SISC for Seasoned Schemers

Scott G. Miller
Matthias RadestockSISC for Seasoned Schemers
by Scott G. Miller and Matthias Radestock
Copyright © 2002-2006 Scott G. Miller,Matthias RadestockThis documentation is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
The source code refers to the DocBook XML source files. The object code refers to the generated HTML, PostScript, PDF, and other forms.
This documentation is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License (in Appendix E) along with this documentation; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

Table of Contents
1. Introduction............................................................................................................................................1
1.1. Features .......................................................................................................................................1
1.2. About this document ...................................................................................................................1
1.2.1. About procedure documentation ....................................................................................1
1.3. Where to obtain this document ...................................................................................................2
1.4. Version Applicability ..................................................................................................................2
2. Installation and Invocation ...................................................................................................................4
2.1. Required Environment ................................................................................................................4
2.2. The Read-Eval-Print-Loop..........................................................................................................4
2.3. Display Conventions ...................................................................................................................4
2.4. Running SISC ..............................................................................................................................5
2.4.1. Command-line Switches.................................................................................................5
2.4.2. Configuration Parameters ...............................................................................................6
2.5. Running Scheme Programs .......................................................................................................122.5.1. Loading.........................................................................................................................122.5.2. Scheme Shell Scripts ....................................................................................................12
3. Scheme Language ................................................................................................................................133.1. Types .........................................................................................................................................133.1.1. Numbers .......................................................................................................................13
3.1.2. Characters .....................................................................................................................13
3.1.3. Symbols ........................................................................................................................14
3.1.4. Strings...........................................................................................................................15
3.1.5. Pairs and Lists...............................................................................................................16
3.1.6. Vectors ..........................................................................................................................16
3.1.7. Boxes ............................................................................................................................16
3.1.8. Parameters ....................................................................................................................17
3.1.9. Immutable types ...........................................................................................................173.2. Equivalence ...............................................................................................................................173.3. Syntax and Lexical Structure ....................................................................................................18
3.3.1. Comments.....................................................................................................................18
3.3.2. Shared Structures..........................................................................................................18
3.3.3. Control Features ...........................................................................................................19
3.3.4. Syntactic Extension ......................................................................................................19
3.4. Errors and Error Handling.........................................................................................................20
3.4.1. Failure Continuations ...................................................................................................20
3.4.2. Error Records................................................................................................................22
3.4.3. Examples ......................................................................................................................25
3.4.4. dynamic-wind ............................................................................................................26
3.5. Symbolic Environments and Property Maps.............................................................................27
3.5.1. Access Functions ..........................................................................................................27
3.5.2. Obtaining and Naming .................................................................................................27
3.5.3. Chained Symbolic Environments .................................................................................28
3.6. Miscellaneous Functions...........................................................................................................28iii

4. Debugging Facilities.............................................................................................................................30
4.1. Passive Debugging ....................................................................................................................30
4.2. Active Debugging......................................................................................................................31
4.2.1. Runtime Tracing ...........................................................................................................32
4.2.2. Breakpoints...................................................................................................................32
5. I/O..........................................................................................................................................................34
5.1. Ports ..........................................................................................................................................34
5.1.1. URLs.............................................................................................................................34
5.1.2. Buffered I/O..................................................................................................................35
5.1.3. Character Ports .............................................................................................................35
5.1.4. String Ports ...................................................................................................................37
5.1.5. Binary Ports and Block IO ...........................................................................................38
5.1.6. Java Ports ......................................................................................................................40
5.1.7. Serialization..................................................................................................................425.2. Networking................................................................................................................................43
5.2.1. IP Addressing ...............................................................................................................44
5.2.2. Socket Operations.........................................................................................................44
5.2.3. TCP...............................................................................................................................45
5.2.4. TLS and SSL ................................................................................................................45
5.2.5. UDP ..............................................................................................................................47
5.2.6. Multicast UDP ..............................................................................................................48
5.3. User Defined Ports ....................................................................................................................49
5.4. Miscellaneous............................................................................................................................52
5.4.1. Pretty-Printing ..............................................................................................................52
5.4.2. Source Loading.............................................................................................................52
5.4.3. Location Tracking.........................................................................................................53
5.4.4. Locating Resources ......................................................................................................53
5.4.5. File Manipulation .........................................................................................................54
5.4.6. Class Loading ...............................................................................................................55
6. Threads and Concurrency ..................................................................................................................56
6.1. Scheme Thread Semantics ........................................................................................................56
6.2. Basic Thread Operations ...........................................................................................................57
6.3. High-level Functions .................................................................................................................59
6.4. Thread Scheduling ....................................................................................................................59
6.5. Synchronization Primitives .......................................................................................................60
6.5.1. Mutex Operations .........................................................................................................61
6.5.2. Condition Variable Operations .....................................................................................61
6.5.3. High-level Concurrency................................................................................................62
7. Types and Objects................................................................................................................................63
7.1. Type System ..............................................................................................................................63
7.1.1. Core Procedures and Predicates ...................................................................................63
7.1.2. Derived Procedures and Predicates ..............................................................................64
7.1.3. Hooks............................................................................................................................65
7.1.4. Standard Types .............................................................................................................66
7.2. Generic Procedures ...................................................................................................................66
7.2.1. Defining Generic Procedures........................................................................................67iv

7.2.2. Defining Methods .........................................................................................................68
7.2.3. Invoking Generic Procedures .......................................................................................70
7.2.4. Procedures on Methods ................................................................................................71
7.2.5. Miscellaneous ...............................................................................................................72
7.3. Object System ...........................................................................................................................76
7.3.1. Classes ..........................................................................................................................76
7.3.2. Slots ..............................................................................................................................78
7.3.3. Instantiation ..................................................................................................................80
7.3.4. Inheritance ....................................................................................................................80
8. Java Interaction ...................................................................................................................................83
8.1. Calling Scheme from Java.........................................................................................................83
8.1.1. Application Contexts ....................................................................................................83
8.1.2. The SISC Heap .............................................................................................................84
8.1.3. The Dynamic Environment ..........................................................................................84
8.1.4. The Interpreter ..............................................................................................................85
8.1.5. Miscellaneous Features ................................................................................................87
8.1.6. Quick Reference ...........................................................................................................88
8.2. Calling Java from Scheme.........................................................................................................89
8.2.1. Classes ..........................................................................................................................89
8.2.2. Methods ........................................................................................................................91
8.2.3. Fields ............................................................................................................................92
8.2.4. Instances .......................................................................................................................93
8.2.5. Arrays ...........................................................................................................................94
8.2.6. Proxies ..........................................................................................................................96
8.2.7. Types and Conversions .................................................................................................97
8.2.8. Multi-threading.............................................................................................................98
8.2.9. Exception Handling ......................................................................................................99
8.2.10. Access Permissions ....................................................................................................99
8.2.11. Common Usage ........................................................................................................100
8.3. Java Reflection Interface .........................................................................................................102
8.3.1. Classes ........................................................................................................................102
8.3.2. Constructors................................................................................................................103
8.3.3. Methods ......................................................................................................................104
8.3.4. Fields ..........................................................................................................................105
8.3.5. Arrays .........................................................................................................................105
8.3.6. Proxies ........................................................................................................................106
9. Additional Libraries ..........................................................................................................................107
9.1. Optional SISC Libraries ..........................................................................................................107
9.1.1. Definitions ..................................................................................................................107
9.1.2. Bitwise Logical Operations ........................................................................................107
9.1.3. Records .......................................................................................................................1089.1.4. Hash Tables.................................................................................................................109
9.1.5. Binary Buffers ............................................................................................................112
9.1.6. Procedure Properties...................................................................................................113
9.1.7. Loadable Scheme Libraries ........................................................................................113
9.1.8. Operating System Interface ........................................................................................114
9.2. Third-Party Libraries...............................................................................................................115v

9.2.1. SRFIs ..........................................................................................................................115
9.2.2. SLIB ...........................................................................................................................117
9.3. Creating Libraries ...................................................................................................................118
10. Modules and Libraries ....................................................................................................................120
10.1. Modules.................................................................................................................................120
10.1.1. Overview ..................................................................................................................120
10.1.2. Style..........................................................................................................................120
10.1.3. Modularizing Existing Code.....................................................................................121
10.1.4. Evaluation.................................................................................................................122
10.2. Libraries ................................................................................................................................122
10.2.1. require-extension .............................................................................................123
11. Extensibility......................................................................................................................................125
11.1. Adding Types ........................................................................................................................125
11.1.1. External Representation of Values............................................................................125
11.1.2. Equality.....................................................................................................................126
11.1.3. Serializable Values....................................................................................................126
11.2. Adding Native Bindings........................................................................................................126
11.2.1. Native Procedures.....................................................................................................126
11.2.2. Fixable Native Procedures........................................................................................127
11.2.3. Indexed Native Libraries ..........................................................................................127
11.3. Serialization ..........................................................................................................................128
11.3.1. Deserializer methods ................................................................................................129
11.3.2. Serializer methods ....................................................................................................129
A. Errata.................................................................................................................................................131
B. R5RS Liberties and Violations .........................................................................................................132
C. Troubleshooting.................................................................................................................................133
C.1. Kaffe .......................................................................................................................................133
D. Backend Details.................................................................................................................................134
D.1. Limits .....................................................................................................................................134
D.1.1. Datastructure Limits ..................................................................................................134
D.1.2. Symbol Uniqueness ...................................................................................................134
D.2. Performance and Efficiency considerations ...........................................................................135
D.2.1. Math...........................................................................................................................135
D.2.2. Strings........................................................................................................................135
D.2.3. Interrupts....................................................................................................................136
E. GNU General Public License ...........................................................................................................137
E.1. Preamble .................................................................................................................................137
E.2. TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
137
E.2.1. Section 0.....................................................................................................................138
E.2.2. Section 1.....................................................................................................................138
E.2.3. Section 2.....................................................................................................................138
E.2.4. Section 3.....................................................................................................................139
E.2.5. Section 4.....................................................................................................................139
E.2.6. Section 5.....................................................................................................................140
E.2.7. Section 6.....................................................................................................................140vi

E.2.8. Section 7.....................................................................................................................140
E.2.9. Section 8.....................................................................................................................140
E.2.10. Section 9...................................................................................................................141
E.2.11. Section 10.................................................................................................................141
E.2.12. NO WARRANTY Section 11 ..................................................................................141
E.2.13. Section 12.................................................................................................................141
E.3. How to Apply These Terms to Your New Programs ..............................................................142
Index of Functions, Parameters, and Syntax ......................................................................................144vii

List of Tables
2-1. SISC Command line Switches..............................................................................................................5
3-1. Named character literals.....................................................................................................................143-2. String escape codes ............................................................................................................................15
8-1. Typical Java to Scheme, External Calls..............................................................................................89
8-2. Typical Java to Scheme, Internal Calls...............................................................................................89
8-3. Common S2J Usage .........................................................................................................................100D-1. SISC Limits......................................................................................................................................134

viii

Chapter 1. Introduction
SISC 1 is a lightweight, platform independent Scheme system whose primary goals are rapid execution of
the complete R5RS and future Scheme standards, plus a useful superset for real-world application
development.
SISC’s development progresses in two directions, to improve the core interpreter to be simpler, moreelegant, and more efficient; and to add necessary functionality through extensions that do not complicate
the core.
SISC as a project began as the successor to the Lightweight Interpreter of Scheme Code (LISC). LISC
was a small, stack-based almost R4RS compliant Scheme. SISC was born out of the desire to create aninterpreter that was of a similar footprint to LISC, but which executed Scheme code much faster,
complied fully to the R5RS standard, and which wasn’t limited by the stack-based model. SISC met thesegoals very quickly, and has since progressed in active development to be a competitive Scheme system.
As a successor to LISC the interpreter was named the Second Interpreter of Scheme Code.

