Normal--20-200-75-75-C-100-ISO8859-1")))− Convert CTRL-h into DEL and DEL into CTRL-h.
(Some older keyboards needed this, although I have not seen the problem
recently.);; Translate 'C-h' to <DEL>.
; (keyboard-translate ?\C-h ?\C-?);; Translate <DEL> to 'C-h'.
(keyboard-translate ?\C-? ?\C-h)− Turn off a blinking cursor!
(if (fboundp 'blink-cursor-mode)(blink-cursor-mode -1))or start GNU Emacs with the command emacs -nbc.

Section 16.13: Miscellaneous Settings for a .emacs File

211

− When using grep‘-i’ Ignore case distinctions‘-n’ Prefix each line of output with line number‘-H’ Print the filename for each match.
‘-e’ Protect patterns beginning with a hyphen character, ‘-’(setq grep-command "grep -i -nH -e ")− Find an existing buffer, even if it has a different name
This avoids problems with symbolic links.
(setq find-file-existing-other-name t)− Set your language environment and default input method(set-language-environment "latin-1");; Remember you can enable or disable multilingual text input;; with the toggle-input-method' (C-\) command(setq default-input-method "latin-1-prefix")If you want to write with Chinese GB characters, set this instead:(set-language-environment "Chinese-GB")(setq default-input-method "chinese-tonepy")

Fixing Unpleasant Key Bindings
Some systems bind keys unpleasantly. Sometimes, for example, the CTRL key appears in an awkward spot rather than at the far left of the home row.
Usually, when people fix these sorts of key bindings, they do not change their~/.emacs file. Instead, they bind the proper keys on their consoles with theloadkeys or install-keymap commands in their boot script and then include
xmodmap commands in their .xinitrc or .Xsession file for X Windows.
For a boot script:
loadkeys /usr/share/keymaps/i386/qwerty/emacs2.kmap.gz
orinstall-keymap emacs2

For a .xinitrc or .Xsession file when the Caps Lock key is at the far left of thehome row:# Bind the key labeled 'Caps Lock' to 'Control'# (Such a broken user interface suggests that keyboard manufacturers# think that computers are typewriters from 1885.)
xmodmap -e "clear Lock"xmodmap -e "add Control = Caps_Lock"

In a .xinitrc or .Xsession file, to convert an ALT key to a META key:
# Some ill designed keyboards have a key labeled ALT and no Meta
xmodmap -e "keysym Alt_L = Meta_L Alt_L"

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16.14 A Modified Mode Line
Finally, a feature I really like: a modified mode line.
When I work over a network, I forget which machine I am using. Also, I tend
to lose track of where I am, and which line point is on.
So I reset my mode line to look like this:-:-- foo.texi

rattlesnake:/home/bob/Line 1(Texinfo Fill) Top

I am visiting a file called foo.texi, on my machine rattlesnake in my/home/bob buffer. I am on line 1, in Texinfo mode, and am at the top of thebuffer.
My .emacs file has a section that looks like this:;; Set a Mode Line that tells me which machine, which directory,;; and which line I am on, plus the other customary information.
(setq-default mode-line-format(quote(#("-" 0 1(help-echo"mouse-1: select window, mouse-2: delete others ..."))
mode-line-mule-infomode-line-modified
mode-line-frame-identification""
mode-line-buffer-identification""(:eval (substring(system-name) 0 (string-match "\\..+" (system-name))))":"default-directory#(" " 0 1
(help-echo"mouse-1: select window, mouse-2: delete others ..."))(line-number-mode " Line %l ")global-mode-string#("%[(" 0 6(help-echo"mouse-1: select window, mouse-2: delete others ..."))(:eval (format-time-string "%F"))
mode-line-processminor-mode-alist#("%n" 0 2 (help-echo "mouse-2: widen" local-map (keymap ...)))")%] "(-3 . "%P")
;;
"-%-"
)))

Here, I redefine the default mode line. Most of the parts are from the original; but
I make a few changes. I set the default mode line format so as to permit various
modes, such as Info, to override it.
Many elements in the list are self-explanatory: mode-line-modified is a variable that tells whether the buffer has been modified, mode-name tells the name ofSection 16.14: A Modified Mode Line213

the mode, and so on. However, the format looks complicated because of two features
we have not discussed.
The first string in the mode line is a dash or hyphen, ‘-’. In the old days, itwould have been specified simply as "-". But nowadays, Emacs can add properties
to a string, such as highlighting or, as in this case, a help feature. If you place
your mouse cursor over the hyphen, some help information appears (By default,
you must wait seven-tenths of a second before the information appears. You canchange that timing by changing the value of tooltip-delay.)
The new string format has a special syntax:#("-" 0 1 (help-echo "mouse-1: select window, ..."))The #( begins a list. The first element of the list is the string itself, just one ‘-’. The
second and third elements specify the range over which the fourth element applies.
A range starts after a character, so a zero means the range starts just before thefirst character; a 1 means that the range ends just after the first character. The
third element is the property for the range. It consists of a property list, a property
name, in this case, ‘help-echo’, followed by a value, in this case, a string. The
second, third, and fourth elements of this new string format can be repeated.
See Section “Text Properties” in The GNU Emacs Lisp Reference Manual, andsee Section “Mode Line Format” in The GNU Emacs Lisp Reference Manual, formore information.
mode-line-buffer-identification displays the current buffer name. It is alist beginning (#("%12b" 0 4 .... The #( begins the list.
The ‘"%12b"’ displays the current buffer name, using the buffer-name function
with which we are familiar; the ‘12’ specifies the maximum number of charactersthat will be displayed. When a name has fewer characters, whitespace is added
to fill out to this number. (Buffer names can and often should be longer than 12
characters; this length works well in a typical 80 column wide window.):eval says to evaluate the following form and use the result as a string todisplay. In this case, the expression displays the first component of the full systemname. The end of the first component is a ‘.’ (period), so I use the string-matchfunction to tell me the length of the first component. The substring from the zerothcharacter to that length is the name of the machine.
This is the expression:(:eval (substring(system-name) 0 (string-match "\\..+" (system-name))))‘%[’ and ‘%]’ cause a pair of square brackets to appear for each recursive editinglevel. ‘%n’ says “Narrow” when narrowing is in effect. ‘%P’ tells you the percentageof the buffer that is above the bottom of the window, or “Top”, “Bottom”, or “All”.
(A lower case ‘p’ tell you the percentage above the top of the window.) ‘%-’ inserts
enough dashes to fill out the line.
Remember, you don’t have to like Emacs to like it—your own Emacs can havedifferent colors, different commands, and different keys than a default Emacs.
On the other hand, if you want to bring up a plain out-of-the-box Emacs, withno customization, type:emacs -q

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Chapter 16: Your .emacs File

This will start an Emacs that does not load your ~/.emacs initialization file. A
plain, default Emacs. Nothing more.

Section 17.1: debug

215

17 Debugging
GNU Emacs has two debuggers, debug and edebug. The first is built into theinternals of Emacs and is always with you; the second requires that you instrumenta function before you can use it.
Both debuggers are described extensively in Section “Debugging Lisp Programs”
in The GNU Emacs Lisp Reference Manual. In this chapter, I will walk through ashort example of each.

17.1 debug
Suppose you have written a function definition that is intended to return the sumof the numbers 1 through a given number. (This is the triangle function discussed
earlier. See “Example with Decrementing Counter”, page 123, for a discussion.)
However, your function definition has a bug. You have mistyped ‘1=’ for ‘1-’.
Here is the broken definition:(defun triangle-bugged (number)"Return sum of numbers 1 through NUMBER inclusive."(let ((total 0))(while (> number 0)(setq total (+ total number))(setq number (1= number))); Error here.
total))

If you are reading this in Info, you can evaluate this definition in the normal
fashion. You will see triangle-bugged appear in the echo area.
Now evaluate the triangle-bugged function with an argument of 4:(triangle-bugged 4)

This will create and enter a *Backtrace* buffer that says:---------- Buffer: *Backtrace* ---------Debugger entered--Lisp error: (void-function 1=)(1= number)(setq number (1= number))(while (> number 0) (setq total (+ total number))(setq number (1= number)))(let ((total 0)) (while (> number 0) (setq total ...)
(setq number ...)) total)
triangle-bugged(4)eval((triangle-bugged 4) nil)eval-expression((triangle-bugged 4) nil nil 127)
funcall-interactively(eval-expression (triangle-bugged 4) nil nil 127)
call-interactively(eval-expression nil nil)command-execute(eval-expression)---------- Buffer: *Backtrace* ----------(I have reformatted this example slightly; the debugger does not fold long lines. As
usual, you can quit the debugger by typing q in the *Backtrace* buffer.)
In practice, for a bug as simple as this, the Lisp error line will tell you what you
need to know to correct the definition. The function 1= is void.

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Chapter 17: Debugging

However, suppose you are not quite certain what is going on? You can read thecomplete backtrace.
Emacs automatically starts the debugger that puts you in the *Backtrace*
buffer. You can also start the debugger manually as described below.
Read the *Backtrace* buffer from the bottom up; it tells you what Emacs didthat led to the error. Emacs made an interactive call to C-x C-e (eval-last-sexp),
which led to the evaluation of the triangle-bugged expression. Each line above
tells you what the Lisp interpreter evaluated next.
The third line from the top of the buffer is(setq number (1= number))Emacs tried to evaluate this expression; in order to do so, it tried to evaluate theinner expression shown on the second line from the top:
(1= number)This is where the error occurred; as the top line says:
Debugger entered--Lisp error: (void-function 1=)

You can correct the mistake, re-evaluate the function definition, and then run yourtest again.

17.2 debug-on-entry
Emacs starts the debugger automatically when your function has an error.
Incidentally, you can start the debugger manually for all versions of Emacs; theadvantage is that the debugger runs even if you do not have a bug in your code.
Sometimes your code will be free of bugs!
You can enter the debugger when you call the function by calling debug-onentry.
Type:
M-x debug-on-entry RET triangle-bugged RET

Section 17.2: debug-on-entry

217

Now, evaluate the following:(triangle-bugged 5)

All versions of Emacs will create a *Backtrace* buffer and tell you that it is beginning to evaluate the triangle-bugged function:---------- Buffer: *Backtrace* ---------Debugger entered--entering a function:* triangle-bugged(5)eval((triangle-bugged 5) nil)eval-expression((triangle-bugged 5) nil nil 127)
funcall-interactively(eval-expression (triangle-bugged 5) nil nil 127)
call-interactively(eval-expression nil nil)command-execute(eval-expression)---------- Buffer: *Backtrace* ----------In the *Backtrace* buffer, type d. Emacs will evaluate the first expression intriangle-bugged; the buffer will look like this:---------- Buffer: *Backtrace* ---------Debugger entered--beginning evaluation of function call form:* (let ((total 0)) (while (> number 0) (setq total ...)
(setq number ...)) total)* triangle-bugged(5)eval((triangle-bugged 5))eval((triangle-bugged 5) nil)eval-expression((triangle-bugged 5) nil nil 127)
funcall-interactively(eval-expression (triangle-bugged 5) nil nil 127)
call-interactively(eval-expression nil nil)command-execute(eval-expression)---------- Buffer: *Backtrace* ----------

Now, type d again, eight times, slowly. Each time you type d, Emacs will evaluate
another expression in the function definition.
Eventually, the buffer will look like this:---------- Buffer: *Backtrace* ---------Debugger entered--beginning evaluation of function call form:* (setq number (1= number))* (while (> number 0) (setq total (+ total number))(setq number (1= number)))* (let ((total 0)) (while (> number 0) (setq total ...)
(setq number ...)) total)* triangle-bugged(5)eval((triangle-bugged 5) nil)eval-expression((triangle-bugged 5) nil nil 127)
funcall-interactively(eval-expression (triangle-bugged 5) nil nil 127)
call-interactively(eval-expression nil nil)command-execute(eval-expression)---------- Buffer: *Backtrace* ----------

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Chapter 17: Debugging

Finally, after you type d two more times, Emacs will reach the error, and the top
two lines of the *Backtrace* buffer will look like this:---------- Buffer: *Backtrace* ---------Debugger entered--Lisp error: (void-function 1=)* (1= number)...
---------- Buffer: *Backtrace* ----------

By typing d, you were able to step through the function.
You can quit a *Backtrace* buffer by typing q in it; this quits the trace, butdoes not cancel debug-on-entry.
To cancel the effect of debug-on-entry, call cancel-debug-on-entry and thename of the function, like this:M-x cancel-debug-on-entry RET triangle-bugged RET(If you are reading this in Info, cancel debug-on-entry now.)

17.3 debug-on-quit and (debug)
In addition to setting debug-on-error or calling debug-on-entry, there are twoother ways to start debug.
You can start debug whenever you type C-g (keyboard-quit) by setting thevariable debug-on-quit to t. This is useful for debugging infinite loops.
Or, you can insert a line that says (debug) into your code where you want the
debugger to start, like this:(defun triangle-bugged (number)"Return sum of numbers 1 through NUMBER inclusive."(let ((total 0))(while (> number 0)(setq total (+ total number))(debug); Start debugger.
(setq number (1= number))); Error here.
total))The debug function is described in detail in Section “The Lisp Debugger” in
The GNU Emacs Lisp Reference Manual.