1.1. Features•

Full R5RS compliance•

Efficient number tower, with support for integers, floating-point numbers, rationals, and complex
numbers of arbitrary precision•

Lightweight Scheme engine, implementing all R5RS functionality in approximately 10,000 lines ofnative code plus approximately 5,000 lines of Scheme code.

•

Flexible runtime extensibility through a scopeable module system, which may add arbitrary bindings
and new first-class types.

1.2. About this document
This document explains the SISC Scheme system. It assumes knowledge of the Scheme language. As
such, when discussing the Scheme language, we will focus primarily on differences between the Scheme
standard and the language implemented by SISC.
Secondly, SISC implements the R5RS standard. As such, any code written to that standard should run
without reading any further.

1.2.1. About procedure documentation
Throughout this document, procedures will be defined using the following syntax:
procedure: (function-name required-argument [optional-argument] [rest-argument] ...) =>return value
Description of the procedure’s semantics.

A procedure is any function that takes zero or more arguments and returns a value.

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Chapter 1. Introduction
parameter: (parameter-name [new-parameter-value])
Description of the parameter.

A Parameter is a special function that is used to store a value in the dynamic-environment. When given
no arguments, a parameter will return its current value. When provided an argument, the value of theparameter is set to the value of that argument.
syntax: (syntactic-keyword structure ...)
Description of the syntactic transform.

Syntax refers to a syntactic keyword that when compiled will be transformed into some expression
composed only of basic Scheme forms. let, cond, and or are all examples of syntactic forms.
Procedures, parameters, and syntax may take one or more arguments. The name of an argument in a
description summarizes the semantics of the expected argument. The type of an argument, when not
clear, should be described in the procedure summary paragraph that follows the prototype. If theargument is enclosed in square brackets ([]), that argument is optional, and may be omitted. If anargument is optional and is followed by ellipses (...), then the argument is a rest argument, and may besatisfied by zero or more values.
Finally, some of the functions described in this document are encapsulated in modules (see Chapter 10).
Sections that describe functions in modules will have a requires statement indicating the expression thatmust be evaluated to make those functions available in the Scheme environment. The requires statement
will appear as follows:
Requires: (import module-name)module-name is will be a name uniquely identifying the name of the module that contains the functions.

1.3. Where to obtain this document
This document is produced from DocBook sources and is made available in HTML, Adobe Acrobat(PDF), and PostScript forms. Other formats, and up-to-date versions of this document should beavailable from the SISC website, http://sisc.sourceforge.net (http://sisc.sourceforge.net).
The DocBook sources are available from the same site, packaged in the source distribution of SISC, andavailable from the project’s CVS tree. Details for accessing both are linked off the project site.
Starting with SISC version 1.8.4, the HTML render of this manual is included in the full binary
distributions of SISC.

1.4. Version Applicability
This document describes functionality present in SISC version 1.16.4. Some of the document may apply
to previous and future versions. When in doubt, the source of this document can be found in the SISC
CVS repository, or in the packaged source distribution that should be available wherever the binary
distribution was obtained.

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Chapter 1. Introduction

Notes
1. The pronunciation is written as sIsk in the International Phonetic Alphabet. This is similar to theEnglish word ’whisk’.

3

Chapter 2. Installation and Invocation
2.1. Required Environment
SISC primarily targets the Java Virtual Machine, and the Java v1.3 and higher class libraries. The 1.2
libraries and VM are required due to a reliance on memory reference functionality used to properly
garbage collect unused symbols while still maintaining pointer equality between those that remain active,
while the 1.3 libraries are needed for the proxy functionality of the Java bridge library.
SISC does not require any particular operating system, though the official distribution currently provides
launching assistance for Windows and Unix based systems. Theoretically any machine that supports Java
1.3 or higher should be able to run SISC.

2.2. The Read-Eval-Print-Loop
As in most Scheme systems, SISC’ss primary human interface is a REPL, or Read-Eval-Print-Loop.
SISC presents the user with a prompt (’#;>’), then reads an s-expression from the console. SISC thenevaluates the s-expression, producing a value or error, which it then prints. Finally the process begins
again with a new prompt.
The process terminates with a forced kill of the JVM (with Control-C or similar), or when an end of fileis detected on the console. Also, the exit procedure may be used to exit the current REPL.
procedure: (exit [return-value]) => does not return
Instructs the current REPL to exit. If the optional return value is given, the value will be passed out of the REPL to
the calling code or environment.

Evaluation may be prematurely interrupted on some platforms by sending SIGQUIT (usually by pressing
Control-C) to the SISC process. If supported, this will cause an interrupted error to be raised at whichever
expression is currently being evaluated in the REPL thread. If not caught, this will cause the error to beraised at the console and a new prompt will be displayed. Issuing the signal twice will terminate SISC.

2.3. Display Conventions
Executing or loading code in the REPL ordinarily produces a value result which is displayed during thePrint phase. However, two other message types may be displayed. First, if an error is raised that is nothandled by the running program, the message will be described in one of several forms, depending on
what combinations of a location, a message, and a parent message the error has:Error.
Error in <error-location>.
Error: <description>
Error in <error-location>: <description>

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Chapter 2. Installation and Invocation

Error in nested call.
<nested-error>
Error in nested call: <description><nested-error>
Error in nested call from <error-location>.
<nested-error>
Error in nested call from <error-location>: <description><nested-error>

The location of an error often refers to the name of a procedure, syntax, or parameter. Errors that do notcontain a location often originate in an anonymous function. Nested errors occur when one function
makes a call to another function and the second raises an error. If the first function wishes, it may catch
this error and generate its own, with the second functions error as a parent. Thus a nested error consistsof an error message on the first line, and the nested error message or messages on following lines.
In addition to errors, it is possible for code to produce warnings during compilation or run-time. A
warning is a message to the user about a condition that is unusual but not terminal to the program flow.
The compiler, for example, does a minimal amount of sanity checking on code and may produce
warnings about code that has the potential to raise errors during execution. Warnings are always
distinguished from ordinary messages by surrounding braces ({}) and starting with the word ’warning’.
{warning: <description>}

2.4. Running SISCSISC’s official distribution provides a startup script for Windows and Unix based hosts. Simply executing
this script starts the SISC REPL. Zero or more Scheme source files can also be specified on the commandline after the script name, which will be loaded in order before the REPL prompt is displayed.
It may be desirable to pass options to the underlying JVM. This can be done by setting the JAVAOPT
environment variable to the options you wish to pass to the JVM. This includes switches for heap size,
system properties, etc. SISC itself has a number of properties that can affect its operation. See
Section 2.4.2 for a list of these properties and their meanings.

2.4.1. Command-line Switches
SISC can, in addition to loading Scheme source programs, also accept a few command-line switches tochange its behavior. Any non-switch argument is considered a Scheme source file to load on startup, until
the end of options characters "--" are reached. Any item after those characters are considered arguments
to the function specified in the "call-with-args" switch.
All command-line switches have both a short and long form. These forms are equivalent in meaning.

5Chapter 2. Installation and Invocation
Table 2-1. SISC Command line SwitchesLong Switch

Short Switch--call-with-args <name>-c <name>

Switch meaning
Call the top-level procedurename with the remaining

command-line arguments afterthe "--" sequence.
--eval <expression>-e <expression>

Evaluate the provided
expression.

--no-repl-x

Exit after loading all the Scheme
source files and processing all
command-line switches.

--heap <heap-file>-h <heap-file>

File containing pre-compiled
code and data for a complete
Scheme top-level environment.
This parameter is mandatory if
the heap cannot be locatedautomatically.

--properties<config-file>-p <config-file>Specifies a Java properties filethat contains application
properties. Typically some ofthese properties define defaults
for configuration parameters(Section 2.4.2. The file can be
specified as a URL.

--listen [<host>:]<port>-l [<host>:]<port>

Listen on host/port for REPL
connections, i.e. connecting to
the specified host/port will create
a new REPL.

The order of processing the command line is as follows:
1. Process the entire command line, noting the settings of each switch and accumulating all Scheme
source files and arguments after the end of options sequence.
2. Load the heap file.
3. Load each Scheme source file found in the order they occured on the command line. Note whether
any errors occurred.
4. If present, evaluate the expression in an --eval switch. Note its success or failure.
5. If present, apply the named function in a --call-with-args switch to the arguments after the endof options sequence. Note its return value.
6. If --no-repl was not specified, invoke the REPL, otherwise exit.
7. If the REPL was run if its return value is an integer, return that integer as SISC’s overall return code.
If the REPL was not run, and any return code supporting step above was run, return the most recent
return code. If no return code step was performed, but a success/failure step was performed, return 1
if any failures occured, 0 otherwise.

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Chapter 2. Installation and Invocation

2.4.2. Configuration Parameters
SISC’s behaviour is affected by a number of configuration parameters. These fall into two categories:
1. Static configuration parameters that can only be specified at system startup and apply across all SISC
applications that use the same classloader.
2. Dynamic configuration parameters that can be altered on a per-thread basis without impacting otherthreads. These kind of parameters are also called "thread locals", or, in Scheme terminology,
dynamic parameters.

2.4.2.1. Static Parameters
Static configuration parameters can be set using Java system properties. Their value can be retrieved witha Scheme function, but it is not possible to alter it. Static configuration parameters default to pre-definedsystem-internal settings if left unspecified.
SISC has the following static configuration parameters:
Java property a

Scheme parameter

default

description

permitInterrupts

permit-interruptstrue

If set to true,
thread/interrupt ispermitted to interrupt
running Scheme code,in addition to sending aninterrupt signal to thehost language.

minFloatPrecision

min-float-precision 16

Specifies the minimum
precision, in decimal
places, to be maintainedby the Quantity lib if
using arbitrary precision
floats.

maxFloatPrecision

max-float-precision 32

Specifies the maximum
precision, in decimal
places, to be maintainedby the Quantity lib if
using arbitrary precision
floats.