17.4 The edebug Source Level Debugger
Edebug is a source level debugger. Edebug normally displays the source of the code
you are debugging, with an arrow at the left that shows which line you are currently
executing.
You can walk through the execution of a function, line by line, or run quickly
until reaching a breakpoint where execution stops.
Edebug is described in Section “Edebug” in The GNU Emacs Lisp Reference
Manual.

Section 17.4: The edebug Source Level Debugger

219

Here is a bugged function definition for triangle-recursively.
See
Section 11.3.4 “Recursion in place of a counter”, page 130, for a review of it.
(defun triangle-recursively-bugged (number)"Return sum of numbers 1 through NUMBER inclusive.
Uses recursion."(if (= number 1)
1(+ number(triangle-recursively-bugged(1= number))))); Error here.

Normally, you would install this definition by positioning your cursor after the function’s closing parenthesis and typing C-x C-e (eval-last-sexp) or else by positioning your cursor within the definition and typing C-M-x (eval-defun). (By default,
the eval-defun command works only in Emacs Lisp mode or in Lisp Interaction
mode.)
However, to prepare this function definition for Edebug, you must first instrument the code using a different command. You can do this by positioning yourcursor within or just after the definition and typing
M-x edebug-defun RETThis will cause Emacs to load Edebug automatically if it is not already loaded, andproperly instrument the function.
After instrumenting the function, place your cursor after the following expression
and type C-x C-e (eval-last-sexp):(triangle-recursively-bugged 3)

You will be jumped back to the source for triangle-recursively-bugged and the
cursor positioned at the beginning of the if line of the function. Also, you will see
an arrowhead at the left hand side of that line. The arrowhead marks the line where
the function is executing. (In the following examples, we show the arrowhead with‘=>’; in a windowing system, you may see the arrowhead as a solid triangle in thewindow fringe.)=>?(if (= number 1)

In the example, the location of point is displayed with a star, ‘?’ (in Info, it isdisplayed as ‘-!-’).
If you now press SPC, point will move to the next expression to be executed; theline will look like this:=>(if ?(= number 1)

As you continue to press SPC, point will move from expression to expression. At
the same time, whenever an expression returns a value, that value will be displayedin the echo area. For example, after you move point past number, you will see thefollowing:
Result: 3 (#o3, #x3, ?\C-c)

This means the value of number is 3, which is octal three, hexadecimal three, andascii Control-C (the third letter of the alphabet, in case you need to know thisinformation).
You can continue moving through the code until you reach the line with theerror. Before evaluation, that line looks like this:

220

Chapter 17: Debugging=>?(1= number))))); Error here.

When you press SPC once again, you will produce an error message that says:
Symbol's function definition is void: 1=

This is the bug.
Press q to quit Edebug.
To remove instrumentation from a function definition, simply re-evaluate it witha command that does not instrument it. For example, you could place your cursor
after the definition’s closing parenthesis and type C-x C-e.
Edebug does a great deal more than walk with you through a function. You can
set it so it races through on its own, stopping only at an error or at specified stopping
points; you can cause it to display the changing values of various expressions; youcan find out how many times a function is called, and more.
Edebug is described in Section “Edebug” in The GNU Emacs Lisp Reference
Manual.

17.5 Debugging Exercises• Install the count-words-example function and then cause it to enter the builtin debugger when you call it. Run the command on a region containing two
words. You will need to press d a remarkable number of times. On your system, is a hook called after the command finishes? (For information on hooks,
see Section “Command Loop Overview” in The GNU Emacs Lisp Reference
Manual.)• Copy count-words-example into the *scratch* buffer, instrument the function for Edebug, and walk through its execution. The function does not needto have a bug, although you can introduce one if you wish. If the function
lacks a bug, the walk-through completes without problems.
• While running Edebug, type ? to see a list of all the Edebug commands. (Theglobal-edebug-prefix is usually C-x X, i.e., CTRL-x followed by an upper
case X; use this prefix for commands made outside of the Edebug debugging
buffer.)• In the Edebug debugging buffer, use the p (edebug-bounce-point) command
to see where in the region the count-words-example is working.
• Move point to some spot further down the function and then type the h(edebug-goto-here) command to jump to that location.
• Use the t (edebug-trace-mode) command to cause Edebug to walk throughthe function on its own; use an upper case T for edebug-Trace-fast-mode.
• Set a breakpoint, then run Edebug in Trace mode until it reaches the stopping
point.

221

18 Conclusion
We have now reached the end of this Introduction. You have now learned enoughabout programming in Emacs Lisp to set values, to write simple .emacs files foryourself and your friends, and write simple customizations and extensions to Emacs.
This is a place to stop. Or, if you wish, you can now go onward, and teachyourself.
You have learned some of the basic nuts and bolts of programming. But onlysome. There are a great many more brackets and hinges that are easy to use that
we have not touched.
A path you can follow right now lies among the sources to GNU Emacs and in
The GNU Emacs Lisp Reference Manual.
The Emacs Lisp sources are an adventure. When you read the sources and comeacross a function or expression that is unfamiliar, you need to figure out or find out
what it does.
Go to the Reference Manual. It is a thorough, complete, and fairly easy-toread description of Emacs Lisp. It is written not only for experts, but for people
who know what you know. (The Reference Manual comes with the standard GNU
Emacs distribution. Like this introduction, it comes as a Texinfo source file, so you
can read it on your computer and as a typeset, printed book.)
Go to the other built-in help that is part of GNU Emacs: the built-in documentation for all functions and variables, and xref-find-definitions, the programthat takes you to sources.
Here is an example of how I explore the sources. Because of its name, simple.elis the file I looked at first, a long time ago. As it happens some of the functions
in simple.el are complicated, or at least look complicated at first sight. The
open-line function, for example, looks complicated.
You may want to walk through this function slowly, as we did with theforward-sentence function. (See Section 12.3 “forward-sentence”, page 143.)
Or you may want to skip that function and look at another, such as split-line.
You don’t need to read all the functions. According to count-words-in-defun, thesplit-line function contains 102 words and symbols.
Even though it is short, split-line contains expressions we have not studied:skip-chars-forward, indent-to, current-column and insert-and-inherit.
Consider the skip-chars-forward function. In GNU Emacs, you can find out
more about skip-chars-forward by typing C-h f (describe-function) and thename of the function. This gives you the function documentation.
You may be able to guess what is done by a well named function such as
indent-to; or you can look it up, too. Incidentally, the describe-function function itself is in help.el; it is one of those long, but decipherable functions. You
can look up describe-function using the C-h f command!
In this instance, since the code is Lisp, the *Help* buffer contains the name of
the library containing the function’s source. You can put point over the name of thelibrary and press the RET key, which in this situation is bound to help-follow, andbe taken directly to the source, in the same way as M-. (xref-find-definitions).

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Chapter 18: Conclusion

The definition for describe-function illustrates how to customize theinteractive expression without using the standard character codes; and it shows
how to create a temporary buffer.
(The indent-to function is written in C rather than Emacs Lisp; it is a built-infunction. help-follow takes you to its source as does xref-find-definitions,when properly set up.)
You can look at a function’s source using xref-find-definitions, which isbound to M-. Finally, you can find out what the Reference Manual has to say byvisiting the manual in Info, and typing i (Info-index) and the name of the function,or by looking up the function in the index to a printed copy of the manual.
Similarly, you can find out what is meant by insert-and-inherit.
Other interesting source files include paragraphs.el, loaddefs.el, andloadup.el. The paragraphs.el file includes short, easily understood functions aswell as longer ones. The loaddefs.el file contains the many standard autoloads
and many keymaps. I have never looked at it all; only at parts. loadup.el is thefile that loads the standard parts of Emacs; it tells you a great deal about how
Emacs is built. (See Section “Building Emacs” in The GNU Emacs Lisp Reference
Manual, for more about building.)
As I said, you have learned some nuts and bolts; however, and very importantly,
we have hardly touched major aspects of programming; I have said nothing about
how to sort information, except to use the predefined sort function; I have said
nothing about how to store information, except to use variables and lists; I havesaid nothing about how to write programs that write programs. These are topics
for another, and different kind of book, a different kind of learning.
What you have done is learn enough for much practical work with GNU Emacs.
What you have done is get started. This is the end of a beginning.

223

Appendix A The the-the Function
Sometimes when you you write text, you duplicate words—as with “you you” near
the beginning of this sentence. I find that most frequently, I duplicate “the”; hence,
I call the function for detecting duplicated words, the-the.
As a first step, you could use the following regular expression to search for
duplicates:\\(\\w+[ \t\n]+\\)\\1

This regexp matches one or more word-constituent characters followed by one ormore spaces, tabs, or newlines. However, it does not detect duplicated words on
different lines, since the ending of the first word, the end of the line, is different
from the ending of the second word, a space. (For more information about regularexpressions, see Chapter 12 “Regular Expression Searches”, page 141, as well as
Section “Syntax of Regular Expressions” in The GNU Emacs Manual, and Section“Regular Expressions” in The GNU Emacs Lisp Reference Manual.)
You might try searching just for duplicated word-constituent characters but that
does not work since the pattern detects doubles such as the two occurrences of “th”
in “with the”.
Another possible regexp searches for word-constituent characters followed by
non-word-constituent characters, reduplicated. Here, ‘\\w+’ matches one or more
word-constituent characters and ‘\\W*’ matches zero or more non-word-constituent
characters.
\\(\\(\\w+\\)\\W*\\)\\1

Again, not useful.
Here is the pattern that I use. It is not perfect, but good enough. ‘\\b’ matches
the empty string, provided it is at the beginning or end of a word; ‘[^@ \n\t]+’matches one or more occurrences of any characters that are not an @-sign, space,
newline, or tab.
\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\bOne can write more complicated expressions, but I found that this expression isgood enough, so I use it.
Here is the the-the function, as I include it in my .emacs file, along with ahandy global key binding:(defun the-the ()"Search forward for for a duplicated word."(interactive)(message "Searching for for duplicated words ...")(push-mark);; This regexp is not perfect;; but is fairly good over all:(if (re-search-forward"\\b\\([^@ \n\t]+\\)[ \n\t]+\\1\\b" nil 'move)(message "Found duplicated word.")(message "End of buffer")));; Bind 'the-the' to C-c \(global-set-key "\C-c\\" 'the-the)224

Appendix A: The the-the Function
Here is test text:
one two two three four five
five six seven

You can substitute the other regular expressions shown above in the function
definition and try each of them on this list.

Section B.1: The current-kill Function

225

Appendix B Handling the Kill Ring
The kill ring is a list that is transformed into a ring by the workings of thecurrent-kill function. The yank and yank-pop commands use the current-kill
function.
This appendix describes the current-kill function as well as both the yankand the yank-pop commands, but first, consider the workings of the kill ring.
The kill ring has a default maximum length of sixty items; this number is toolarge for an explanation. Instead, set it to four. Please evaluate the following:(setq old-kill-ring-max kill-ring-max)(setq kill-ring-max 4)

Then, please copy each line of the following indented example into the kill ring.
You may kill each line with C-k or mark it and copy it with M-w.
(In a read-only buffer, such as the *info* buffer, the kill command, C-k(kill-line), will not remove the text, merely copy it to the kill ring. However,your machine may beep at you. Alternatively, for silence, you may copy the regionof each line with the M-w (kill-ring-save) command. You must mark each line
for this command to succeed, but it does not matter at which end you put point
or mark.)
Please invoke the calls in order, so that five elements attempt to fill the kill ring:first some text
second piece of text
third line
fourth line of text
fifth bit of text

Then find the value of kill-ring by evaluating
kill-ring

It is:("fifth bit of text" "fourth line of text""third line" "second piece of text")

The first element, ‘first some text’, was dropped.
To return to the old value for the length of the kill ring, evaluate:(setq kill-ring-max old-kill-ring-max)

B.1 The current-kill Function
The current-kill function changes the element in the kill ring to which
kill-ring-yank-pointer points. (Also, the kill-new function sets kill-ringyank-pointer to point to the latest element of the kill ring. The kill-new
function is used directly or indirectly by kill-append, copy-region-as-kill,
kill-ring-save, kill-line, and kill-region.)

226

Appendix B: Handling the Kill Ring

The current-kill function is used by yank and by yank-pop. Here is the codefor current-kill:(defun current-kill (n &optional do-not-move)"Rotate the yanking point by N places, and then return that kill.
If N is zero and `interprogram-paste-function' is set to afunction that returns a string or a list of strings, and if thatfunction doesn't return nil, then that string (or list) is added
to the front of the kill ring and the string (or first string in
the list) is returned as the latest kill.
If N is not zero, and if `yank-pop-change-selection' is
non-nil, use `interprogram-cut-function' to transfer thekill at the new yank point into the window system selection.
If optional arg DO-NOT-MOVE is non-nil, then don't actually
move the yanking point; just return the Nth kill forward."(let ((interprogram-paste (and (= n 0)
interprogram-paste-function(funcall interprogram-paste-function))))(if interprogram-paste(progn;; Disable the interprogram cut function when we add the new;; text to the kill ring, so Emacs doesn't try to own the;; selection, with identical text.
(let ((interprogram-cut-function nil))(if (listp interprogram-paste)(mapc 'kill-new (nreverse interprogram-paste))(kill-new interprogram-paste)))(car kill-ring))(or kill-ring (error "Kill ring is empty"))(let ((ARGth-kill-element(nthcdr (mod (- n (length kill-ring-yank-pointer))(length kill-ring))
kill-ring)))(unless do-not-move(setq kill-ring-yank-pointer ARGth-kill-element)(when (and yank-pop-change-selection(> n 0)interprogram-cut-function)(funcall interprogram-cut-function (car ARGth-kill-element))))(car ARGth-kill-element)))))

Remember also that the kill-new function sets kill-ring-yank-pointer tothe latest element of the kill ring, which means that all the functions that call it
set the value indirectly: kill-append, copy-region-as-kill, kill-ring-save,kill-line, and kill-region.