Notes:
a. The names of all properties for configuration parameters must be prefixed with sisc..

When static configuration parameters are retrieved with their associated Scheme function, the value is of
the type specified for the parameter. See Section 2.4.2.3.

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Chapter 2. Installation and Invocation

2.4.2.2. Dynamic Parameters
There are four ways to specify a configuration parameter, in decreasing order of precedence:
1. Invoking a scheme function. Dynamic configuration parameters are special Scheme parameters (seeSection 3.1.8. Invoking the parameter with a value sets the parameter for the current thread without
affecting other dynamic contexts / threads.
2. Defining an application property. A single SISC runtime can host multiple applications
simultaneously. Application properties define default values for dynamic configuration parametersacross all dynamic contexts / threads of an application. They can be specified at application
initialisation time. See Section 8.1. When SISC is started from the command line, the location of aJava properties file containing application properties can be specified with a command line option.
See Table 2-1.
3. Defining a Java system property. Java system properties define default values for dynamic
configuration parameters that apply across all applications inside a single SISC runtime.
4. System defaults. All dynamic configuration parameters have a reasonable default value.

SISC has the following dynamic configuration parameters:
Java property a

Scheme parameter

default

description

caseSensitive

case-sensitive

false

Determines whether
symbols read via the
Scheme reader are to betreated case sensitively.

characterSet

character-set"UTF8"

Defines the default
character set used bycharacter ports (seeSection 5.1.3) if nocharacter set isotherwise specified.

emitAnnotations

emit-annotations

true

If set to true, thisparameter causes source
files loaded with load
or import, as well assource entered in the
console, to be annotatedby the Scheme reader.
Annotations include
source file location
information, whichsimplifies debugging.
See Section 4.1.

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Chapter 2. Installation and Invocation
Java property a

Scheme parameter

default
emit-debugging-symbols
truedescription
If set to true,
additional annotations
useful for debugging,such as function andvariable names, areproduced by SISC’s
compiler. See
Section 4.1.

max-stack-trace-depth0Specifies the maximum
depth of virtual callstacks, which are used
to obtain proper stack
traces even in thepresence of tailrecursion andcontinuation capture andinvocation, while stillpreserving tail-callsemantics andsafe-for-spaceguarantees. See
Chapter 4. Collecting
this information iscomputationallyexpensive, so thisfeature is turned off bydefault. A value of 16 isabout right for
debugging mostapplications. Large
values allow collectionof more information but
carry a large
performance penalty,
small values result insome call information
being lost.

emitDebuggingSymbols

maxStackTraceDepth

9Chapter 2. Installation and Invocation
Java property apermissiveParsing

Scheme parameter

default
permissive-parsing false

printSharedprint-shared

replPrompt

repl-prompt

String to be displayed
as part of SISC’s REPL
prompt.

stackTraceOnError

stack-trace-on-errortrue

If set to true,
whenever an uncaught
error is encountered afull stack trace isdisplayed automatically.
See Section 4.1.

truedescription
If set to true, theScheme parser will warn
rather than raise an error
for various syntactic
errors such as
unbalanced parentheses.
This allows one tocontinue parsing a
syntactically invalid file,finding many errors at
once.
If set to true, write
and the REPL detectshared structures in dataand invoke a version of
write capable ofemitting the shared
structure’s external
representation of data.
See Section 3.3.2.2. The
user may wish to set thisparameter to false
because of the overheadof both scanning all
data, and constructing
this representation whenshared structures aredetected.

10Chapter 2. Installation and Invocation
Java property a
Scheme parameter
default
strictR5RSCompliance strict-r5rs-compliance
false

description
If set to true, strict
R5RS syntax andsemantics are followed.
This will cause SISC toraise errors in all
situations described as"an error" in the Scheme
standard. This will
override and invalidate
all the interpretation
liberties described inAppendix B.

synopsisLength

synopsis-length

Limit on the length (incharacters) of theexternal representationof data structures inerror messages andwarnings. When thelimit is reached, an
ellipsis (...) isappended to thecurtailed external
representation.

vectorLengthPrefixing

vector-length-prefixing
true

30

If set to true, thisparameter will instruct
the pretty-printer to emit
length prefixed, trailingduplicate-eliminatedvectors in its output. If
false, ordinary
full-length vectors
without prefixes will beemitted. See
Section 3.1.6.

When dynamic configuration parameters are retrieved or set with their associated Scheme function, thevalue is of the type specified for the parameter. See Section 2.4.2.3.

2.4.2.3. Value Conversion
When specifying configuration parameters via Java properties, a Java-like notation is used, e.g. boolean
parameters are specified as true and false. By contrast, when getting and setting configuration
parameters from Scheme, their values are of the appropriate Scheme type, e.g. boolean parameters are
specified as #t and #f. Strings and symbols undergo a similiar conversion; they are specified without
their double/single quotes in the Java properties.

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Chapter 2. Installation and Invocation
For parameter types other than boolean, string and symbol, Java properties are read as Scheme values,
i.e. Scheme literal notation should be used in the properties.

2.5. Running Scheme Programs
2.5.1. Loading
SISC supports loading Scheme programs using the R5RS optional procedure load. A number of filetypes for loading Scheme code are supported. The common extensions are:• scm - Scheme source code
• sce - Scheme code expanded. This type of code has been processed by the syntax expander andcontains only core Scheme forms.
• scc - SISC Compiled Code. This contains expanded and compiled code in a SISC specificrepresentation. The code gets executed when loaded.
In addition to code loaded, SISC requires a heap, which contains the default set of libraries and functions
for the initial environment. SISC will look for a heap file called sisc.shp in the current directory, thedirectory referenced by the SISC_HOME environment variable, and as a resource paired with the
HeapAnchor class in the sisc.boot package of the classpath. The standard SISC distribution contains
sisc.shp in the same directory as the supporting .jar files.
It isn’t uncommon to want the heap to reside on the classpath, where it can be more easily resolved inapplets or in web applications. This file, usually called sisc-heap.jar can be added to the classpath in
any usual fashion, and the heap loading routines will discover it if not found elsewhere. To create a jar
file containing the heap, create the following file structure in the jar:sisc/
sisc/boot/sisc/boot/HeapAnchor.classsisc/boot/sisc.shp

The HeapAnchor.class class file is distributed in the sisc-opt.jar file of the full binary
distribution.

2.5.2. Scheme Shell Scripts
On Unix or Unix-like systems, SISC supports SRFI-22, a mechanism for writing shell-like scripts thatcan be invoked directly as executable programs. The text of the SRFI, which can be found at
http://srfi.schemers.org (http://srfi.schemers.org/srfi-22/srfi-22.html), describes how such programs are
written.

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Chapter 3. Scheme Language
In this chapter we will examine the language that SISC interprets, which is a superset of the R5RS
Scheme Standard.

3.1. Types
3.1.1. Numbers
The full Scheme number tower is supported:•

Integers•

Floating Point numbers•

Rational numbers•

Complex numbers

Depending on the numeric library compiled into SISC, floating point numbers have either 32 or 64 bit
IEEE precision, or arbitrary 1 precision. Regardless, SISC’s complex numbers have floating point
components of the same precision as the reals. Integers have arbitrary precision in all numeric libraries,
and rational numbers are built with arbitrary precision components.

3.1.1.1. Numeric constants
The precision specifying exponents (S, F, L, and D) are ignored in SISC, all inexact numbers are kept
in the precision of the numeric library. The exponents are read and used to scale the real number as
expected. In the case of arbitrary precision floats, specific precision constraints are maintained to preventa runaway increase of precision. The constraints can be set by minFloatPrecision and maxFloatPrecisionconfiguration parameters on startup. See Section 2.4.2.
All four base specifiers (#x, #o, #d, #b) are supported for integers and rationals. Only decimal (#d),
the default, is supported for floating point and complex numbers.
SISC will produce infinite or not-a-number quantities from some operations. Those quantities are
represented and can be used in Scheme programs as #!+inf (positive infinity), #!-inf (negative
infinity), and #!nan (not-a-number).

3.1.1.2. Exactness
Exactness and inexactness contagion behaves as expected. Rational’s are made inexact through division.
Floats are made exact by conversion to a rational number. SISC attempts as accurate a conversion as
possible, by converting the decimal portion of the number to a ratio with a denominator of the form
10^n, where n is the scale of the floating point number. Then the fraction is reduced as usual.
Since complex numbers must have floating point components currently, conversion to an exact merely
rounds the components to integers.

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Chapter 3. Scheme Language

3.1.2. Characters
SISC’s characters are double-byte wide. This means that they are capable of representing the full rangeof unicode characters. Unicode characters can be created with number->character; #\nnnnnn, where
nnnnnn is an octal number in the range 000000 -> 177777; or #\uxxxx, where xxxx is a hexadecimal
number in the range 0000 -> ffff. At least two zeros must be specified to distinguish from the ’0’
character when using an octal character literal. At least one zero must be specified to distinguish ahexadecimal character from the ’u’ character.
SISC also provides additional named characters, to add to the Scheme standard’s space and newline:
Table 3-1. Named character literals
Character NameUnicode Value (hex)backspace0008

newline

000a

nul

0000

page

000c

return000d

rubout

007f

space

0020

tab

0009

Formally, SISC’s lexer modifies the R5RS grammar with the following productions for character literals:<character> --> #\ <any character>| #\u <uinteger 16>| #\ <uinteger 8>| #\ <character name><character name> --> backspace | newline | nul| page | return | rubout | space | tab

Characters are not compared with respect to the locale of the running system. Character comparison isequivalent to numeric comparison of the character value as returned by char->integer.
There are a number of reasons why a full Unicode system is non-trivial, especially within the frameworkof the R5RS string and character functions. Such a discussion is outside the scope of this document.
Unicode compliant processing may be made available in the future as a library, however.

3.1.3. Symbols
SISC’s symbols are ordinarily case-insensitive. SISC maintains true pointer equality between symbols
with like contents, unless the symbol is created uninterned. An uninterned symbol is one which isguaranteed to be pointer distinct from any other symbol in the Scheme system, even another with thesame contents. Uninterned symbols can be generated with:procedure: (string->uninterned-symbol string) => symbol
Converts the provided string into an uninterned, pointer distinct symbol.

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Chapter 3. Scheme Language
Uninterned symbols, while always pointer-distinct, may still be equal? to another symbol if it’srepresentation matches another.

3.1.3.1. Case Sensitivity
SISC also allows symbols to be created that are case-sensitive. This can be done one of two ways. The
first is by setting the caseSensitive configuration parameter (see Section 2.4.2. The second method is via
a non-standard symbol syntax. If a symbol is enclosed in pipe (’|’) characters, the reader will treat that
individual symbol as cased. The syntax extends the R5RS grammar with the following production:<cased symbol> --> |<identifier>|

Example 3-1. Case sensitive Symbol literals(eq? ’a ’|A|) ; => #f(eq? ’a ’|a|) ; => #t(eq? ’|A| ’|a|) ; => #f

3.1.3.2. Printed Representation
Symbols may contain characters that are disallowed by R5RS using symbol->string. In such a case,the printed representation of that symbol will contain those characters, prefaced with the escape (’\’)
character. Likewise, such symbols may be created without symbol->string by escaping non-standard
characters.
Symbols which contain characters that could only be present in a case-sensitive environment will be
printed in one of two ways, depending on the value of the case-sensitive parameter. If true, thesymbols will be printed as is, containing the upper and lower case letters. If false, the symbol will be
printed surrounded by pipe characters.

3.1.4. Strings
Strings are built from Unicode characters, and are compared lexicographically in a manner derived fromcharacter comparison. In addition to using backslash to escape the double-quote (") character and thebackspace character itself, SISC provides several escape codes to ease string literal construction.
Table 3-2. String escape codes
EscapeValue\fInserts the formfeed character (unicode 000c)\nInserts the newline character (unicode 000a)\r

Inserts the rubout character (unicode 007f)\t

Inserts the tab character (unicode 0009)\uxxxxInserts the unicode character described by the hex
number ’xxxx’. All four hex digits must be
specified.

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Chapter 3. Scheme Language
EscapeValue\\

Inserts the backslash (’\’) character\"

Inserts the double quote (’"’) character

3.1.5. Pairs and Lists
A function is provided to determine if a given pair is a proper list.
procedure: (proper-list? datum) => #t/#f
Returns #t if the given argument is a proper-list. That is, if the argument is a pair, whose cdr is either the empty-list
or also a proper-list, and which contains no references to itself (is not circular).

3.1.6. Vectors
SISC supports the length prefix method of creating Vector constants. For example, ’#5(x) creates avector constant containing five identical symbols. In addition, the length-prefix form is used whenprinting vectors, and if elements repeat at the end of a Vector, only the last unique element is printed.
This form is referred to as the compact vector representation. The unprefixed form with all elements
displayed is called the verbose representation.
Vectors are displayed differently depending on the call used. When called with display, in addition tothe ordinary R5RS rules regarding the output of values displayed with display, the verbose
representation is displayed. Using write, on the other hand produces the compact representation.
Displaying a vector with pretty-print may output either the verbose or compact representation of avector. The behavior in this regard is controlled by the vectorLengthPrefixing configuration parameter(see Section 2.4.2). If set to #t, pretty-print will emit the compact representation. If #f, the verbose
representation is produced.

3.1.7. Boxes
SISC supports boxes, a container for a Scheme value. Boxing is often used to implement
call-by-reference semantics. Boxes are created and accessed using the following three functions:procedure: (box value) => box
Creates a box filled with the given value.
procedure: (unbox box) => value
Returns the value contained in the given box.
procedure: (set-box! box value) => undefinedReplaces the value contained in the given box with the value provided.

In addition to the box function for creating boxes, SISC provides an external representation for boxes
and boxed values. It extends the R5RS grammar with the following:

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Chapter 3. Scheme Language<boxed value> --> #&<datum>This syntax denotes a boxed value, with <datum> as the contained value.
Boxes are a distinct first class type. The box? predicate tests a value to see if is a box.
procedure: (box? value) => #t/#f
Returns #t only if the given value is a box.

Boxes, like pairs, are only equal in the sense of eq? and eqv? when a box is compared with itself. A box
is equal to another in the sense of equal? if the value contained within the box is equal? to the value
contained in the other.

3.1.8. Parameters
A parameter is a named dynamic variable that is accessed through a function. The function, when given
no arguments, returns the current value of the parameter. When given an argument, the value of theparameter is set to the provided value.
SISC’s parameters are fully compatible with those specified by SRFI-39. Consult the SRFI-39
specification at srfi.schemers.org (http://srfi.schemers.org) for documentation on how to construct anduse parameters. SRFI-39 does not specify the semantics for parameters in the presence of threads. SISC’s
parameters bind into the dynamic environment, which means their semantics are defined based on thesemantics of dynamic environments’ interactions with threads, specified in Section 6.1.

3.1.9. Immutable types
SISC follows the R5RS recommendation of immutable list, string, and vector constants. Quoted lists andvectors are immutable. Attempting to modify elements in these constants will raise an error. String
constants are immutable as well when created with symbol->string.

3.2. Equivalence
SISC’s storage model maintains true pointer equality between symbols, booleans, the end-of-file object,
void, and the empty list. Thus two instances of any of those types is guaranteed to return #t from eq? ifthey would have produced #t from equal?.
Numbers and characters are not pointer equal ordinarily (unless actually occupying the same storage).
SISC will return #t from eqv? if two numbers are both exact, or both inexact, and are numerically equal.
Two characters are equivalent from eqv? if they occupy the same code-point in the unicode characterset. This is the behavior specified by R5RS.
Strings, vectors, lists, and boxes are containers for other Scheme types. As such they are not pointerequal unless they are referenced by two variables that point to the same storage location (i.e. they are
actually pointer equal). SISC holds that only equal? will return #t if two objects are the same type andtheir contents contain equivalent values with respect to equal?.

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Chapter 3. Scheme Language

3.3. Syntax and Lexical Structure
3.3.1. Comments
In addition to the single line comments of the Scheme standard, SISC supports both s-expressioncommenting and nested, multiline comments. An s-expression comment is used to comment out an entire
s-expression. To do this, the sharp sequence #; is used. It extends the R5RS grammar with the following
production:<expression-comment> --> #;<datum>

The reader, upon encountering this sharp sequence, will read and discard the next datum. The expressioncommented out must still be a valid s-expression, however.
Nested, multiline comments are as defined in SRFI-30. Briefly, a multiline comment begins with the
sharp sequence #| and ends with the sequence |#. The comment may contain nested comments as well.
Unfortunately, this extension cannot be represented in a stateless grammar for the lexical structure.

3.3.2. Shared Structures
3.3.2.1. Reader Syntax
SISC provides a parse-time syntax for creating data (primarily vectors and lists) that contain references
to themselves or data which contains several pointer-equal elements. This can be useful to create
streams, graphs, and other self-referencing structures while maintaining readability and avoiding
complex construction code.
The reader syntax has two parts, defining a pointer, and later referencing the pointer to create the circularreference.
Below is an additional production in the R5RS formal syntax (specifically section 7.1.2, external
representations) to support circular structures:<pointer definition> --> #<uinteger 10>=<datum><pointer reference> --> #<uinteger 10>#

The first form instructs the reader to create a pointer identified by the specified integer, which maps to thedatum that follows, and is active during the reading of the datum on the right-hand side of the definition.
If a second definition occurs during the reading of the datum with the same integral identifier, theprevious definition is overwritten for the duration of the read. The definitions are not scoped in any way.
The pointer identifiers should be kept unique by the programmer to prevent any unintended effects ofidentifier collisions.

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Chapter 3. Scheme Language
The second form references a previously created pointer definition. It is an error to reference an
undefined pointer. The reader will handle a valid reference by placing a pointer at the current readposition back to the location of the definition.
At this point some examples might be helpful:
Example 3-2. Circular Structures(define x ’#0=(1 2 . #0#))(caddr x); => 1(list-ref x 15) ; => 2
(define y ’(1 2 #1=#(3 4) . #1#))(eq? (caddr y) (cdddr y)) ; => #t

3.3.2.2. Writing
Ordinarily, the display of cyclical data would cause a problem for a Read-Eval-Print-Loop. For thisreason, the REPL will attempt to check the structure it is about to print for circularities before printing. If
a cycle is found in the structure, the REPL will refuse to print if the printShared configuration parameter,
described below, is false. In that case the REPL will issue a warning to the user that the structure
contains a cycle. If a circular structure is printed with display, write, etc, and the printSharedparameter is set to false, the environment may enter an infinite loop which may or may not cause the
Scheme system to exit with an error.
The printShared configuration parameter (see Section 2.4.2), if set to true enables SISC to scan data forcircularity and data sharing before writing values. If such sharing is found, an alternate printer is invokedwhich will emit a representation compatible with the circular structure representation described in theprevious section.
Alternately, SISC also supports SRFI-38, which describes the functions write-showing-shared andread-with-shared-structure.

3.3.3. Control Features
In addition to the R5RS standard control features, two additional forms, when and unless, are supported
by SISC.
syntax: (when condition expression [expressions] ...) => value
Evaluates condition, an expression. If true, the expressions that follow are evaluated, in order, the value of the last
being returned. If not true, the result is unspecified.
syntax: (unless condition expression [expressions] ...) => value
Evaluates condition, an expression. If false, the expressions that follow are evaluated, in order, the value of the last
being returned. If true, the result is unspecified.

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Chapter 3. Scheme Language

3.3.4. Syntactic Extension
SISC provides a hygienic macro system that fully conforms to the R5RS standard. The macro system isprovided by the portable syntax-case macro expander. In addition to R5RS macros, the expander provides
a more flexible macro definition tool called syntax-case. A full description of the capabilities of the
expander is best found in the Chez Scheme Users Guide (http://www.scheme.com/csug.html),
specifically Section 9.2, Syntax-Case (http://www.scheme.com/csug/syntax.html#g2154).
In addition, SISC supports non-hygienic, legacy macro support in two forms; define-macro anddefmacro. These forms, found in older Scheme code written for R4RS compliant Scheme systems,
should be used only for executing legacy code which relies on it. New code should use the safer andmore flexible syntax-case or the standard syntax-rules macros.
syntax: (define-macro (name . args) body ...)
syntax: (define-macro name transformer)
In the first form, define-macro creates a macro transformer bound to name, which when applied will have raw
s-expressions bound to one or more parameters (args). The (name . args) name and formal parameter form isidentical to the short form for procedure definition with define.
The transformer’s body will then, using the s-expressions bound to its arguments, return a new s-expression that is
the result of the macro transformation.
The second form binds an arbitrary procedure to the syntactic keyword name, using that procedure to transform
occurences of that named syntax during future evaluations.
syntax: (defmacro name args body ...)
defmacro is another macro definition form supported by some Scheme systems. Its semantics are equivalent to:(define-macro (name . args) body ...)

3.4. Errors and Error Handling
Errors can be raised by primitives in libraries and Scheme-level code. SISC provides a sophisticatedmechanism for handling these errors when they occur during program execution.