Section B.1: The current-kill Function

227

Here is the line in kill-new, which is explained in “The kill-new function”,
page 97.
(setq kill-ring-yank-pointer kill-ring)

The current-kill function looks complex, but as usual, it can be understoodby taking it apart piece by piece. First look at it in skeletal form:(defun current-kill (n &optional do-not-move)"Rotate the yanking point by N places, and then return that kill."(let varlist
body...)

This function takes two arguments, one of which is optional. It has a documentation string. It is not interactive.
The body of the function definition is a let expression, which itself has a bodyas well as a varlist.
The let expression declares a variable that will be only usable within the boundsof this function. This variable is called interprogram-paste and is for copying toanother program. It is not for copying within this instance of GNU Emacs. Most
window systems provide a facility for interprogram pasting. Sadly, that facility usually provides only for the last element. Most windowing systems have not adopted
a ring of many possibilities, even though Emacs has provided it for decades.
The if expression has two parts, one if there exists interprogram-paste andone if not.
Let us consider the else-part of the current-kill function. (The then-partuses the kill-new function, which we have already described. See “The kill-new
function”, page 97.)(or kill-ring (error "Kill ring is empty"))(let ((ARGth-kill-element(nthcdr (mod (- n (length kill-ring-yank-pointer))(length kill-ring))
kill-ring)))(or do-not-move(setq kill-ring-yank-pointer ARGth-kill-element))(car ARGth-kill-element))

The code first checks whether the kill ring has content; otherwise it signals an error.
Note that the or expression is very similar to testing length with an if:(if (zerop (length kill-ring))(error "Kill ring is empty"));; No else-part; if-part; then-part

If there is not anything in the kill ring, its length must be zero and an error message
sent to the user: ‘Kill ring is empty’. The current-kill function uses an or
expression which is simpler. But an if expression reminds us what goes on.
This if expression uses the function zerop which returns true if the value it
is testing is zero. When zerop tests true, the then-part of the if is evaluated.
The then-part is a list starting with the function error, which is a function thatis similar to the message function (see Section 1.8.5 “The message Function”,
page 14) in that it prints a one-line message in the echo area. However, in addition
to printing a message, error also stops evaluation of the function within which it

228

Appendix B: Handling the Kill Ringis embedded. This means that the rest of the function will not be evaluated if thelength of the kill ring is zero.
Then the current-kill function selects the element to return. The selection depends on the number of places that current-kill rotates and on where kill-ringyank-pointer points.
Next, either the optional do-not-move argument is true or the current value ofkill-ring-yank-pointer is set to point to the list. Finally, another expression
returns the first element of the list even if the do-not-move argument is true.
In my opinion, it is slightly misleading, at least to humans, to use the term
“error” as the name of the error function. A better term would be “cancel”.
Strictly speaking, of course, you cannot point to, much less rotate a pointer to alist that has no length, so from the point of view of the computer, the word “error”
is correct. But a human expects to attempt this sort of thing, if only to find out
whether the kill ring is full or empty. This is an act of exploration.
From the human point of view, the act of exploration and discovery is notnecessarily an error, and therefore should not be labeled as one, even in the bowels
of a computer. As it is, the code in Emacs implies that a human who is acting
virtuously, by exploring his or her environment, is making an error. This is bad.
Even though the computer takes the same steps as it does when there is an error,a term such as “cancel” would have a clearer connotation.
Among other actions, the else-part of the if expression sets the value ofkill-ring-yank-pointer to ARGth-kill-element when the kill ring has something in it and the value of do-not-move is nil.
The code looks like this:(nthcdr (mod (- n (length kill-ring-yank-pointer))(length kill-ring))
kill-ring)))

This needs some examination. Unless it is not supposed to move the pointer,the current-kill function changes where kill-ring-yank-pointer points. That
is what the (setq kill-ring-yank-pointer ARGth-kill-element)) expression
does. Also, clearly, ARGth-kill-element is being set to be equal to some cdrof the kill ring, using the nthcdr function that is described in an earlier section.
(See Section 8.3 “copy-region-as-kill”, page 93.) How does it do this?
As we have seen before (see Section 7.3 “nthcdr”, page 80), the nthcdr function
works by repeatedly taking the cdr of a list—it takes the cdr of the cdr of the
cdr . . .

Section B.1: The current-kill Function

229

The two following expressions produce the same result:(setq kill-ring-yank-pointer (cdr kill-ring))(setq kill-ring-yank-pointer (nthcdr 1 kill-ring))

However, the nthcdr expression is more complicated. It uses the mod function
to determine which cdr to select.
(You will remember to look at inner functions first; indeed, we will have to go
inside the mod.)The mod function returns the value of its first argument modulo the second; thatis to say, it returns the remainder after dividing the first argument by the second.
The value returned has the same sign as the second argument.
Thus,(mod 12 4)⇒ 0 ;; because there is no remainder(mod 13 4)⇒ 1

In this case, the first argument is often smaller than the second. That is fine.
(mod 0 4)⇒ 0(mod 1 4)⇒ 1

We can guess what the - function does. It is like + but subtracts instead of adds;
the - function subtracts its second argument from its first. Also, we already knowwhat the length function does (see Section 7.2.1 “length”, page 79). It returns
the length of a list.
And n is the name of the required argument to the current-kill function.
So when the first argument to nthcdr is zero, the nthcdr expression returns thewhole list, as you can see by evaluating the following:;; kill-ring-yank-pointer and kill-ring have a length of four;; and (mod (- 0 4) 4) ⇒ 0(nthcdr (mod (- 0 4) 4)'("fourth line of text""third line""second piece of text""first some text"))

When the first argument to the current-kill function is one, the nthcdr expression returns the list without its first element.
(nthcdr (mod (- 1 4) 4)'("fourth line of text""third line""second piece of text""first some text"))

Incidentally, both kill-ring and kill-ring-yank-pointer are global variables. That means that any expression in Emacs Lisp can access them. They are
not like the local variables set by let or like the symbols in an argument list. Local
variables can only be accessed within the let that defines them or the function thatspecifies them in an argument list (and within expressions called by them).

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(See Section 3.5 “let Prevents Confusion”, page 32, and Section 3.1 “Thedefun Macro”, page 26.)

B.2 yank
After learning about current-kill, the code for the yank function is almost easy.
The yank function does not use the kill-ring-yank-pointer variable directly.
It calls insert-for-yank which calls current-kill which sets the kill-ringyank-pointer variable.
The code looks like this:(defun yank (&optional arg)
"Reinsert (\"paste\") the last stretch of killed text.
More precisely, reinsert the stretch of killed text most recently
killed OR yanked. Put point at end, and set mark at beginning.
With just \\[universal-argument] as argument, same but put point at beginning (and mark at end).
With argument N, reinsert the Nth most recently killed stretch of killed
text.
When this command inserts killed text into the buffer, it honors`yank-excluded-properties' and `yank-handler' as described in the
doc string for `insert-for-yank-1', which see.
See also the command `yank-pop' (\\[yank-pop])."(interactive "*P")(setq yank-window-start (window-start));; If we don't get all the way thru, make last-command indicate that;; for the following command.
(setq this-command t)(push-mark (point))(insert-for-yank (current-kill (cond((listp arg) 0)((eq arg '-) -2)(t (1- arg)))))(if (consp arg);; This is like exchange-point-and-mark, but doesn't activate the mark.
;; It is cleaner to avoid activation, even though the command;; loop would deactivate the mark because we inserted text.
(goto-char (prog1 (mark t)(set-marker (mark-marker) (point) (current-buffer)))));; If we do get all the way thru, make this-command indicate that.
(if (eq this-command t)(setq this-command 'yank))nil)

The key expression is insert-for-yank, which inserts the string returned bycurrent-kill, but removes some text properties from it.
However, before getting to that expression, the function sets the value ofyank-window-start to the position returned by the (window-start) expression,
the position at which the display currently starts. The yank function also setsthis-command and pushes the mark.

Section B.3: yank-pop

231

After it yanks the appropriate element, if the optional argument is a cons rather
than a number or nothing, it puts point at beginning of the yanked text and markat its end.
(The prog1 function is like progn but returns the value of its first argument
rather than the value of its last argument. Its first argument is forced to return thebuffer’s mark as an integer. You can see the documentation for these functions byplacing point over them in this buffer and then typing C-h f (describe-function)
followed by a RET; the default is the function.)
The last part of the function tells what to do when it succeeds.

B.3 yank-pop
After understanding yank and current-kill, you know how to approach theyank-pop function. Leaving out the documentation to save space, it looks likethis:(defun yank-pop (&optional arg)"..."
(interactive "*p")(if (not (eq last-command 'yank))(error "Previous command was not a yank"))(setq this-command 'yank)(unless arg (setq arg 1))(let ((inhibit-read-only t)(before (< (point) (mark t))))(if before(funcall (or yank-undo-function 'delete-region) (point) (mark t))(funcall (or yank-undo-function 'delete-region) (mark t) (point)))(setq yank-undo-function nil)(set-marker (mark-marker) (point) (current-buffer))(insert-for-yank (current-kill arg));; Set the window start back where it was in the yank command,;; if possible.
(set-window-start (selected-window) yank-window-start t)(if before;; This is like exchange-point-and-mark,;;but doesn't activate the mark.
;; It is cleaner to avoid activation, even though the command;; loop would deactivate the mark because we inserted text.
(goto-char (prog1 (mark t)(set-marker (mark-marker)(point)(current-buffer))))))
nil)

The function is interactive with a small ‘p’ so the prefix argument is processedand passed to the function. The command can only be used after a previous yank;otherwise an error message is sent. This check uses the variable last-command which
is set by yank and is discussed elsewhere. (See Section 8.3 “copy-region-as-kill”,
page 93.)

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The let clause sets the variable before to true or false depending whether point
is before or after mark and then the region between point and mark is deleted. Thisis the region that was just inserted by the previous yank and it is this text that will
be replaced.
funcall calls its first argument as a function, passing remaining arguments to
it. The first argument is whatever the or expression returns. The two remainingarguments are the positions of point and mark set by the preceding yank command.
There is more, but that is the hardest part.

B.4 The ring.el File
Interestingly, GNU Emacs possesses a file called ring.el that provides many of thefeatures we just discussed. But functions such as kill-ring-yank-pointer do not
use this library, possibly because they were written earlier.

233

Appendix C A Graph with Labeled Axes
Printed axes help you understand a graph. They convey scale. In an earlier chapter(see Chapter 15 “Readying a Graph”, page 187), we wrote the code to print the
body of a graph. Here we write the code for printing and labeling vertical andhorizontal axes, along with the body itself.
Since insertions fill a buffer to the right and below point, the new graph printing
function should first print the Y or vertical axis, then the body of the graph, andfinally the X or horizontal axis. This sequence lays out for us the contents of thefunction:
1. Set up code.
2. Print Y axis.
3. Print body of graph.
4. Print X axis.
Here is an example of how a finished graph should look:
10 *
* *
* **
* ***
5 *
******** *** ***********************************
1 - ****************
|
|
|
|
1
5
10
15

In this graph, both the vertical and the horizontal axes are labeled with numbers.
However, in some graphs, the horizontal axis is time and would be better labeledwith months, like this:5 -** ** ****************** **1 - **************|^|Jan JuneJan

Indeed, with a little thought, we can easily come up with a variety of vertical andhorizontal labeling schemes. Our task could become complicated. But complications
breed confusion. Rather than permit this, it is better choose a simple labelingscheme for our first effort, and to modify or replace it later.

234

Appendix C: A Graph with Labeled Axes
These considerations suggest the following outline for the print-graph function:(defun print-graph (numbers-list)"documentation..."(let ((height ...
...))(print-Y-axis height ... )(graph-body-print numbers-list)(print-X-axis ... )))We can work on each part of the print-graph function definition in turn.

C.1 The print-graph Varlist
In writing the print-graph function, the first task is to write the varlist in the let
expression. (We will leave aside for the moment any thoughts about making thefunction interactive or about the contents of its documentation string.)
The varlist should set several values. Clearly, the top of the label for the verticalaxis must be at least the height of the graph, which means that we must obtain
this information here. Note that the print-graph-body function also requires thisinformation. There is no reason to calculate the height of the graph in two different
places, so we should change print-graph-body from the way we defined it earlier
to take advantage of the calculation.
Similarly, both the function for printing the X axis labels and the print-graphbody function need to learn the value of the width of each symbol. We can performthe calculation here and change the definition for print-graph-body from the way
we defined it in the previous chapter.
The length of the label for the horizontal axis must be at least as long as thegraph. However, this information is used only in the function that prints the horizontal axis, so it does not need to be calculated here.
These thoughts lead us directly to the following form for the varlist in the letfor print-graph:(let ((height (apply 'max numbers-list)) ; First version.
(symbol-width (length graph-blank)))

As we shall see, this expression is not quite right.