3.4.1. Failure Continuations
During the execution of any program, there is always a continuation that represents the rest of acomputation. In addition, one can imagine all the activities that will occur as a result of an error. Thissequence of actions is explicitly represented in SISC as a failure continuation.
Two values must be applied to a failure continuation. The first is an error record, a datastructure which
describes the error (and may contain information about the name of the function that generated the error,a descriptive message about the error, etc.). The second is the continuation of the expression that raised
the error. All errors raised in SISC automatically and implicitly obtain and apply these values to the20Chapter 3. Scheme Language
active failure continuation. Applying the error record and error continuation to the failure continuation
will not return to the continuation of the application, unless that continuation was captured and later
invoked in a non-local entrance.

3.4.1.1. Creation
A programmer may wish to augment current failure continuation, choosing a different set of actions tooccur for a body of code if it raises an error. To facilitate this, SISC provides thewith-failure-continuation procedure.
procedure: (with-failure-continuation handler thunk) => value
procedure: (with/fc handler thunk) => value
with-failure-continuation takes as arguments a thunk (a zero-argument procedure) to be evaluated. The
thunk will be evaluated in the continuation of the with/fc function, and with a failure continuation defined by theprovided error handler. If during the evaluation of the thunk an error is raised, the first, two argument procedure iscalled with values describing the error and its context. If no error occurs, value of the thunk is applied to thecontinuation of the with/fc expression.

The error handler required as an argument to with-failure-continuation must accept two values.
The first is a value containing information about the error that occurred. This is often an association list
containing a number of attributes of the error. The second is a procedure encapsulating the continuationthat was in place at the site of the error. This continuation is referred to as the error continuation
When an error occurs, the error handler may choose one of three courses in dealing with the error. First,
the handler may choose to return an alternate value to be applied to the continuation of the with/fc
expression. Second, the handler may restart the computation from the error site by invoking the error
continuation with a value that should be returned in place of the expression that caused the error. Finally,
the handler may choose to propagate the error (or a new error) to the failure continuation of the with/fc
expression. This can be done with the throw function described in Section 3.4.2.3.

3.4.1.2. Capture
The currently active failure continuation may be obtained explicitly using thecall-with-failure-continuation procedure. This continuation may be applied to appropriate
values at any time in the future.
procedure: (call-with-failure-continuation procedure) => value
procedure: (call/fc procedure) => value
Calls the given one-argument procedure with the currently active failure continuation.

3.4.1.3. Interaction with Ordinary Continuations
Failure continuations exist as an attribute of the ordinary continuations of Scheme expressions. Becauseof this, the invocation of a continuation may cause a different failure continuation to become active in theregion of the captured continuation. Specifically, the failure continuation in place at the call/cc
expression will be reinstated when that continuation is later invoked.

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Chapter 3. Scheme Language
Similarly, invoking a continuation that escapes a region of code will cause any created failure
continuations to be abandoned, unless the region is itself captured in a continuation and later invoked.
See also Section 3.4.4.

3.4.2. Error Records
An error record is the value usually propagated with an error in SISC. It is a datastructure containing
such information as the location of the error, a descriptive message about the error, and possibly othererror metadata.

3.4.2.1. Creating Error Records
Error records can be created in advance of actually raising an error with the make-error function. The
function allows the programmer to create error records that contain a location and a message or value.
No field of an error record is required.
procedure: (make-error [location] [message] [arguments] ...) => error-record
Constructs an error record. If present, a symbol, and not #f, the first argument is the location of the error, which may
be a symbol equivalent to a function identifier. If present, the message is a format-string processed with the optional
arguments that follow as by format in SRFI-28. The remaining arguments must only be present if the format-string
is present as the message.
procedure: (make-error [location] error-value) => error-record
Constructs an error record. If present, a symbol, and not #f, the first argument is the location of the error. The
second argument is an arbitrary Scheme value that will be the error value. This value will be accessible with theerror-message function.

None of the fields of an error-record are required. One may create an error record with no information,
an error record with only a location, or an error record with only a message or value. Below are someexamples (for an explanation of the throw procedure see Section 3.4.2.3).
(throw (make-error)); => Error.
(throw (make-error ’foo)); => Error in foo.
(throw (make-error "something ~a happened" ’bad)); => Error: something bad happened(throw (make-error 3)); => Error: 3(throw (make-error #f ’foo)); => Error: foo(throw (make-error ’foo "something ~a happened" ’bad)); => Error in foo: something bad happened

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Chapter 3. Scheme Language
In addition, an error record may be created that adds additional information to an error record that was
already created. This is useful when an error was caught in an error handler, and one wishes to raise anerror from the handler that contains additional information about the local location or error message aswell as the error that was caught.
procedure: (make-nested-error local-error parent-error parent-error-continuation) =>error-record
procedure: (make-nested-error local-error exception) => error-record
The first version creates an error record which has parent-error (and it’s associatedparent-error-continuation) as the root cause of an error-record passed as local-error .
The second version creates an error record which has exception (see Section 3.4.2.4) as the root cause of an error
passed as local-error .

An example of the creating, throwing, and display of a nested error follows.
(with-failure-continuation(lambda (m e)(throw (make-nested-error(make-error ’foo "could not call bar.") m e)))(lambda ()(error ’bar "something went wrong.")));=> Error in foo: could not call bar.
;
Caused by Error in bar: something went wrong.

3.4.2.2. Accessors
An error record contains several useful pieces of information. The following functions allow the
programmer to access that information.
procedure: (error-location error-record) => symbol
Obtains the location of the error, a symbol which may be a function identifier. If there is no location specified, #f isreturned.
procedure: (error-message error-record) => value
Obtains the message of the error, which may be a string which is a descriptive message of the error, or an arbitraryvalue (as created by the second form of make-error). If there is no message specified, #f is returned.
procedure: (error-parent-error error-record) => error-record
Obtains the parent error of the error. This is the value of the second argument to the make-nested-error function.
If there is no parent specified, #f is returned.
procedure: (error-parent-continuation error-record) => error-continuation
Obtains the parent error continuation of the error. This is the value of the third argument to the
make-nested-error function. If there is no parent specified, #f is returned.

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Chapter 3. Scheme Language

3.4.2.3. Raising Errors
The fundamental mechanism for raising an error in application code is provided by the throw procedure.
procedure: (throw error-record [error-continuation]) => does not returnprocedure: (throw exception) => does not return
The first verison applies the given error record to the current failure continuation. If provided, the error continuation
is designated by the optional parameter. If not, the continuation of the throw expression is used.
The second form applies the current failure continuation to the error record and error continuation extracted from the
supplied exception (see Section 3.4.2.4).

If invoked from an error-handler with the values of the handler’s formal parameters, throw has the effectof propagating the error in a manner that is equivalent to the absence of the modified failure-continuation.
throw could be defined in terms of call-with-failure-continuation as:(define (throw error . args)(call-with-failure-continuation(lambda (fk)(if (null? args)(call-with-current-continuation (lambda (k) (fk error k)))(fk error (car args))))))For convenience and compatibility with SRFI-23, the function error is provided. Its syntax is identicalto make-error, but it immediately applies the resulting error record to the current failure continuation
with the current continuation as the error continuation.
procedure: (error [location] [message] [arguments] ...) => does not return
Raises an error record whose location, if provided, is location, a symbol; and whose error message, if present, ismessage. If provided, the error message is a format-string that is processed, with the optional arguments, as withthe format function in SRFI 28.
procedure: (error [location] error-value) => does not return
Raises an error record whose location, if present, is the symbol location, and and whose error-value is any
arbitrary Scheme value.
error can be implemented in terms of throw and make-error:(define (error . args)(throw (apply make-error args)))

3.4.2.4. Exceptions
Exceptions in SISC are a simple wrapper around an error record and an associated error continuation.

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Chapter 3. Scheme Language
Exceptions are created withprocedure: (make-exception error-record error-continuation) => exception
Constructs an exception from an error-record and an error-continuation, e.g. as obtained from thearguments of a handler procedure passed to with-fc.

Accessors and a type-test are provided by the following procedures:procedure: (exception-error exception) => error-record
Returns the exception’s error record.
procedure: (exception-continuation exception) => error-continuation
Returns the exception’s error continuation.
procedure: (exception? value) => #t/#f
Returns #t if value is an exception object, #f otherwise.

3.4.3. Examples
At this point, a few examples may be helpful:(+ 1 (/ 1 0) 3); => A divide by zero error is raised

Example 3-3. Return a new value(with-failure-continuation(lambda (error-record error-k)’error)(lambda () (+ 1 (/ 1 0) 3)))
; => The symbol ’error

Example 3-4. Restart with a different value(with-failure-continuation(lambda (error-record error-k)(error-k 2))(lambda () (+ 1 (/ 1 0) 3)))
; => 6

Example 3-5. Propagate the error(with-failure-continuation(lambda (error-record error-k)(throw error-record error-k))(lambda () (+ 1 (/ 1 0) 3)))
; => A divide by zero error is raised25

Chapter 3. Scheme Language
Example 3-6. Propagate a different error with the same error continuation(with-failure-continuation(lambda (error-record error-k)(throw (make-error ’/ "could not perform the division.") error-k))(lambda () (+ 1 (/ 1 0) 3)))
; => An error is raised: Error in /: could not perform the division.

Example 3-7. Raise a new error(with-failure-continuation(lambda (error-record error-k)(error ’example-function "could not evaluate the expression."))(lambda () (+ 1 (/ 1 0) 3)))
; => An error is raised: Error in example-function: could not evaluate the expression.

Note that the difference between Example 3-6 and Example 3-7 is that in the latter, the computation canstill be restarted from the second argument of the addition if an outside handler catches the newly raised
exception and applies the continuation. This is not true in the last example, as its a new error whose
continuation is the same as the with-failure-continuation expression.

3.4.4. dynamic-wind
R5RS does not specify the behavior of dynamic-wind in the case where an error is raised while
evaluating the during thunk. SISC chooses to view an error raised in that section as an instance of the
dynamic extent being exited. In other words, if an error is raised in the dynamic extent of a
dynamic-wind expression, SISC will ensure that the after thunk is evaluated before the error is
propagated to the failure-continuation of the dynamic-wind expression.
Example 3-8. Errors and dynamic-wind(define x 0)
(dynamic-wind (lambda () (set! x (+ x 1)))(lambda () (/ 1 0))(lambda () (set! x (+ x 1)))); => A divide by zero error is raised, and the value of x is 2

If an error is raised in either the before or after thunks, no additional measures are taken. The error is
propagated to the failure-continuation of the dynamic-wind as if the dynamic-wind call was an ordinary
function application. Explicitly, if an error is raised from before, neither during nor after will be
executed. If an error is raised in after, the results of evaluating before and during remain valid.
Also noteworthy is what happens if a continuation is invoked that exits from either the before or after
thunks. Such a case is treated just as if a continuation was invoked during the evaluation of an operand toan application. This is to say that no additional steps will be taken by SISC. If before is escaped by acontinuation invocation, neither during nor after will be executed. If after is escaped, the results ofbefore and during remain valid.

26

Chapter 3. Scheme Language
In summary, extraordinary evaluation is only possible during the evaluation of the during thunk. The
before and after thunks are evaluated with the dynamic environment and dynamic-wind stack of the callto dynamic-wind itself.

3.5. Symbolic Environments and Property Maps
Symbolic environments and property maps provide additional named global environments useful forstoring program specific data without exposing it to the general purpose top-level environment.
A property map is dictionary structure tied to the interaction environment which maps symbolic names to
Scheme values. First-class symbolic environments provide a similar mapping, but can be used as firstclass values (including as an argument to eval). Symbolic environments are used to implement SISC’s
global (top level)and report environments.

3.5.1. Access Functions
Access to symbolic environments is performed through the getprop and putprop functions. Allsymbolic environment operations are thread safe.
procedure: (getprop binding-name plist-name [default-value]) => value
procedure: (getprop binding-name environment [default-value]) => value
Attempts a lookup of binding-name in an environment.
In the first form, the the binding is resolved in the interaction-environment’s property list named plist-name, asymbol. If the environment is not found or the binding doesn’t exist, default-value is returned if provided,
otherwise #f is returned.
In the second form, the binding is resolved in a first-class symbolic environment.
procedure: (putprop binding-name plist-name value) => undefinedprocedure: (putprop binding-name environment value) => undefined
Sets the value of a binding named with the symbol binding-name in a property list or first class symbolic
environment.
In the first form, the binding is resolved using a symbolic name (plist-name) in the interaction environment’s
property lists. If the map does not yet exist, it is created as an empty map.
In the second form, the binding is resolved in the provided first class symbolic environment. If the binding does not
yet exist in the given environment, it is created. If a binding previously existed, its previous value is discarded.

3.5.2. Obtaining and Naming
Symbolic environments are a first class datatype in SISC. The top-level environment itself is merely aspecial cased symbolic environment. To obtain the top-level environment as a first class value, one can27

Chapter 3. Scheme Language
use the interaction-environment function that is an optional procedure in R5RS. Another usefulenvironment is the R5RS report environment available by calling:(scheme-report-environment 5)

Each call to scheme-report-environment returns a new environment that contains only the bindings
available in the Scheme report. Finally, the initial environment available to the programmer when SISC
starts can be retrieved using the sisc-initial-environment function:
procedure: (sisc-initial-environment) => environment
Returns the initial SISC interaction environment.

Like scheme-report-environment, each call to sisc-initial-environment returns a distinct
environment which contains only the bindings initially available when SISC starts. An interesting use ofthis would be to define one or more distinct initial-environments, bound to toplevel variables. One could
then define Scheme code and data in each environment that can use the full SISC language but cannot see
any bindings in other environments.
Finally, R5RS states that it is an error to modify the contents of a top-level variable that has not yet beencreated. SISC adheres to the standard, and raises an error when any unbound variable in a symbolic
environment (including the top-level) is modified using set!. This differs from some Scheme systems
that will silently create the binding and set it to the new value.

3.5.3. Chained Symbolic Environments
SISC contains a mechanism for creating a symbolic environment which is chained to anotherenvironment, such that new and modified bindings are created in the new, child environment, but
bindings may also be resolved from the parent if not present in the child. SISC uses this functionality toprotect the contents of the R5RS and SISC initial environments from modification. One can use it in asimilar way, protecting the bindings in the parent for sandboxing or other purposes.
procedure: (make-child-environment parent-environment) => environment
Creates a new environment, initially empty of its own bindings, but which chains to the provided
parent-environment when resolving a binding.
procedure: (parent-environment environment) => environment
Obtains the parent environment of a symbolic environment. If the given environment has no parent (e.g. is notchained), #f is returned.

3.6. Miscellaneous Functions
The remaining functions in this chapter are not easily classified, but nevertheless are useful and worth
describing.
procedure: (circular? datum) => #t/#f

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Chapter 3. Scheme Language
Returns #t if the given datum is circular. A datum is circular if it is a compound datum (lists or vectors for example),and one of its elements is a reference to itself, or a reference to a sub-element which creates a cycle.
procedure: (compose [function] ...) => procedurecompose takes zero or more functions of one argument and returns a new function of one argument that will apply tothat argument to the selected functions in reverse order. If no functions are provided, the identity function is returned.
For example, the function caddr could be simply defined as:(define caddr (compose car cdr cdr))

procedure: (iota n) => pair
The iota function produces a list whose elements are the integers 0 to n-1 inclusive.
syntax: (time [iterations] expression) => list
Evaluates the given expression iterations times, or if iterations is not provided, only once. When complete, alist is returned of the following form:(result (n ms))

where result is the Scheme value that resulted from the last evaluation of the expression, and n is the number ofmilliseconds taken to evaluate the expression. If more than one iteration occurred, then the average number ofmilliseconds elapsed during each iteration is returned.

Notes
1. Essentially arbitrary, see Section D.1 for a discussion of the physical limits of number representation

29

Chapter 4. Debugging Facilities
No Scheme system would be complete without facilities to assist the programmer in debugging his or her
code. SISC provides aid for passive debugging (requiring no action on the part of the programmer) andactive debugging (requiring code instrumentation to facilitate debugging).

4.1. Passive Debugging
Passive debugging facilities are provided that collect information on an error that occurred at runtime andwas not caught by the executing code. The programmer can then inspect the last error, obtain information
about the call stack of the error, or even attempt to restart the computation.
procedure: (get-last-exception) => exception
Retrieves the last exception that occurred in SISC.

One of the most common desires is to obtain a trace of the call stack, to determine what sequence of calls
resulted in the error. SISC provides procedures for accessing the call stack of any continuation.
Requires: (import debugging)procedure: (stack-trace continuation) => list
Returns the stack trace for continuation in form of a list. The format of the list is
stack-trace:=(call-frame ...)

call-frame:=

sisc-expr | (sisc-expr overflown .
stack)

overflown:=#t | #fstack:=(stack-entry ...)

stack-entry:=

sisc-expr | (repetitions . stack)repetitions:=

integer

Each element in the list represents one level in the SISC interpreter’s call stack, starting from the top. The element
contains the SISC virtual machine expression that would be executed next in that frame, and, if available, a compact
representation of a virtual stack created by collecting information on tail calls carried out in that frame.
The virtual stack is bounded in size by the value of the see maxStackTraceDepth configuration parameter (seeSection 2.4.2). If old information was dropped due to the bound being exceeded then the overflown flag is set.
Each entry in the virtual stack contains either the SISC virtual machine expression that was executed, or a sub-stackannotated with a repetition count, indicating that the entries in that sub-stack were repeated several times.
SISC expressions are annotated with source locations if the emitAnnotations parameter is set to true. Additionalannotations are produced when emitDebuggingSymbols is set to true. See Section 2.4.2. The annotations can be
retrieved using the annotation function with the keys source-file, line-number, column-number,source-kind, and proc-name.
Stack trace entries for expressions with a source-kind mentioned on
suppressed-stack-trace-source-kinds are suppressed.

30

Chapter 4. Debugging Facilities
procedure: (suppressed-stack-trace-source-kinds [list]) => list
Retrieves or sets the list of source kinds to suppress in stack traces returned by stack-trace. This is a dynamic
parameter.
The default value is (#f) which causes all stack trace entries for expressions with no specified source kind to be
suppressed.
The annotation of expressions with source kinds and other information is controlled by the source-annotations
parameter.
procedure: (source-annotations [alist]) => alist
Retrieves or sets the association list of additional annotations for expressions that are being read. This is a dynamic
parameter.
All system and core library code is loaded with this parameter set to (), resulting in no additional annotations being
produced. However, on entry to the REPL, the parameter is set to ((source-kind . user)). In combination withthe default settings for suppressed-stack-trace-source-kinds this results in system code being omitted
from stack traces.
procedure: (print-stack-trace continuation) => void
Displays the call stack of the continuation in a human-readable form.

The error message, location information and call stack associated with an exception can be displayed inhuman-readable form using the following procedure.
procedure: (print-exception exception [stack-trace?]) => void
Displays the error message and location of exception. A stack trace is displayed if stack-trace? is absent or set
to #t. Furthermore the procedure calls itself recursively in order to display similar information for nested exceptions.

In order to obtain the source file location of a call, your Scheme code must have been loaded while
SISC’s reader had annotations enabled. Annotations are a means of attaching metadata to compiled
Scheme code. To allow the reader to attach annotations related to the source file position of the code it
reads, enable the emission of annotations with the emitAnnotations configuration parameter (seeSection 2.4.2).
SISC can also produce more detailed stack traces if code was generated with debugging symbols. Theseare extra annotations generated by the compiler that track function and variable names that would
ordinarily be discarded. By including these annotations, the stack trace can display the name of more of
the calls involved. To enable the generation of debugging symbols, the emitDebuggingSymbols
configuration parameter must be set to true (see Section 2.4.2).
Finally, when debugging a program for a long period of time, it may be desirable to have stack traces
displayed whenever an error occurs, rather than needing to invoke print-exception or other functions
each time. For this, the stackTraceOnError configuratin parameter must be set to true (seeSection 2.4.2).

4.2. Active Debugging
Requires: (import debugging)

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Chapter 4. Debugging Facilities
SISC provides active debugging aids; procedures and syntax that can be used in source code to assist in
tracing the activities of running Scheme code.

4.2.1. Runtime Tracing
When a function is traced, each call to the function will be displayed to the console with the function’s
trace identifier and the values of the operands the function is being applied to. Each nested call isindented slightly, so as to illustrate the depth of calls. When the function application returns, the value ofthe function-call is displayed at the same indentation as the call itself. Once indented to a certain depth,
the same indentation is kept for further nesting, but the depth of the call is displayed as an integer
preceding the call.
syntax: (trace-lambda trace-name formals body) => procedure
When replaced with a trace-lambda, all calls to a lambda defined procedure are traced on the console. trace-nameis an identifier which will disambiguate the procedure in the trace. formals and body have identical semantics tolambda.
syntax: (trace-let loop-identifier formal-bindings body) => value
Replaces a named-let expression in a similar manner to trace-lambda.
procedure: (trace [symbol] ...) => undefined
Begins traces on the procedures named by the symbols given. The procedures must be defined in the top-level
environment.
If no procedures are given, a message is displayed indicating the names of top-level procedures currently being
traced.
If a traced procedure is redefined, it will not retain the instrumenting installed by trace, until trace or untrace iscalled again (with any arguments). At that time, the traced procedures are reinspected and instrumenting reinstalled
on redefined procedures.
procedure: (untrace [symbol] ...) => undefinedStops tracing the top-level procedures named by the symbols given.
If no procedures are given, a message is displayed indicating the names of top-level procedures currently being
traced.
trace-lambda and trace-let are useful for debugging anonymous lambdas and named-lets
respectively. trace and untrace ar useful for tracing any top-level bound procedure, including calls tobuiltin procedures and stored continuations.
Note: Tracing a function installs instrumentation code around the procedure which does not preserve
the continuation of a call to that function. Thus, tail calls made in a traced function are no longer tailcalls. This may affect the memory usage characteristics of running code.