C.2 The print-Y-axis Function
The job of the print-Y-axis function is to print a label for the vertical axis thatlooks like this:
10 -

5 -

1 -

Section C.2: The print-Y-axis Function

235

The function should be passed the height of the graph, and then should construct
and insert the appropriate numbers and marks.
It is easy enough to see in the figure what the Y axis label should look like; but
to say in words, and then to write a function definition to do the job is anothermatter. It is not quite true to say that we want a number and a tic every five lines:
there are only three lines between the ‘1’ and the ‘5’ (lines 2, 3, and 4), but fourlines between the ‘5’ and the ‘10’ (lines 6, 7, 8, and 9). It is better to say that wewant a number and a tic mark on the base line (number 1) and then that we want
a number and a tic on the fifth line from the bottom and on every line that is amultiple of five.
The next issue is what height the label should be? Suppose the maximum heightof tallest column of the graph is seven. Should the highest label on the Y axis be ‘5-’, and should the graph stick up above the label? Or should the highest label be‘7 -’, and mark the peak of the graph? Or should the highest label be 10 -, which
is a multiple of five, and be higher than the topmost value of the graph?
The latter form is preferred. Most graphs are drawn within rectangles whosesides are an integral number of steps long—5, 10, 15, and so on for a step distance offive. But as soon as we decide to use a step height for the vertical axis, we discoverthat the simple expression in the varlist for computing the height is wrong. The
expression is (apply 'max numbers-list). This returns the precise height, not the
maximum height plus whatever is necessary to round up to the nearest multiple offive. A more complex expression is required.
As usual in cases like this, a complex problem becomes simpler if it is divided
into several smaller problems.
First, consider the case when the highest value of the graph is an integral multipleof five—when it is 5, 10, 15, or some higher multiple of five. We can use this value
as the Y axis height.
A fairly simply way to determine whether a number is a multiple of five isto divide it by five and see if the division results in a remainder. If there is no
remainder, the number is a multiple of five. Thus, seven divided by five has aremainder of two, and seven is not an integral multiple of five. Put in slightly
different language, more reminiscent of the classroom, five goes into seven once,with a remainder of two. However, five goes into ten twice, with no remainder: ten
is an integral multiple of five.
C.2.1 Side Trip: Compute a Remainder
In Lisp, the function for computing a remainder is %. The function returns theremainder of its first argument divided by its second argument. As it happens,% is a function in Emacs Lisp that you cannot discover using apropos: you findnothing if you type M-x apropos RET remainder RET. The only way to learn of theexistence of % is to read about it in a book such as this or in the Emacs Lisp sources.
You can try the % function by evaluating the following two expressions:(% 7 5)(% 10 5)

236

Appendix C: A Graph with Labeled Axes

The first expression returns 2 and the second expression returns 0.
To test whether the returned value is zero or some other number, we can use thezerop function. This function returns t if its argument, which must be a number,is zero.
(zerop (% 7 5))⇒ nil(zerop (% 10 5))⇒ tThus, the following expression will return t if the height of the graph is evenly
divisible by five:(zerop (% height 5))(The value of height, of course, can be found from (apply 'max numbers-list).)On the other hand, if the value of height is not a multiple of five, we want toreset the value to the next higher multiple of five. This is straightforward arithmetic
using functions with which we are already familiar. First, we divide the value ofheight by five to determine how many times five goes into the number. Thus, fivegoes into twelve twice. If we add one to this quotient and multiply by five, we willobtain the value of the next multiple of five that is larger than the height. Five goes
into twelve twice. Add one to two, and multiply by five; the result is fifteen, whichis the next multiple of five that is higher than twelve. The Lisp expression for thisis:(* (1+ (/ height 5)) 5)

For example, if you evaluate the following, the result is 15:(* (1+ (/ 12 5)) 5)

All through this discussion, we have been using 5 as the value for spacing labelson the Y axis; but we may want to use some other value. For generality, we should
replace 5 with a variable to which we can assign a value. The best name I can thinkof for this variable is Y-axis-label-spacing.

Section C.2: The print-Y-axis Function

237

Using this term, and an if expression, we produce the following:(if (zerop (% height Y-axis-label-spacing))height;; else(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing))

This expression returns the value of height itself if the height is an even multiple ofthe value of the Y-axis-label-spacing or else it computes and returns a value ofheight that is equal to the next higher multiple of the value of the Y-axis-labelspacing.
We can now include this expression in the let expression of the print-graphfunction (after first setting the value of Y-axis-label-spacing):(defvar Y-axis-label-spacing 5"Number of lines from one Y axis label to next.")...
(let* ((height (apply 'max numbers-list))(height-of-top-line(if (zerop (% height Y-axis-label-spacing))height;; else(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing)))(symbol-width (length graph-blank))))...

(Note use of the let* function: the initial value of height is computed once by the(apply 'max numbers-list) expression and then the resulting value of height isused to compute its final value. See “The let* expression”, page 148, for more
about let*.)C.2.2 Construct a Y Axis Element
When we print the vertical axis, we want to insert strings such as ‘5 -’ and ‘10 - ’
every five lines. Moreover, we want the numbers and dashes to line up, so shorter
numbers must be padded with leading spaces. If some of the strings use two digit
numbers, the strings with single digit numbers must include a leading blank space
before the number.
To figure out the length of the number, the length function is used. But thelength function works only with a string, not with a number. So the number has
to be converted from being a number to being a string. This is done with thenumber-to-string function. For example,(length (number-to-string 35))
⇒ 2
(length (number-to-string 100))⇒ 3(number-to-string is also called int-to-string; you will see this alternative namein various sources.)

238

Appendix C: A Graph with Labeled Axes

In addition, in each label, each number is followed by a string such as ‘ - ’, which
we will call the Y-axis-tic marker. This variable is defined with defvar:(defvar Y-axis-tic " - ""String that follows number in a Y axis label.")The length of the Y label is the sum of the length of the Y axis tic mark andthe length of the number of the top of the graph.
(length (concat (number-to-string height) Y-axis-tic)))

This value will be calculated by the print-graph function in its varlist as
full-Y-label-width and passed on. (Note that we did not think to include thisin the varlist when we first proposed it.)
To make a complete vertical axis label, a tic mark is concatenated with a number;and the two together may be preceded by one or more spaces depending on how
long the number is. The label consists of three parts: the (optional) leading spaces,
the number, and the tic mark. The function is passed the value of the number forthe specific row, and the value of the width of the top line, which is calculated (justonce) by print-graph.
(defun Y-axis-element (number full-Y-label-width)"Construct a NUMBERed label element.
A numbered element looks like this ` 5 - ',and is padded as needed so all line up withthe element for the largest number."(let* ((leading-spaces(- full-Y-label-width(length(concat (number-to-string number)
Y-axis-tic)))))(concat(make-string leading-spaces ? )(number-to-string number)
Y-axis-tic)))

The Y-axis-element function concatenates together the leading spaces, if any;
the number, as a string; and the tic mark.
To figure out how many leading spaces the label will need, the function subtracts
the actual length of the label—the length of the number plus the length of the tic
mark—from the desired label width.
Blank spaces are inserted using the make-string function. This function takes
two arguments: the first tells it how long the string will be and the second is asymbol for the character to insert, in a special format. The format is a question
mark followed by a blank space, like this, ‘? ’. See Section “Character Type” in The
GNU Emacs Lisp Reference Manual, for a description of the syntax for characters.
(Of course, you might want to replace the blank space by some other character . . .
You know what to do.)
The number-to-string function is used in the concatenation expression, toconvert the number to a string that is concatenated with the leading spaces and thetic mark.

Section C.2: The print-Y-axis Function

239

C.2.3 Create a Y Axis Column
The preceding functions provide all the tools needed to construct a function thatgenerates a list of numbered and blank strings to insert as the label for the verticalaxis:(defun Y-axis-column (height width-of-label)"Construct list of Y axis labels and blank strings.
For HEIGHT of line above base and WIDTH-OF-LABEL."(let (Y-axis)(while (> height 1)(if (zerop (% height Y-axis-label-spacing));; Insert label.
(setq Y-axis(cons(Y-axis-element height width-of-label)
Y-axis));; Else, insert blanks.
(setq Y-axis(cons(make-string width-of-label ? )Y-axis)))(setq height (1- height)));; Insert base line.
(setq Y-axis(cons (Y-axis-element 1 width-of-label) Y-axis))(nreverse Y-axis)))In this function, we start with the value of height and repetitively subtract
one from its value. After each subtraction, we test to see whether the value is an
integral multiple of the Y-axis-label-spacing. If it is, we construct a numberedlabel using the Y-axis-element function; if not, we construct a blank label using
the make-string function. The base line consists of the number one followed by atic mark.

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Appendix C: A Graph with Labeled Axes

C.2.4 The Not Quite Final Version of print-Y-axis
The list constructed by the Y-axis-column function is passed to the print-Y-axis
function, which inserts the list as a column.
(defun print-Y-axis (height full-Y-label-width)"Insert Y axis using HEIGHT and FULL-Y-LABEL-WIDTH.
Height must be the maximum height of the graph.
Full width is the width of the highest label element.";; Value of height and full-Y-label-width;; are passed by print-graph.
(let ((start (point)))(insert-rectangle(Y-axis-column height full-Y-label-width));; Place point ready for inserting graph.
(goto-char start);; Move point forward by value of full-Y-label-width(forward-char full-Y-label-width)))The print-Y-axis uses the insert-rectangle function to insert the Y axis
labels created by the Y-axis-column function. In addition, it places point at thecorrect position for printing the body of the graph.
You can test print-Y-axis:
1. InstallY-axis-label-spacing
Y-axis-tic
Y-axis-element
Y-axis-column
print-Y-axis

2. Copy the following expression:(print-Y-axis 12 5)

3. Switch to the *scratch* buffer and place the cursor where you want the axis
labels to start.
4. Type M-: (eval-expression).
5. Yank the graph-body-print expression into the minibuffer with C-y (yank).
6. Press RET to evaluate the expression.
Emacs will print labels vertically, the top one being ‘10 - ’. (The print-graphfunction will pass the value of height-of-top-line, which in this case will end up
as 15, thereby getting rid of what might appear as a bug.)Section C.3: The print-X-axis Function

241

C.3 The print-X-axis Function
X axis labels are much like Y axis labels, except that the ticks are on a line above
the numbers. Labels should look like this:
|
1

|
5

|
10

|
15

The first tic is under the first column of the graph and is preceded by several
blank spaces. These spaces provide room in rows above for the Y axis labels. The
second, third, fourth, and subsequent ticks are all spaced equally, according to thevalue of X-axis-label-spacing.
The second row of the X axis consists of numbers, preceded by several blankspaces and also separated according to the value of the variable X-axis-labelspacing.
The value of the variable X-axis-label-spacing should itself be measured inunits of symbol-width, since you may want to change the width of the symbols
that you are using to print the body of the graph without changing the ways thegraph is labeled.
The print-X-axis function is constructed in more or less the same fashion as
the print-Y-axis function except that it has two lines: the line of tic marks andthe numbers. We will write a separate function to print each line and then combine
them within the print-X-axis function.
This is a three step process:1. Write a function to print the X axis tic marks, print-X-axis-tic-line.
2. Write a function to print the X numbers, print-X-axis-numbered-line.
3. Write a function to print both lines, the print-X-axis function, using
print-X-axis-tic-line and print-X-axis-numbered-line.
C.3.1 X Axis Tic Marks
The first function should print the X axis tic marks. We must specify the tic marks
themselves and their spacing:(defvar X-axis-label-spacing(if (boundp 'graph-blank)(* 5 (length graph-blank)) 5)"Number of units from one X axis label to next.")(Note that the value of graph-blank is set by another defvar. The boundp
predicate checks whether it has already been set; boundp returns nil if it
has not. If graph-blank were unbound and we did not use this conditional
construction, we would enter the debugger and see an error message saying‘Debugger entered--Lisp error: (void-variable graph-blank)’.)242

Appendix C: A Graph with Labeled Axes
Here is the defvar for X-axis-tic-symbol:(defvar X-axis-tic-symbol "|""String to insert to point to a column in X axis.")The goal is to make a line that looks like this:
|

|

|

|

The first tic is indented so that it is under the first column, which is indented
to provide space for the Y axis labels.
A tic element consists of the blank spaces that stretch from one tic to the next
plus a tic symbol. The number of blanks is determined by the width of the tic
symbol and the X-axis-label-spacing.
The code looks like this:;;; X-axis-tic-element...
(concat(make-string;; Make a string of blanks.
(- (* symbol-width X-axis-label-spacing)(length X-axis-tic-symbol))? );; Concatenate blanks with tic symbol.
X-axis-tic-symbol)...

Next, we determine how many blanks are needed to indent the first tic mark to
the first column of the graph. This uses the value of full-Y-label-width passed
it by the print-graph function.
The code to make X-axis-leading-spaces looks like this:;; X-axis-leading-spaces...
(make-string full-Y-label-width ? )...

We also need to determine the length of the horizontal axis, which is the lengthof the numbers list, and the number of ticks in the horizontal axis:;; X-length...
(length numbers-list);; tic-width...
(* symbol-width X-axis-label-spacing);; number-of-X-ticks(if (zerop (% (X-length tic-width)))(/ (X-length tic-width))(1+ (/ (X-length tic-width))))Section C.3: The print-X-axis Function
All this leads us directly to the function for printing the X axis tic line:(defun print-X-axis-tic-line(number-of-X-tics X-axis-leading-spaces X-axis-tic-element)"Print ticks for X axis."(insert X-axis-leading-spaces)(insert X-axis-tic-symbol) ; Under first column.
;; Insert second tic in the right spot.
(insert (concat(make-string(- (* symbol-width X-axis-label-spacing);; Insert white space up to second tic symbol.
(* 2 (length X-axis-tic-symbol)))? )X-axis-tic-symbol));; Insert remaining ticks.
(while (> number-of-X-tics 1)(insert X-axis-tic-element)(setq number-of-X-tics (1- number-of-X-tics))))The line of numbers is equally straightforward:

243

244

Appendix C: A Graph with Labeled Axes
First, we create a numbered element with blank spaces before each number:(defun X-axis-element (number)"Construct a numbered X axis element."(let ((leading-spaces(- (* symbol-width X-axis-label-spacing)(length (number-to-string number)))))(concat (make-string leading-spaces ? )(number-to-string number))))Next, we create the function to print the numbered line, starting with the number 1 under the first column:(defun print-X-axis-numbered-line(number-of-X-tics X-axis-leading-spaces)"Print line of X-axis numbers"(let ((number X-axis-label-spacing))(insert X-axis-leading-spaces)(insert "1")(insert (concat(make-string;; Insert white space up to next number.
(- (* symbol-width X-axis-label-spacing) 2)? )(number-to-string number)));; Insert remaining numbers.
(setq number (+ number X-axis-label-spacing))(while (> number-of-X-tics 1)(insert (X-axis-element number))(setq number (+ number X-axis-label-spacing))(setq number-of-X-tics (1- number-of-X-tics)))))

Finally, we need to write the print-X-axis that uses print-X-axis-tic-line
and print-X-axis-numbered-line.
The function must determine the local values of the variables used by bothprint-X-axis-tic-line and print-X-axis-numbered-line, and then it must callthem. Also, it must print the carriage return that separates the two lines.
The function consists of a varlist that specifies five local variables, and calls toeach of the two line printing functions:(defun print-X-axis (numbers-list)"Print X axis labels to length of NUMBERS-LIST."(let* ((leading-spaces(make-string full-Y-label-width ? ));; symbol-width is provided by graph-body-print(tic-width (* symbol-width X-axis-label-spacing))(X-length (length numbers-list))(X-tic(concat(make-string;; Make a string of blanks.
(- (* symbol-width X-axis-label-spacing)(length X-axis-tic-symbol))? )Section C.4: Printing the Whole Graph

245;; Concatenate blanks with tic symbol.
X-axis-tic-symbol))(tic-number(if (zerop (% X-length tic-width))(/ X-length tic-width)(1+ (/ X-length tic-width)))))(print-X-axis-tic-line tic-number leading-spaces X-tic)(insert "\n")(print-X-axis-numbered-line tic-number leading-spaces)))

You can test print-X-axis:
1. Install X-axis-tic-symbol, X-axis-label-spacing, print-X-axis-ticline, as well as X-axis-element, print-X-axis-numbered-line, andprint-X-axis.
2. Copy the following expression:(progn(let ((full-Y-label-width 5)(symbol-width 1))(print-X-axis'(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16))))

3. Switch to the *scratch* buffer and place the cursor where you want the axis
labels to start.
4. Type M-: (eval-expression).
5. Yank the test expression into the minibuffer with C-y (yank).
6. Press RET to evaluate the expression.
Emacs will print the horizontal axis like this:
|
1

|
5

|
10

|
15

|
20

C.4 Printing the Whole Graph
Now we are nearly ready to print the whole graph.
The function to print the graph with the proper labels follows the outline wecreated earlier (see Appendix C “A Graph with Labeled Axes”, page 233), butwith additions.
Here is the outline:(defun print-graph (numbers-list)"documentation..."(let ((height ...
...))(print-Y-axis height ... )(graph-body-print numbers-list)(print-X-axis ... )))The final version is different from what we planned in two ways: first, it contains additional values calculated once in the varlist; second, it carries an option to
specify the labels’ increment per row. This latter feature turns out to be essential;
otherwise, a graph may have more rows than fit on a display or on a sheet of paper.

246

Appendix C: A Graph with Labeled Axes

This new feature requires a change to the Y-axis-column function, to add
vertical-step to it. The function looks like this:;;; Final version.
(defun Y-axis-column(height width-of-label &optional vertical-step)"Construct list of labels for Y axis.
HEIGHT is maximum height of graph.
WIDTH-OF-LABEL is maximum width of label.
VERTICAL-STEP, an option, is a positive integerthat specifies how much a Y axis label increments
for each line. For example, a step of 5 meansthat each line is five units of the graph."(let (Y-axis(number-per-line (or vertical-step 1)))(while (> height 1)(if (zerop (% height Y-axis-label-spacing));; Insert label.
(setq Y-axis(cons(Y-axis-element(* height number-per-line)
width-of-label)
Y-axis));; Else, insert blanks.
(setq Y-axis(cons(make-string width-of-label ? )Y-axis)))(setq height (1- height)));; Insert base line.
(setq Y-axis (cons (Y-axis-element(or vertical-step 1)
width-of-label)
Y-axis))(nreverse Y-axis)))The values for the maximum height of graph and the width of a symbol arecomputed by print-graph in its let expression; so graph-body-print must bechanged to accept them.
;;; Final version.
(defun graph-body-print (numbers-list height symbol-width)"Print a bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values.
HEIGHT is maximum height of graph.
SYMBOL-WIDTH is number of each column."(let (from-position)(while numbers-list(setq from-position (point))(insert-rectangle(column-of-graph height (car numbers-list)))(goto-char from-position)(forward-char symbol-width)

Section C.4: Printing the Whole Graph247;; Draw graph column by column.
(sit-for 0)(setq numbers-list (cdr numbers-list)));; Place point for X axis labels.
(forward-line height)(insert "\n")))Finally, the code for the print-graph function:;;; Final version.
(defun print-graph(numbers-list &optional vertical-step)"Print labeled bar graph of the NUMBERS-LIST.
The numbers-list consists of the Y-axis values.
Optionally, VERTICAL-STEP, a positive integer,
specifies how much a Y axis label increments foreach line. For example, a step of 5 means thateach row is five units."(let* ((symbol-width (length graph-blank));; height is both the largest number;; and the number with the most digits.
(height (apply 'max numbers-list))(height-of-top-line(if (zerop (% height Y-axis-label-spacing))height;; else(* (1+ (/ height Y-axis-label-spacing))
Y-axis-label-spacing)))(vertical-step (or vertical-step 1))(full-Y-label-width(length(concat(number-to-string(* height-of-top-line vertical-step))
Y-axis-tic))))(print-Y-axis
height-of-top-line full-Y-label-width vertical-step)(graph-body-print
numbers-list height-of-top-line symbol-width)(print-X-axis numbers-list)))C.4.1 Testing print-graphWe can test the print-graph function with a short list of numbers:
1. Install the final versions of Y-axis-column, graph-body-print, andprint-graph (in addition to the rest of the code.)
2. Copy the following expression:(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1))

3. Switch to the *scratch* buffer and place the cursor where you want the axis
labels to start.

248

Appendix C: A Graph with Labeled Axes

4. Type M-: (eval-expression).
5. Yank the test expression into the minibuffer with C-y (yank).
6. Press RET to evaluate the expression.
Emacs will print a graph that looks like this:
10 -

*
**
*
5 **** ***** **** *********************
1 - *************
|
1

|
5

|
10

|
15

On the other hand, if you pass print-graph a vertical-step value of 2, by
evaluating this expression:(print-graph '(3 2 5 6 7 5 3 4 6 4 3 2 1) 2)

The graph looks like this:
20 -

*
**
*
10 **** ***** **** *********************
2 - *************
|
1

|
5

|
10

|
15

(A question: is the ‘2’ on the bottom of the vertical axis a bug or a feature? If you
think it is a bug, and should be a ‘1’ instead, (or even a ‘0’), you can modify thesources.)C.4.2 Graphing Numbers of Words and Symbols
Now for the graph for which all this code was written: a graph that shows how many
function definitions contain fewer than 10 words and symbols, how many containbetween 10 and 19 words and symbols, how many contain between 20 and 29 words
and symbols, and so on.
This is a multi-step process. First make sure you have loaded all the requisitecode.

Section C.4: Printing the Whole Graph249

It is a good idea to reset the value of top-of-ranges in case you have set it tosome different value. You can evaluate the following:(setq top-of-ranges'(10 20 30 40 50
60 70 80 90 100
110 120 130 140 150
160 170 180 190 200
210 220 230 240 250
260 270 280 290 300)

Next create a list of the number of words and symbols in each range.
Evaluate the following:(setq list-for-graph(defuns-per-range(sort(recursive-lengths-list-many-files(directory-files "/usr/local/emacs/lisp"
t ".+el$"))'<)top-of-ranges))

On my old machine, this took about an hour. It looked though 303 Lisp files in mycopy of Emacs version 19.23. After all that computing, the list-for-graph had
this value:(537 1027 955 785 594 483 349 292 224 199 166 120 116 99
90 80 67 48 52 45 41 33 28 26 25 20 12 28 11 13 220)

This means that my copy of Emacs had 537 function definitions with fewer than 10
words or symbols in them, 1,027 function definitions with 10 to 19 words or symbols
in them, 955 function definitions with 20 to 29 words or symbols in them, and soon.
Clearly, just by looking at this list we can see that most function definitions
contain ten to thirty words and symbols.
Now for printing. We do not want to print a graph that is 1,030 lines high . . .
Instead, we should print a graph that is fewer than twenty-five lines high. A graphthat height can be displayed on almost any monitor, and easily printed on a sheetof paper.
This means that each value in list-for-graph must be reduced to one-fiftieth
its present value.
Here is a short function to do just that, using two functions we have not yet
seen, mapcar and lambda.
(defun one-fiftieth (full-range)"Return list, each number one-fiftieth of previous."(mapcar (lambda (arg) (/ arg 50)) full-range))

C.4.3 A lambda Expression: Useful Anonymity
lambda is the symbol for an anonymous function, a function without a name. Everytime you use an anonymous function, you need to include its whole body.

250

Appendix C: A Graph with Labeled Axes

Thus,(lambda (arg) (/ arg 50))

is a function that returns the value resulting from dividing whatever is passed to it
as arg by 50.
Earlier, for example, we had a function multiply-by-seven; it multiplied its
argument by 7. This function is similar, except it divides its argument by 50; and,it has no name. The anonymous equivalent of multiply-by-seven is:(lambda (number) (* 7 number))(See Section 3.1 “The defun Macro”, page 26.)
If we want to multiply 3 by 7, we can write:(multiply-by-seven 3)

functionargument

This expression returns 21.
Similarly, we can write:((lambda (number) (* 7 number)) 3)

anonymous function

argument

If we want to divide 100 by 50, we can write:((lambda (arg) (/ arg 50)) 100)

anonymous function argument

This expression returns 2. The 100 is passed to the function, which divides that
number by 50.
See Section “Lambda Expressions” in The GNU Emacs Lisp Reference Manual,
for more about lambda. Lisp and lambda expressions derive from the Lambda
Calculus.
C.4.4 The mapcar Function
mapcar is a function that calls its first argument with each element of its second
argument, in turn. The second argument must be a sequence.