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Chapter 4. Debugging Facilities

4.2.2. Breakpoints
A user may wish to halt execution of a running Scheme program when a given procedure is called. SISC
provides means to install breakpoints at top-level visible functions without having to redefine thefunction.
When a breakpoint is set using set-breakpoint!, and the function is called, execution will halt,
returning to the REPL and displaying an informational message indicating a break, the procedure called,
the arguments passed to the breakpointed procedure, and, if possible, the location in a source file of thecall. The user may then continue execution using the continue procedure.
procedure: (set-breakpoint! symbol) => undefined
Instruments the top-level procedure named by the given symbol, such that when called, execution will halt andreturn to the REPL and the name of the breakpointed function and its arguments are displayed.
procedure: (clear-breakpoint! symbol) => undefined
Removes the instrumentation on the named top-level procedure, if present. Execution will continue normallythrough occurances of the formally breakpointed procedure.
procedure: (continue) => does not return
Continues execution from the most recent break. It is an error to call this procedure if a breakpoint has not been hit,or to call this procedure more than once for a given break.
procedure: (current-breakpoint-continuation) => continuation
Returns the continuation of the most recent breakpoint, or #f if execution is not currently interrupted at a breakpoint.
The continuation is useful for obtaining stack traces, e.g. with (print-stack-trace(current-breakpoint-continuation)).
procedure: (current-breakpoint-args) => list
Returns a list containing the current breakpoint’s continuation procedure and arguments that will be used when
execution is resumed with continue, or #f if execution is not currently interrupted at a breakpoint.
This procedure is useful for performing deep inspection of the breakpointed procedure and its arguments. The
returned values are also handy for constructing modified breakpoint continuations with
set-current-breakpoint-args!.
procedure: (set-current-breakpoint-args! procedure value ...) => #t/#f
Sets the current breakpoint’s continuation procedure and arguments that will be used when execution is resumed
with continue. If execution is not currently interrupted at a breakpoint then invoking this procedure has no effectand it returns #f. Otherwise it returns #t.

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Chapter 5. I/O
SISC’s I/O routines are implemented in a flexible manner, allowing extensions to create new I/O sourcesthat will behave as standard Scheme port objects. The ports can then be operated on with all R5RS port
operations, as well as some SISC specific port functions.

5.1. Ports
5.1.1. URLs
In SISC all procedures that create ports for accessing files, e.g. open-input-file,open-output-file accept URLs in addition to ordinary file names. Here are some examples of valid
URLs:
http://foo.com/bar/bar1.scm
file:/tmp/foo.scm
file:c:\bar\baz.scm
file:foo.scm
jar:http://foo.com/bar.jar!/bar/bar1.scm

The last is a URL referring to a file stored in a JAR on a remote web server. For further details on theformat of URLs please consult this specification (http://www.ietf.org/rfc/rfc2396.txt). The format of JAR
URLs is defined in the JDK API documentation(http://java.sun.com/j2se/1.4/docs/api/java/net/JarURLConnection.html). What types of URLs aresupported by a particular installation of Java depends on the configured protocol handlers. See the JDK
API documentation (http://java.sun.com/j2se/1.4/docs/api/java/net/URL.html) for details. 1
Relative file names or URLs are resolved in relation to the following parameter:parameter: (current-url [url]) => url
Retrieves or sets the URL which forms the basis for resolving relative filenames and URLs. It is initialized on startup with the path to the current directory. All parameters and the returned value are strings.

The algorithm for resolving relative URLs is defined in this specification(http://www.ietf.org/rfc/rfc2396.txt). For compatibility with other Schemes, SISC also supports thecurrent-directory procedure, which is a simple wrapper around current-url.
A convenience procedure exists for executing a procedure while the current-url is temporarily set toa different value:
procedure: (with-current-url url thunk) => value
Sets the current URL to the URL obtained by normalizing url in relation to the current URL, then executes thunk,and then sets the current URL back to the previous value.

URLs can be normalized using
procedure: (normalize-url url1 [url2]) => url
When called with one string argument, normalize-url returns the normalized version of the given URL.
Normalization involves, amongst other things, the replacement of relative path references such as . and ...

34

Chapter 5. I/OWhen called with two string arguments, the procedure returns the normalized version of the second URL wheninterpreted as a being relative to the first URL.

5.1.2. Buffered I/O
Buffered ports are provided in SISC to layer over any existing port, and read or write in larger, more
efficient chunks. Buffered ports are created with the following constructors, which accept the underlying
port and an optional size, indicating the number of bytes or characters to buffer before making an
underlying read or write.
procedure: (open-buffered-binary-input-port binary-input-port) => binary-input-port
Creates a binary-input-port which is buffered, and reads its bytes from the provided port.
procedure: (open-buffered-binary-output-port binary-output-port) => binary-output-port
Creates a binary-output-port which is buffered, and writes its bytes to the provided port.
procedure: (open-buffered-character-input-port character-input-port) => character-input-port
Creates a character-input-port which is buffered, and reads its characters from the provided port.
procedure: (open-buffered-character-output-port character-output-port) => character-output-port
Creates a character-output-port which is buffered, and writes its characters to the provided port.

With buffered output ports, individual writes may not actually reach the eventual output source, so theprogrammer must explicitly flush the port when it the output data must reach its destination.
procedure: (flush-output-port [output-port]) => undefined
Causes the specified output-port’s buffered data to be written immediately. This operation is allowed on any
output port, but may have no affect on some. output-port defaults to current-output-port.

5.1.3. Character Ports
The R5RS I/O primitives implemented by SISC create character ports. Character ports read charactersfrom input sources and treat the data as characters in a given character set. Correspondingly, character
ports output bytes from characters according to a given character set’s encoding rules.
By default character ports use the value of the string parameter character-set as the character
encoding name. A list of many possible encoding names can be found in the Java Platform
Documentation (http://java.sun.com/j2se/1.4/docs/guide/intl/encoding.doc.html)
One may temporarily change the default character set using the with-character-set function.
procedure: (with-character-set encoding thunk) => value
Changes the value of the character-set parameter, and the default character set to the named encoding while
executing the body of thunk.

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Chapter 5. I/O

5.1.3.1. Port Creation
procedure: (open-input-file url [encoding]) => input-port
Creates an input port from the specified url. If the optional encoding parameter, a string, is supplied input will be
decoded from the specified encoding rather than the default.
procedure: (open-output-file url [encoding] [auto-flush]) => output-port
Creates an output port to the specified url. If the optional encoding parameter, a string, is supplied output will be
encoded in the specified encoding rather than the default. If the optional auto-flush argument is provided and is
non-false, the port will automatically flush after each write call. If the specified file exists, it will be overwritten
silently when the port is opened.
procedure: (open-character-input-port binary-input-port [character-set]) => input-port
Creates an input port which reads from the provided binary input port. One may specify the desired character set as astring, otherwise the character set is retrieved from the character-set dynamic parameter.
procedure: (open-character-output-port binary-output-port [character-set] [auto-flush])=> output-port
Creates an output port which writes to the provided binary output port. One may specify the desired character set as
a string, otherwise the character set is retrieved from the character-set dynamic parameter. If the optional
auto-flush argument is provided and is non-false, the port will automatically flush after each write call.

5.1.3.2. Bulk Character I/O
In addition to the R5RS I/O primitives, SISC provides two functions for reading and writing blocks ofcharacters from a character port.
procedure: (read-string buffer offset count [character-input-port]) => integer
Reads up to count characters from the implicit or specified character port into the string buffer starting from theposition specified by the integer offset. The number of characters successfully read (which may be fewer thancount) is returned, or the end-of-file value if the end-of-file was reached before any characters were encountered.
procedure: (write-string buffer offset count [character-output-port]) => undefined
Writes exactly count characters from the given string buffer starting from the integer position offset to theimplicit or specified character output port.

5.1.3.3. Port Creation Wrappers
The next set of procedures assists in creating a port, followed by calling a given procedure with that port.
When the procedure returns, the port is closed. Invoking escaping continuations from inside theprocedure does not close the port, and invoking a continuation captured inside the procedure does notopen the port.
procedure: (call-with-input-file url [encoding] procedure) => value
Calls procedure with a new input port attached to url. The result of the thunk is returned.
If the optional encoding parameter is provided, the character port created will use the specified encoding rather
than the default.

36

Chapter 5. I/Oprocedure: (call-with-output-file url [encoding] procedure) => value
Calls procedure with a new character output port attached to url. The result of the thunk is returned.
If the optional encoding parameter is provided, the character port created will use the specified encoding rather
than the default.

5.1.3.4. Replacing Standard Ports
The following procedures wrap a thunk, redirecting the input and output of the thunk while it isevaluating to an input or output port other than the current-input-port andcurrent-output-port. Invoking escaping continuations from inside the procedure restores theoriginal port, and invoking a continuation captured inside the procedure restores the redirection.
procedure: (with-input-from-port input-port thunk) => value
Evaluates thunk with input-port as the current-input-port for the duration of the evaluation.
procedure: (with-output-to-port output-port thunk) => value
Evaluates thunk with output-port as the current-output-port for the duration of the evaluation.
procedure: (with-input-from-file url [encoding] thunk) => value
Evaluates thunk with an input port attached to a file opened for reading from url as the current-input-port
for the duration of the evaluation. The port is closed when thunk returns normally.
If the optional encoding parameter is provided, the character port created will use the specified encoding rather
than the default.
procedure: (with-output-to-file url [encoding] thunk) => value
Evaluates thunk with an input port attached to a file opened for writing to url as the current-output-port forthe duration of the evaluation. The port is closed when thunk returns normally.
If the optional encoding parameter is provided, the character port created will use the specified encoding rather
than the default.

5.1.3.5. Port Predicatesprocedure: (input-port? value) => #t/#f
Returns #t if value is an input port, #f otherwise.
procedure: (output-port? value) => #t/#f
Returns #t if value is an output port, #f otherwise.