Section C.4: Printing the Whole Graph251

The ‘map’ part of the name comes from the mathematical phrase, “mapping overa domain”, meaning to apply a function to each of the elements in a domain. The
mathematical phrase is based on the metaphor of a surveyor walking, one step at atime, over an area he is mapping. And ‘car’, of course, comes from the Lisp notionof the first of a list.
For example,(mapcar '1+ '(2 4 6))⇒ (3 5 7)

The function 1+ which adds one to its argument, is executed on each element of thelist, and a new list is returned.
Contrast this with apply, which applies its first argument to all the remaining.
(See Chapter 15 “Readying a Graph”, page 187, for an explanation of apply.)In the definition of one-fiftieth, the first argument is the anonymous function:(lambda (arg) (/ arg 50))

and the second argument is full-range, which will be bound to list-for-graph.
The whole expression looks like this:(mapcar (lambda (arg) (/ arg 50)) full-range))

See Section “Mapping Functions” in The GNU Emacs Lisp Reference Manual,
for more about mapcar.
Using the one-fiftieth function, we can generate a list in which each element
is one-fiftieth the size of the corresponding element in list-for-graph.
(setq fiftieth-list-for-graph(one-fiftieth list-for-graph))

The resulting list looks like this:(10 20 19 15 11 9 6 5 4 3 3 2 2
1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 4)

This, we are almost ready to print! (We also notice the loss of information: many of
the higher ranges are 0, meaning that fewer than 50 defuns had that many words orsymbols—but not necessarily meaning that none had that many words or symbols.)
C.4.5 Another Bug . . . Most Insidious
I said “almost ready to print”! Of course, there is a bug in the print-graphfunction . . . It has a vertical-step option, but not a horizontal-step option.
The top-of-range scale goes from 10 to 300 by tens. But the print-graph function
will print only by ones.
This is a classic example of what some consider the most insidious type of bug,the bug of omission. This is not the kind of bug you can find by studying the code,for it is not in the code; it is an omitted feature. Your best actions are to try yourprogram early and often; and try to arrange, as much as you can, to write codethat is easy to understand and easy to change. Try to be aware, whenever you can,that whatever you have written, will be rewritten, if not soon, eventually. A hard
maxim to follow.
It is the print-X-axis-numbered-line function that needs the work; and thenthe print-X-axis and the print-graph functions need to be adapted. Not much

252

Appendix C: A Graph with Labeled Axes

needs to be done; there is one nicety: the numbers ought to line up under the tic
marks. This takes a little thought.
Here is the corrected print-X-axis-numbered-line:(defun print-X-axis-numbered-line(number-of-X-tics X-axis-leading-spaces&optional horizontal-step)"Print line of X-axis numbers"(let ((number X-axis-label-spacing)(horizontal-step (or horizontal-step 1)))
(insert X-axis-leading-spaces);; Delete extra leading spaces.
(delete-char(- (1(length (number-to-string horizontal-step)))))(insert (concat(make-string;; Insert white space.
(- (* symbol-width
X-axis-label-spacing)(1(length(number-to-string horizontal-step)))2)? )(number-to-string(* number horizontal-step))));; Insert remaining numbers.
(setq number (+ number X-axis-label-spacing))(while (> number-of-X-tics 1)(insert (X-axis-element(* number horizontal-step)))(setq number (+ number X-axis-label-spacing))(setq number-of-X-tics (1- number-of-X-tics)))))

If you are reading this in Info, you can see the new versions of print-X-axis
print-graph and evaluate them. If you are reading this in a printed book, you can
see the changed lines here (the full text is too much to print).
(defun print-X-axis (numbers-list horizontal-step)...
(print-X-axis-numbered-line
tic-number leading-spaces horizontal-step))(defun print-graph(numbers-list&optional vertical-step horizontal-step)...
(print-X-axis numbers-list horizontal-step))Section C.4: Printing the Whole Graph253

C.4.6 The Printed Graph
When made and installed, you can call the print-graph command like this:(print-graph fiftieth-list-for-graph 50 10)

Here is the graph:1000 -

*
**
**
**
**
750 - ***
***
***
***
****
500 - ******************************
250 - *****************************************
50 - ***************** *
|
|
|
|
|
10 50 100 150 200

|
250

*
*
*
*
|
|
300 350

The largest group of functions contain 10–19 words and symbols each.

254

Appendix D: Free Software and Free Manuals

Appendix D Free Software and Free Manuals
by Richard M. Stallman
The biggest deficiency in free operating systems is not in the software—it is thelack of good free manuals that we can include in these systems. Many of our mostimportant programs do not come with full manuals. Documentation is an essential
part of any software package; when an important free software package does notcome with a free manual, that is a major gap. We have many such gaps today.
Once upon a time, many years ago, I thought I would learn Perl. I got a copyof a free manual, but I found it hard to read. When I asked Perl users aboutalternatives, they told me that there were better introductory manuals—but thosewere not free.
Why was this? The authors of the good manuals had written them for O’Reilly
Associates, which published them with restrictive terms—no copying, no modification, source files not available—which exclude them from the free software community.
That wasn’t the first time this sort of thing has happened, and (to our community’s great loss) it was far from the last. Proprietary manual publishers haveenticed a great many authors to restrict their manuals since then. Many times I
have heard a GNU user eagerly tell me about a manual that he is writing, withwhich he expects to help the GNU project—and then had my hopes dashed, as heproceeded to explain that he had signed a contract with a publisher that would
restrict it so that we cannot use it.
Given that writing good English is a rare skill among programmers, we can ill
afford to lose manuals this way.
Free documentation, like free software, is a matter of freedom, not price. The
problem with these manuals was not that O’Reilly Associates charged a price forprinted copies—that in itself is fine. The Free Software Foundation sells printed
copies (https://shop.fsf.org) of free GNU manuals (https://www.gnu.org/
doc/doc.html), too. But GNU manuals are available in source code form, whilethese manuals are available only on paper. GNU manuals come with permission tocopy and modify; the Perl manuals do not. These restrictions are the problems.
The criterion for a free manual is pretty much the same as for free software: it isa matter of giving all users certain freedoms. Redistribution (including commercial
redistribution) must be permitted, so that the manual can accompany every copyof the program, on-line or on paper. Permission for modification is crucial too.
As a general rule, I don’t believe that it is essential for people to have permission
to modify all sorts of articles and books. The issues for writings are not necessarily
the same as those for software. For example, I don’t think you or I are obliged togive permission to modify articles like this one, which describe our actions and ourviews.
But there is a particular reason why the freedom to modify is crucial for documentation for free software. When people exercise their right to modify the software,and add or change its features, if they are conscientious they will change the man-

255
ual too—so they can provide accurate and usable documentation with the modified
program. A manual which forbids programmers to be conscientious and finish thejob, or more precisely requires them to write a new manual from scratch if theychange the program, does not fill our community’s needs.
While a blanket prohibition on modification is unacceptable, some kinds of limits
on the method of modification pose no problem. For example, requirements topreserve the original author’s copyright notice, the distribution terms, or the list of
authors, are ok. It is also no problem to require modified versions to include noticethat they were modified, even to have entire sections that may not be deleted
or changed, as long as these sections deal with nontechnical topics. (Some GNU
manuals have them.)
These kinds of restrictions are not a problem because, as a practical matter,they don’t stop the conscientious programmer from adapting the manual to fit themodified program. In other words, they don’t block the free software communityfrom making full use of the manual.
However, it must be possible to modify all the technical content of the manual,and then distribute the result in all the usual media, through all the usual channels;
otherwise, the restrictions do block the community, the manual is not free, and so
we need another manual.
Unfortunately, it is often hard to find someone to write another manual when aproprietary manual exists. The obstacle is that many users think that a proprietarymanual is good enough—so they don’t see the need to write a free manual. They
do not see that the free operating system has a gap that needs filling.
Why do users think that proprietary manuals are good enough? Some have notconsidered the issue. I hope this article will do something to change that.
Other users consider proprietary manuals acceptable for the same reason somany people consider proprietary software acceptable: they judge in purely practical terms, not using freedom as a criterion. These people are entitled to theiropinions, but since those opinions spring from values which do not include freedom,
they are no guide for those of us who do value freedom.
Please spread the word about this issue. We continue to lose manuals to proprietary publishing. If we spread the word that proprietary manuals are not sufficient,perhaps the next person who wants to help GNU by writing documentation willrealize, before it is too late, that he must above all make it free.
We can also encourage commercial publishers to sell free, copylefted manuals
instead of proprietary ones. One way you can help this is to check the distribution
terms of a manual before you buy it, and prefer copylefted manuals to non-copylefted
ones.

Note: The Free Software Foundation maintains a page on its Web site that lists
free books available from other publishers:
https://www.gnu.org/doc/other-free-books.html

256

Appendix E: GNU Free Documentation License

Appendix E GNU Free Documentation License
Version 1.3, 3 November 2008
Copyright c 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.
https://fsf.org/
Everyone is permitted to copy and distribute verbatim copiesof this license document, but changing it is not allowed.
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional
and useful document free in the sense of freedom: to assure everyone theeffective freedom to copy and redistribute it, with or without modifying it,either commercially or noncommercially. Secondarily, this License preserves
for the author and publisher a way to get credit for their work, while not being
considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the
document must themselves be free in the same sense. It complements the GNU
General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software,because free software needs free documentation: a free program should comewith manuals providing the same freedoms that the software does. But thisLicense is not limited to software manuals; it can be used for any textual work,
regardless of subject matter or whether it is published as a printed book. We
recommend this License principally for works whose purpose is instruction orreference.
1. APPLICABILITY AND DEFINITIONSThis License applies to any manual or other work, in any medium, that contains
a notice placed by the copyright holder saying it can be distributed under theterms of this License. Such a notice grants a world-wide, royalty-free license,
unlimited in duration, to use that work under the conditions stated herein.
The “Document”, below, refers to any such manual or work. Any member ofthe public is a licensee, and is addressed as “you”. You accept the license if
you copy, modify or distribute the work in a way requiring permission undercopyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/ortranslated into another language.
A “Secondary Section” is a named appendix or a front-matter section of theDocument that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters) and contains nothing that could fall directly within that overall subject.
(Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matterof historical connection with the subject or with related matters, or of legal,commercial, philosophical, ethical or political position regarding them.

257
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that theDocument is released under this License. If a section does not fit the above
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The Document may contain zero Invariant Sections. If the Document does not
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The “Cover Texts” are certain short passages of text that are listed, as FrontCover Texts or Back-Cover Texts, in the notice that says that the Document
is released under this License. A Front-Cover Text may be at most 5 words,
and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that issuitable for revising the document straightforwardly with generic text editorsor (for images composed of pixels) generic paint programs or (for drawings)
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Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTEX input format, SGML or XML using
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SGML or XML for which the DTD and/or processing tools are not generally
available, and the machine-generated HTML, PostScript or PDF produced bysome word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such
following pages as are needed to hold, legibly, the material this License requires
to appear in the title page. For works in formats which do not have any title
page as such, “Title Page” means the text near the most prominent appearanceof the work’s title, preceding the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of theDocument to the public.
A section “Entitled XYZ” means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following textthat translates XYZ in another language. (Here XYZ stands for a specificsection name mentioned below, such as “Acknowledgements”, “Dedications”,“Endorsements”, or “History”.) To “Preserve the Title” of such a section
when you modify the Document means that it remains a section “Entitled
XYZ” according to this definition.

258

Appendix E: GNU Free Documentation License
The Document may include Warranty Disclaimers next to the notice whichstates that this License applies to the Document. These Warranty Disclaimers
are considered to be included by reference in this License, but only as regards
disclaiming warranties: any other implication that these Warranty Disclaimers
may have is void and has no effect on the meaning of this License.

2. VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices,and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to thoseof this License. You may not use technical measures to obstruct or control
the reading or further copying of the copies you make or distribute. However,
you may accept compensation in exchange for copies. If you distribute a large
enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you
may publicly display copies.
3. COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed
covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers thatcarry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front
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present the full title with all words of the title equally prominent and visible.
You may add other material on the covers in addition. Copying with changes
limited to the covers, as long as they preserve the title of the Document andsatisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you
should put the first ones listed (as many as fit reasonably) on the actual cover,and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering morethan 100, you must either include a machine-readable Transparent copy alongwith each Opaque copy, or state in or with each Opaque copy a computernetwork location from which the general network-using public has access todownload using public-standard network protocols a complete Transparent
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distribute an Opaque copy (directly or through your agents or retailers) of that
edition to the public.
It is requested, but not required, that you contact the authors of the Document
well before redistributing any large number of copies, to give them a chance toprovide you with an updated version of the Document.

259
4. MODIFICATIONS
You may copy and distribute a Modified Version of the Document under theconditions of sections 2 and 3 above, provided that you release the Modified
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Version to whoever possesses a copy of it. In addition, you must do thesethings in the Modified Version:
A. Use in the Title Page (and on the covers, if any) a title distinct from thatof the Document, and from those of previous versions (which should, if
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C. State on the Title page the name of the publisher of the Modified Version,as the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to
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F. Include, immediately after the copyright notices, a license notice giving
the public permission to use the Modified Version under the terms of thisLicense, in the form shown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled “History”, Preserve its Title, and add toit an item stating at least the title, year, new authors, and publisher ofthe Modified Version as given on the Title Page. If there is no section
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J. Preserve the network location, if any, given in the Document for publicaccess to a Transparent copy of the Document, and likewise the network
locations given in the Document for previous versions it was based on.
These may be placed in the “History” section. You may omit a network
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K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve
the Title of the section, and preserve in the section all the substance260

Appendix E: GNU Free Documentation License
and tone of each of the contributor acknowledgements and/or dedications
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L. Preserve all the Invariant Sections of the Document, unaltered in theirtext and in their titles. Section numbers or the equivalent are not considered part of the section titles.
M. Delete any section Entitled “Endorsements”. Such a section may not be
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If the Modified Version includes new front-matter sections or appendices that
qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections asinvariant. To do this, add their titles to the list of Invariant Sections in the
Modified Version’s license notice. These titles must be distinct from any othersection titles.
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You may add a passage of up to five words as a Front-Cover Text, and apassage of up to 25 words as a Back-Cover Text, to the end of the list of CoverTexts in the Modified Version. Only one passage of Front-Cover Text and oneof Back-Cover Text may be added by (or through arrangements made by) any
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The author(s) and publisher(s) of the Document do not by this License give
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5. COMBINING DOCUMENTS
You may combine the Document with other documents released under thisLicense, under the terms defined in section 4 above for modified versions,provided that you include in the combination all of the Invariant Sectionsof all of the original documents, unmodified, and list them all as Invariant
Sections of your combined work in its license notice, and that you preserve all
their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple
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the name of the original author or publisher of that section if known, or else a

261
unique number. Make the same adjustment to the section titles in the list ofInvariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in thevarious original documents, forming one section Entitled “History”; likewise
combine any sections Entitled “Acknowledgements”, and any sections Entitled“Dedications”. You must delete all sections Entitled “Endorsements.”
6. COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents
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in the various documents with a single copy that is included in the collection,
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You may extract a single document from such a collection, and distribute it
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A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution
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this License does not apply to the other works in the aggregate which are notthemselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of theDocument, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket theDocument within the aggregate, or the electronic equivalent of covers if theDocument is in electronic form. Otherwise they must appear on printed covers
that bracket the whole aggregate.
8. TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders,
but you may include translations of some or all Invariant Sections in addition
to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any
Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers.
In case of a disagreement between the translation and the original version ofthis License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”,
or “History”, the requirement (section 4) to Preserve its Title (section 1) will
typically require changing the actual title.

262

Appendix E: GNU Free Documentation License

9. TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as
expressly provided under this License. Any attempt otherwise to copy, modify,
sublicense, or distribute it is void, and will automatically terminate your rights
under this License.
However, if you cease all violation of this License, then your license from aparticular copyright holder is reinstated (a) provisionally, unless and until the
copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by somereasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonablemeans, this is the first time you have received notice of violation of this License(for any work) from that copyright holder, and you cure the violation prior to
30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licensesof parties who have received copies or rights from you under this License. If
your rights have been terminated and not permanently reinstated, receipt of acopy of some or all of the same material does not give you any rights to use it.
10. FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU
Free Documentation License from time to time. Such new versions will besimilar in spirit to the present version, but may differ in detail to address new
problems or concerns. See https://www.gnu.org/licenses/.
Each version of the License is given a distinguishing version number. If theDocument specifies that a particular numbered version of this License “orany later version” applies to it, you have the option of following the terms
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any version ever published (not as a draft) by the Free Software Foundation.
If the Document specifies that a proxy can decide which future versions of thisLicense can be used, that proxy’s public statement of acceptance of a version
permanently authorizes you to choose that version for the Document.
11. RELICENSING“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World
Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can
edit is an example of such a server. A “Massive Multiauthor Collaboration”(or “MMC”) contained in the site means any set of copyrightable works thus
published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license
published by Creative Commons Corporation, a not-for-profit corporation with263
a principal place of business in San Francisco, California, as well as futurecopyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part,as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, andif all works that were first published under this License somewhere other than
this MMC, and subsequently incorporated in whole or in part into the MMC,
(1) had no cover texts or invariant sections, and (2) were thus incorporated
prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009, provided
the MMC is eligible for relicensing.

264

Appendix E: GNU Free Documentation License

ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License
in the document and put the following copyright and license notices just after thetitle page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;with no Invariant Sections, no Front-Cover Texts, and no Back-CoverTexts. A copy of the license is included in the section entitled ``GNUFree Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace
the “with. . . Texts.” line with this:with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.

If you have Invariant Sections without Cover Texts, or some other combinationof the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend
releasing these examples in parallel under your choice of free software license, suchas the GNU General Public License, to permit their use in free software.

265

Index
%

A

% (remainder function) . . . . . . . . . . . . . . . 235

Accumulate, type ofrecursive pattern . . . . . . . . . . . . . . . . . . . 135
add-hook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101, 149
Anonymous function . . . . . . . . . . . . . . . . . 249
apostrophe for quoting . . . . . . . . . . . . . . . . . . 3
append-to-buffer . . . . . . . . . . . . . . . . . . . . . 52
apply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
apropos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
Argument as local variable . . . . . . . . . . . 124‘argument’ defined . . . . . . . . . . . . . . . . . . . . . 11
‘argument list’ defined . . . . . . . . . . . . . . . 27
Argument, wrong type of . . . . . . . . . . . . . . 13
Arguments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Arguments’ data types. . . . . . . . . . . . . . . . . 12
Arguments, variable number of . . . . . . . . 13
Asterisk for read-only buffer . . . . . . . . . . . 61
Auto Fill mode turned on . . . . . . . . . . . . 201
autoload . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
Automatic mode selection . . . . . . . . . . . . 200
Axis, print horizontal . . . . . . . . . . . . . . . . . 241
Axis, print vertical . . . . . . . . . . . . . . . . . . . 234’' for quoting . . . . . . . . . . . . . . . . . . . . . . . . . . . 3((debug) in code . . . . . . . . . . . . . . . . . . . . . . 218** (multiplication) . . . . . . . . . . . . . . . . . . . . . . 27
* for read-only buffer . . . . . . . . . . . . . . . . . . 61
*scratch* buffer . . . . . . . . . . . . . . . . . . . . . 117.
.emacs file . . . . . . . . . . . . . . . . . . . . . . . . . . . 196.emacs file, beginning of . . . . . . . . . . . . . . 199// (division) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69<<= (less than or equal) . . . . . . . . . . . . . . . . 120>> (greater than) . . . . . . . . . . . . . . . . . . . . . . . 38

B
beginning-of-buffer . . . . . . . . . . . . . . . . . 65
‘bind’ defined . . . . . . . . . . . . . . . . . . . . . . . . . 16
Binding, dynamic . . . . . . . . . . . . . . . . . . . . . . 36
Binding, lexical . . . . . . . . . . . . . . . . . . . . . . . . 36
Bindings, key, fixing unpleasant . . . . . . . 211‘body’ defined . . . . . . . . . . . . . . . . . . . . . . . . . 26
Body of graph . . . . . . . . . . . . . . . . . . . . . . . . 187
Buffer size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Buffer, history of word . . . . . . . . . . . . . . . . . 21
buffer-file-name . . . . . . . . . . . . . . . . . . . . . 20
buffer-menu, bound to key . . . . . . . . . . . 204
buffer-name . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Bug, most insidious type . . . . . . . . . . . . . 251
Building robots. . . . . . . . . . . . . . . . . . . . . . . 127
Byte compiling . . . . . . . . . . . . . . . . . . . . . . . . . 7

266

Index

C

defsubst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
defun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
defvar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
defvar for a user
customizable variable. . . . . . . . . . . . . . . 105
defvar with an asterisk . . . . . . . . . . . . . . 105
delete-and-extract-region . . . . . . . . . 102
Deleting text . . . . . . . . . . . . . . . . . . . . . . . . . . 85
describe-function . . . . . . . . . . . . . . . . . . . 50
describe-function, introduced. . . . . . . . 48
Digression into C . . . . . . . . . . . . . . . . . . . . . 102
directory-files . . . . . . . . . . . . . . . . . . . . . 179
Division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
dolist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
dotimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
dotted pair . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Drawers, Chest of, metaphor
for a symbol . . . . . . . . . . . . . . . . . . . . . . . 111
Duplicated words function . . . . . . . . . . . . 223
Dynamic binding . . . . . . . . . . . . . . . . . . . . . . 36

C language primitives . . . . . . . . . . . . . . . . . 26
C, a digression into . . . . . . . . . . . . . . . . . . . 102‘call’ defined . . . . . . . . . . . . . . . . . . . . . . . . . 24
cancel-debug-on-entry . . . . . . . . . . . . . . 218
car, introduced . . . . . . . . . . . . . . . . . . . . . . . . 77
cdr, introduced . . . . . . . . . . . . . . . . . . . . . . . . 77
Changing a function definition . . . . . . . . . 29
Chest of Drawers, metaphor
for a symbol . . . . . . . . . . . . . . . . . . . . . . . 111
Clipping text . . . . . . . . . . . . . . . . . . . . . . . . . . 85
Code installation . . . . . . . . . . . . . . . . . . . . . . 32
‘command’ defined . . . . . . . . . . . . . . . . . . . . . . 20
Comments in Lisp code . . . . . . . . . . . . . . . . 29
Common Lisp . . . . . . . . . . . . . . . . . . . . . . . . . . 3
compare-windows . . . . . . . . . . . . . . . . . . . . . 203
concat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
cond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
condition-case . . . . . . . . . . . . . . . . . . . . . . . 91
Conditional ’twixt two
versions of Emacs . . . . . . . . . . . . . . . . . . 208
Conditional with if . . . . . . . . . . . . . . . . . . . 37
cons cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
cons, introduced . . . . . . . . . . . . . . . . . . . . . . 78
copy-region-as-kill . . . . . . . . . . . . . . . . . 93
copy-to-buffer . . . . . . . . . . . . . . . . . . . . . . . 59
Count words recursively . . . . . . . . . . . . . . 161
count-words-example . . . . . . . . . . . . . . . . 155
count-words-in-defun . . . . . . . . . . . . . . . 171
Counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Counting words in a defun . . . . . . . 167, 169
curly quotes . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
current-buffer . . . . . . . . . . . . . . . . . . . . . . . 22
current-kill . . . . . . . . . . . . . . . . . . . . . . . . 225
curved quotes . . . . . . . . . . . . . . . . . . . . . . . . . 86
Customizing your .emacs file . . . . . . . . . 196
Cutting and storing text . . . . . . . . . . . . . . . 85

D
Data types . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
debug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
debug-on-entry . . . . . . . . . . . . . . . . . . . . . . 216
debug-on-quit . . . . . . . . . . . . . . . . . . . . . . . 218
debugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
default.el init file . . . . . . . . . . . . . . . . . . . 196
defconst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
defcustom. . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
Deferment in recursion . . . . . . . . . . . . . . . 136
Definition installation . . . . . . . . . . . . . . . . . 28
Definition writing . . . . . . . . . . . . . . . . . . . . . 26
Definition, how to change . . . . . . . . . . . . . . 29

Eedebug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
Else . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Emacs version, choosing . . . . . . . . . . . . . . 208‘empty list’ defined . . . . . . . . . . . . . . . . . . . . 2‘empty string’ defined . . . . . . . . . . . . . . . . . 46
eobp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
eq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
eq (example of use) . . . . . . . . . . . . . . . . . . . . 95
equal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Erasing text . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
Error for symbol without function . . . . . 10
Error for symbol without value . . . . . . . . 10
Error message generation . . . . . . . . . . . . . . . 4‘evaluate’ defined . . . . . . . . . . . . . . . . . . . . . . 3
Evaluating inner lists . . . . . . . . . . . . . . . . . . . 8
Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Evaluation practice . . . . . . . . . . . . . . . . . . . . 20
Every, type of recursive pattern . . . . . . . 133
Example variable, fill-column . . . . . . . . 10
‘expression’ defined . . . . . . . . . . . . . . . . . . . . 2

267

F

I

Falsehood and truth in Emacs Lisp . . . . 41
FDL, GNU Free
Documentation License . . . . . . . . . . . . . 256
files-in-below-directory . . . . . . . . . . 179
fill-column, an example variable . . . . . 10
filter-buffer-substring. . . . . . . . . . . . . 95
Find a File . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
Find function documentation . . . . . . . . . . 48
Find source of function . . . . . . . . . . . . . . . . 48
Flowers in a field . . . . . . . . . . . . . . . . . . . . . . . 1
Focusing attention (narrowing) . . . . . . . . 73
‘form’ defined . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Formatting convention . . . . . . . . . . . . . . . . . 55
Formatting help . . . . . . . . . . . . . . . . . . . . . . . . 3
forward-paragraph . . . . . . . . . . . . . . . . . . 147
forward-sentence . . . . . . . . . . . . . . . . . . . 143‘function’ defined . . . . . . . . . . . . . . . . . . . . . . 5‘function definition’ defined. . . . . . . . . 26
Function definition installation. . . . . . . . . 28
Function definition writing. . . . . . . . . . . . . 26
Function definition, how to change . . . . . 29
Functions, primitive . . . . . . . . . . . . . . . . . . . 26

if . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
‘if-part’ defined . . . . . . . . . . . . . . . . . . . . . . 38
indent-tabs-mode . . . . . . . . . . . . . . . . . . . 202
Indentation for formatting . . . . . . . . . . . . . 55
Initialization file . . . . . . . . . . . . . . . . . . . . . . 196
Initializing a variable . . . . . . . . . . . . . . . . . 104
Inner list evaluation . . . . . . . . . . . . . . . . . . . . 8
insert-buffer . . . . . . . . . . . . . . . . . . . . . . . . 60
insert-buffer, new version body . . . . . 65
insert-buffer-substring. . . . . . . . . . . . . 52
Insidious type of bug . . . . . . . . . . . . . . . . . 251
Install a Function Definition . . . . . . . . . . . 28
Install code permanently. . . . . . . . . . . . . . . 32
interactive . . . . . . . . . . . . . . . . . . . . . . . . . . 29
‘interactive function’ defined . . . . . . . 20
Interactive functions . . . . . . . . . . . . . . . . . . . 29
Interactive options . . . . . . . . . . . . . . . . . . . . . 31
interactive, example use of . . . . . . . . . . 61
Interpreter, Lisp, explained . . . . . . . . . . . . . 4
Interpreter, what it does . . . . . . . . . . . . . . . . 6

GGenerate an error message . . . . . . . . . . . . . . 4
Getting a buffer . . . . . . . . . . . . . . . . . . . . . . . 22
Global set key . . . . . . . . . . . . . . . . . . . . . . . . 203‘global variable’ defined . . . . . . . . . . . . 229
global-set-key . . . . . . . . . . . . . . . . . . . . . . 203
global-unset-key . . . . . . . . . . . . . . . . . . . 203
Graph prototype . . . . . . . . . . . . . . . . . . . . . 187
Graph, printing all . . . . . . . . . . . . . . . . . . . 245
graph-body-print . . . . . . . . . . . . . . . . . . . 192
graph-body-print Final version. . . . . . 246

H
Handling the kill ring . . . . . . . . . . . . . . . . . 225
Help typing lists . . . . . . . . . . . . . . . . . . . . . . . . 3
Horizontal axis printing . . . . . . . . . . . . . . 241

K
Keep, type of recursive pattern . . . . . . . 135
Key bindings, fixing . . . . . . . . . . . . . . . . . . 211
Key setting globally . . . . . . . . . . . . . . . . . . 203
Key unbinding . . . . . . . . . . . . . . . . . . . . . . . 203
Keymaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
Keyword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Kill ring handling . . . . . . . . . . . . . . . . . . . . 225
Kill ring overview . . . . . . . . . . . . . . . . . . . . 113
kill-append . . . . . . . . . . . . . . . . . . . . . . . . . . 95
kill-new . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
kill-region . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Killing text . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Llambda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249
length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
lengths-list-file . . . . . . . . . . . . . . . . . . 174
lengths-list-many-files . . . . . . . . . . . 176
let. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
let expression sample . . . . . . . . . . . . . . . . . 34
let expression, parts of . . . . . . . . . . . . . . . . 34
let variables uninitialized . . . . . . . . . . . . . 35
let* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56, 148
Lexical binding . . . . . . . . . . . . . . . . . . . . . . . . 36
Library, as term for “file” . . . . . . . . . . . . . . 48
line-to-top-of-window . . . . . . . . . . . . . . 207
Lisp Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

268
Lisp history . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Lisp interpreter, explained . . . . . . . . . . . . . . 4
Lisp interpreter, what it does . . . . . . . . . . . 6
Lisp Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Lisp macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
list-buffers, rebound . . . . . . . . . . . . . . . 204
Lists in a computer . . . . . . . . . . . . . . . . . . . 108
load-library . . . . . . . . . . . . . . . . . . . . . . . . 206
load-path. . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
Loading files . . . . . . . . . . . . . . . . . . . . . . . . . 205‘local variable’ defined . . . . . . . . . . . . . . 33
Local variables list, per-buffer, . . . . . . . . 200
Location of point . . . . . . . . . . . . . . . . . . . . . . 24
looking-at . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Loops and recursion . . . . . . . . . . . . . . . . . . 115

M
Maclisp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Macro, lisp . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Mail aliases . . . . . . . . . . . . . . . . . . . . . . . . . . 202
make-string . . . . . . . . . . . . . . . . . . . . . . . . . 238
mapcar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
mark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
mark-whole-buffer . . . . . . . . . . . . . . . . . . . 50
match-beginning . . . . . . . . . . . . . . . . . . . . . 152
max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
message . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
Mode line format . . . . . . . . . . . . . . . . . . . . . 212
Mode selection, automatic . . . . . . . . . . . . 200
mode-line-format . . . . . . . . . . . . . . . . . . . 212
Motion by sentence and paragraph . . . . 141

N
Narrowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
‘narrowing’ defined . . . . . . . . . . . . . . . . . . . . 25
new version body for insert-buffer . . . 65
nil. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
nil, history of word . . . . . . . . . . . . . . . . . . . 21
No deferment solution . . . . . . . . . . . . . . . . 137
nreverse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
nth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
nthcdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80, 93
nthcdr, example. . . . . . . . . . . . . . . . . . . . . . 100
number-to-string . . . . . . . . . . . . . . . . . . . 237

Index

O
occur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
optional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Optional arguments . . . . . . . . . . . . . . . . . . . 66
Options for interactive . . . . . . . . . . . . . . 31
or . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
other-buffer . . . . . . . . . . . . . . . . . . . . . . . . . 22

P
Paragraphs, movement by . . . . . . . . . . . . 141
Parts of a Recursive Definition. . . . . . . . 128
Parts of let expression . . . . . . . . . . . . . . . . 34
Passing information to functions . . . . . . . 11
Pasting text . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Patterns, searching for . . . . . . . . . . . . . . . . 141
Per-buffer, local variables list . . . . . . . . . 200
Permanent code installation . . . . . . . . . . . 32
point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Point and buffer preservation . . . . . . . . . . 42
‘point’ defined . . . . . . . . . . . . . . . . . . . . . . . . 24
Point location . . . . . . . . . . . . . . . . . . . . . . . . . 24
Practicing evaluation . . . . . . . . . . . . . . . . . . 20
‘predicate’ defined . . . . . . . . . . . . . . . . . . . . 14
Preserving point and buffer . . . . . . . . . . . . 42
Primitive functions . . . . . . . . . . . . . . . . . . . . 26
Primitives written in C . . . . . . . . . . . . . . . . 26
Print horizontal axis . . . . . . . . . . . . . . . . . . 241
Print vertical axis . . . . . . . . . . . . . . . . . . . . 234
print-elements-of-list. . . . . . . . . . . . . 117
print-elements-recursively . . . . . . . . 129
print-graph Final version. . . . . . . . . . . . 247
print-graph varlist . . . . . . . . . . . . . . . . . . 234
print-X-axis . . . . . . . . . . . . . . . . . . . . . . . . 244
print-X-axis-numbered-line . . . . . . . . 244
print-X-axis-tic-line . . . . . . . . . . . . . . 243
print-Y-axis . . . . . . . . . . . . . . . . . . . . . . . . 240
Printing the whole graph . . . . . . . . . . . . . 245
progn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
Program, running one . . . . . . . . . . . . . . . . . . 3
Properties, in mode line example . . . . . 213
Properties, mention of
buffer-substring-no-properties . . 76
Prototype graph . . . . . . . . . . . . . . . . . . . . . . 187
push, example . . . . . . . . . . . . . . . . . . . . . . . . 100

Q
quote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
quoting using apostrophe . . . . . . . . . . . . . . . 3

269

R
re-search-forward . . . . . . . . . . . . . . . . . . 142
Read-only buffer. . . . . . . . . . . . . . . . . . . . . . . 61
Readying a graph . . . . . . . . . . . . . . . . . . . . 187
Rebinding keys . . . . . . . . . . . . . . . . . . . . . . . 204
Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Recursion and loops . . . . . . . . . . . . . . . . . . 115
Recursion without Deferments . . . . . . . . 136
Recursive Definition Parts . . . . . . . . . . . . 128
Recursive pattern - accumulate . . . . . . . 135
Recursive pattern - every . . . . . . . . . . . . . 133
Recursive pattern - keep . . . . . . . . . . . . . . 135
Recursive Patterns . . . . . . . . . . . . . . . . . . . 133
recursive-count-words . . . . . . . . . . . . . . 166
recursive-graph-body-print . . . . . . . . 194
recursive-lengths-listmany-files . . . . . . . . . . . . . . . . . . . . . . . . 177
Recursively counting words . . . . . . . . . . . 161
regexp-quote . . . . . . . . . . . . . . . . . . . . . . . . 149
Region, what it is . . . . . . . . . . . . . . . . . . . . . 42
Regular expression searches . . . . . . . . . . . 141
Regular expressions forword counting . . . . . . . . . . . . . . . . . . . . . . 155
Remainder function, % . . . . . . . . . . . . . . . . 235
Repetition (loops) . . . . . . . . . . . . . . . . . . . . 115
Repetition for word counting . . . . . . . . . 155
Retrieving text . . . . . . . . . . . . . . . . . . . . . . . 113‘returned value’ explained . . . . . . . . . . . . . 8
reverse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
Ring, making a list like a . . . . . . . . . . . . . 225
ring.el file . . . . . . . . . . . . . . . . . . . . . . . . . . 232
Robots, building . . . . . . . . . . . . . . . . . . . . . 127
Run a program . . . . . . . . . . . . . . . . . . . . . . . . . 3

S
Sample let expression . . . . . . . . . . . . . . . . . 34
save-excursion . . . . . . . . . . . . . . . . . . . . . . . 42
save-restriction . . . . . . . . . . . . . . . . . . . . . 73
search-forward . . . . . . . . . . . . . . . . . . . . . . . 87
Searches, illustrating . . . . . . . . . . . . . . . . . 141
sentence-end . . . . . . . . . . . . . . . . . . . . . . . . 141
Sentences, movement by . . . . . . . . . . . . . . 141
set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
set-buffer . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
set-variable . . . . . . . . . . . . . . . . . . . . . . . . 105
setcar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
setcdr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
setcdr, example. . . . . . . . . . . . . . . . . . . . . . 100
Setting a key globally . . . . . . . . . . . . . . . . 203
Setting value of variable . . . . . . . . . . . . . . . 16
‘side effect’ defined . . . . . . . . . . . . . . . . . . . 8

Simple extension in .emacs file . . . . . . . 207
simplified-beginning-of-buffer . . . . 49
site-init.el init file . . . . . . . . . . . . . . . . 196
site-load.el init file . . . . . . . . . . . . . . . . 196
Size of buffer . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Solution without deferment . . . . . . . . . . . 137
sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
Source level debugger . . . . . . . . . . . . . . . . . 218
Special form . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Storing and cutting text . . . . . . . . . . . . . . . 85
‘string’ defined . . . . . . . . . . . . . . . . . . . . . . . . 2
substring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
switch-to-buffer . . . . . . . . . . . . . . . . . . . . . 23
Switching to a buffer . . . . . . . . . . . . . . . . . . 23
Symbol names . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Symbol without function error . . . . . . . . . 10
Symbol without value error . . . . . . . . . . . . 10
Symbolic expressions, introduced. . . . . . . . 2
Symbols as a Chest of Drawers . . . . . . . 111
Syntax categories and tables . . . . . . . . . . 168

T
Tabs, preventing . . . . . . . . . . . . . . . . . . . . . 202
Text between double quotation marks . . . 2
Text Mode turned on . . . . . . . . . . . . . . . . . 201
Text retrieval. . . . . . . . . . . . . . . . . . . . . . . . . 113
the-the . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223‘then-part’ defined . . . . . . . . . . . . . . . . . . . . 38
top-of-ranges . . . . . . . . . . . . . . . . . . . . . . . 183
triangle-bugged . . . . . . . . . . . . . . . . . . . . . 215
triangle-recursively . . . . . . . . . . . . . . . 130
Truth and falsehood in Emacs Lisp . . . . 41
Types of data . . . . . . . . . . . . . . . . . . . . . . . . . 12

U
Unbinding key. . . . . . . . . . . . . . . . . . . . . . . . 203
Uninitialized let variables . . . . . . . . . . . . . 35

V
Variable initialization . . . . . . . . . . . . . . . . . 104
Variable number of arguments . . . . . . . . . 13
Variable, example of, fill-column . . . . . 10
‘variable, global’, defined . . . . . . . . . . 229‘variable, local’, defined. . . . . . . . . . . . . 33
Variable, setting value . . . . . . . . . . . . . . . . . 16
Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9‘varlist’ defined . . . . . . . . . . . . . . . . . . . . . . 34
Version of Emacs, choosing . . . . . . . . . . . 208
Vertical axis printing . . . . . . . . . . . . . . . . . 234

270

Index

W

Y

what-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
while . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
Whitespace in lists . . . . . . . . . . . . . . . . . . . . . 2
Whole graph printing . . . . . . . . . . . . . . . . . 245
Widening. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
Widening, example of. . . . . . . . . . . . . . . . . . 74
Word counting in a defun . . . . . . . . . . . . 167
Words and symbols in defun . . . . . . . . . . 167
Words, counted recursively . . . . . . . . . . . 161
Words, duplicated . . . . . . . . . . . . . . . . . . . . 223
Writing a function definition . . . . . . . . . . . 26
Wrong type of argument . . . . . . . . . . . . . . . 13

Y axis printing . . . . . . . . . . . . . . . . . . . . . . . 234
Y-axis-column . . . . . . . . . . . . . . . . . . . . . . . 239
Y-axis-column Final version. . . . . . . . . . 246
Y-axis-label-spacing . . . . . . . . . . . . . . . 237
Y-axis-tic . . . . . . . . . . . . . . . . . . . . . . . . . . 238
yank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113, 230
yank-pop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

X
X axis printing . . . . . . . . . . . . . . . . . . . . . . . 241
X-axis-element . . . . . . . . . . . . . . . . . . . . . . 244

Z
zap-to-char . . . . . . . . . . . . . . . . . . . . . . . . . . 85
zerop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

About the Author
Robert J. Chassell (1946–2017) started working with GNU Emacs in 1985.
He wrote and edited, taught Emacs and Emacs Lisp, and spoke throughout the world on software freedom. Chassell was a founding Director andTreasurer of the Free Software Foundation, Inc. He was co-author of the
Texinfo manual, and edited more than a dozen other books. He graduated from Cambridge University, in England. He had an abiding interest
in social and economic history and flew his own airplane.
"Goodbye to Bob Chassell" (https://www.fsf.org/blogs/community/
goodbye-to-bob-chassell)