5.1.4. String Ports
Requires: (import string-io)
String ports are input or output ports that read or write to a string rather than a file or other stream. String
ports can be used to parse or emit formatted strings using the standard Scheme port operations. A String37

Chapter 5. I/O
Input port will read from a given string until the end of string is reached, at which point #!eof isreturned.
String ports deal with characters as the atomic unit, and as such preserve full unicode width characters at
all times.
procedure: (open-input-string string) => string-input-port
Creates a string input port whose characters are read from the provided string. Characters will be returned from any
read operation on the port until the end of the string is reached. Read calls after reaching the end of the string will
return #!eof.
procedure: (open-output-string) => string-output-port
Creates a string output port, which behaves as an ordinary output port, except that writes are used to create a string
as output. The results of all the write operations are retrieved using get-output-string.
procedure: (get-output-string string-output-port) => string
Returns the string that was created by zero or more writes to a string output port. If no writes were performed on thestring output port, an empty string ("") is returned. After this call, the provided string output port is reset to its initial,
empty state.
procedure: (call-with-input-string string procedure) => value
Calls procedure with a new string input port created from string . The result of the thunk is returned.
procedure: (call-with-output-string procedure) => string
Calls procedure with a new string output port. The contents of the string-output-port are returned when theprocedure returns.
procedure: (with-input-from-string string thunk) => value
Evaluates thunk with a string-input-port created from string as the current-input-port for the duration of
the evaluation.
procedure: (with-output-to-string thunk) => string
Evaluates thunk with a string-output-port created as the current-output-port for the duration of the evaluation.
When the thunk returns, the contents of the string-output-port are returned.
procedure: (string-input-port? value) => #t/#f
Returns #t if value is a string input port, #f otherwise.
procedure: (string-output-port? value) => #t/#f
Returns #t if value is a string output port, #f otherwise.
Note: This interface complies with SRFI-6 (Basic String Ports).

5.1.5. Binary Ports and Block IO
Requires: (import binary-io)38

Chapter 5. I/O
In addition to the R5RS I/O functions, SISC provides a symmetric set of functions for reading and writing
binary data to and from ports with no character set translation. These ports are operated on using thebinary I/O functions described below. Using character-oriented operations (such as the traditional R5RS
functions read, read-char, display, etc.) is an error. Binary ports also provide block input/output
functions, that allow a Scheme program to read blocks of more than one byte of data at a time frombinary ports. SISC stores data that is read or is to be written in block fashion in a binary buffer (seeSection 9.1.5).
procedure: (open-binary-input-file url) => binary-input-port
Creates an input port in the same manner as R5RS open-input-file, producing an input port that does no
character-set decoding on the bytes read as input.
procedure: (open-binary-output-file url [auto-flush]) => binary-output-port
Creates an output port in the same manner as open-output-file, producing an output port that does no
character-set encoding.
procedure: (call-with-binary-input-file url procedure) => value
Calls procedure with a new binary input port attached to url. The result of the thunk is returned.
procedure: (call-with-binary-output-file url procedure) => value
Calls procedure with a new binary output port attached to url. The result of the thunk is returned.
procedure: (with-binary-input-from-file url thunk) => value
Evaluates thunk with a binary input port attached to a file opened for reading from url as thecurrent-input-port for the duration of the evaluation. The port is closed when thunk returns normally.
procedure: (with-binary-output-to-file url thunk) => value
Evaluates thunk with a binary input port attached to a file opened for writing to url as thecurrent-output-port for the duration of the evaluation. The port is closed when thunk returns normally.
There are several operations specifically available for use on binary ports.
procedure: (peek-byte [binary-input-port]) => integer
Similar to peek-char, reads ahead one byte in the stream, returning the next byte available but not advancing thestream. The byte is returned as an integer. The current input port is used unless specified.
procedure: (read-byte [binary-input-port]) => integer
Reads a single byte from the stream, advancing the stream and returning the byte as an integer. The current input
port is used unless specified.
procedure: (read-block buffer offset count [binary-input-port]) => integer
Reads up to count bytes of data from the current input port or the binary-input-port parameter if provided,
into the binary buffer buffer starting at position offset. Note that less than count bytes may be read. The
number of bytes actually read is returned. If the end-of-file is encountered before any bytes could be read, #!eof
will be returned.
procedure: (write-byte integer [binary-output-port]) => undefined
Writes a single byte specified as an integer to the given binary-output-port if provided, current output portotherwise.

39

Chapter 5. I/Oprocedure: (write-block buffer offset count [binary-output-port]) => undefined
Writes count bytes of data from the provided buffer at starting point offset to the given
binary-output-port or to the current output port if unspecified. Exactly count bytes will be written.
procedure: (binary-input-port? value) => #t/#f
Returns #t if value is a binary input port, #f otherwise.
procedure: (binary-output-port? value) => #t/#f
Returns #t if value is a binary output port, #f otherwise.

5.1.5.1. Buffer I/O
Requires: (import buffer-io)
A module is also provided for input and output to and from binary buffers using buffer ports, similar tocharacter I/O to and from strings using string ports.
procedure: (open-input-buffer buffer) => buffer-input-port
Creates a buffer input port whose bytes are read from the provided buffer. Bytes will be returned from any read
operation on the port until the end of the buffer is reached. Read calls after reaching the end of the buffer will return#!eof.
procedure: (open-output-buffer) => buffer-output-port
Creates a buffer output port, which behaves as an ordinary output port, except that writes are used to create a buffer
as output. The results of all the write operations are retrieved using get-output-buffer.
procedure: (get-output-buffer buffer-output-port) => buffer
Returns the buffer that was created by zero or more writes to a buffer output port. If no writes were performed on thebuffer output port, an empty buffer is returned. After this call, the provided buffer output port is reset to its initial,
empty state.
procedure: (call-with-input-buffer buffer procedure) => value
Calls procedure with a new buffer input port created from buffer . The result of the thunk is returned.
procedure: (call-with-output-buffer procedure) => buffer
Calls procedure with a new buffer output port. The contents of the buffer-output-port are returned when theprocedure returns.
procedure: (with-input-from-buffer buffer thunk) => value
Evaluates thunk with a buffer-input-port created from buffer as the current-input-port for the duration of
the evaluation.
procedure: (with-output-to-buffer thunk) => buffer
Evaluates thunk with a buffer-output-port created as the current-output-port for the duration of theevaluation. When the thunk returns, the contents of the buffer-output-port are returned.

40

Chapter 5. I/O

5.1.6. Java Ports
Requires: (import java-io)
This module provides procedures to convert between Scheme and Java I/O types. In general, binary
Scheme ports map to plain Java streams, while character Scheme ports map to Java readers and writers.
The following example is somewhat contrived, but illustrates a common usage pattern:;; A convoluted Hello World example.
(import s2j)
(import java-io)(define-java-classes(<java.lang.system> |java.lang.System|))(define-generic-java-field-accessors(:jout out))(let ((stdout (open-character-output-port(->binary-output-port(:jout (java-null <java.lang.system>)) #t) #t)))(display "Hello, world!" stdout))

procedure: (->binary-input-port jinput-stream) => binary-input-port
Returns a binary input port associated to the java.io.InputStream object passed as the jinput-stream
parameter.
procedure: (->binary-output-port joutput-stream [aflush?]) => binary-output-port
Returns a binary input port associated to the java.io.OutputStream object passed as the joutput-stream
parameter. If the optional boolean aflush? parameter is not provided, the port will not autoflush by default.
procedure: (->character-input-port jreader) => character-input-port
Returns a character input port associated to the java.io.Reader object passed as the jreader parameter.
procedure: (->character-output-port jwriter [aflush?]) => character-output-port
Returns a binary input port associated to the java.io.Writer object passed as the joutput-stream parameter. If
the optional boolean aflush? parameter is not provided, the port will not autoflush by default.
procedure: (->jinput-stream input-port) => jinput-stream
Returns a java.io.InputStream object associated to the given Scheme input-port. The function produces anerror if a character port is passed.
procedure: (->joutput-stream output-port) => joutput-stream
Returns a java.io.OutputStream object associated to the given Scheme output-port. The function produces
an error if a character port is passed.

41

Chapter 5. I/Oprocedure: (->jreader character-input-port) => jreader
Returns a java.io.Reader object associated to the given Scheme character-input-port.
procedure: (->jwriter character-output-port) => jwriter
Returns a java.io.Writer object associated to the given Scheme character-output-port.

Scheme ports can also be used from Java. See Section 8.1.5.3 for information.

5.1.7. Serialization
Requires: (import serial-io)
With read and write, Scheme values are read and written in a standardized, textual external
representation. However, this external representation only fully describes a limited subset of Scheme
types. For instance it is impossible to read/write a procedure, or closure, or continuation.
SISC provides a special composed port type and procedures for reading and writing any Scheme value
using a binary representation. The (de)serialization preserves the referential structure of the object graphcomprising the serialized values.
Serial ports are composed onto and thus are binary ports, i.e. all operations applicable to binary ports
also apply to serial ports.

5.1.7.1. Port Creation and Identification
procedure: (open-serial-input-port binary-input-port) => serial-input-port
Creates a serial input port which reads external representations of Scheme values from the given binary input port.
procedure: (open-serial-output-port binary-output-port [auto-flush]) => serial-output-port
Creates a serial output port which can be used to write external representations of Scheme values to the provided
binary output port.
procedure: (serial-input-port? value) => #t/#f
Returns #t if value is a serial input port, #f otherwise.
procedure: (serial-output-port? value) => #t/#f
Returns #t if value is a serial output port, #f otherwise.

5.1.7.2. Serial Port Wrappers
procedure: (call-with-serial-input-port binary-input-port procedure) => value
Calls procedure with a new serial input port attached to binary-input-port. The result of the thunk is returned.
procedure: (call-with-serial-output-port binary-output-port procedure) => value
Calls procedure with a new serial output port attached to binary-output-port. The result of the thunk isreturned.

42Chapter 5. I/Oprocedure: (with-serial-input-from-port binary-input-port thunk) => value
Evaluates thunk with a serial input port attached to the specified binary-input-port as thecurrent-input-port for the duration of the evaluation. The port is not closed when thunk returns.
procedure: (with-serial-output-to-port binary-output-port thunk) => value
Evaluates thunk with a serial input port attached to the specified binary-output-port as thecurrent-output-port for the duration of the evaluation. The port is not closed when thunk returns.
procedure: (call-with-serial-input-file url procedure) => value
Calls procedure with a new serial input port attached to url. The result of the thunk is returned.
procedure: (call-with-serial-output-file url procedure) => value
Calls procedure with a new serial output port attached to url. The result of the thunk is returned.
procedure: (with-serial-input-from-file url thunk) => value
Evaluates thunk with a serial input port attached to a file opened for reading from url as thecurrent-input-port for the duration of the evaluation. The port is closed when thunk returns normally.
procedure: (with-serial-output-to-file url thunk) => value
Evaluates thunk with a serial input port attached to a file opened for writing to url as the current-output-port
for the duration of the evaluation. The port is closed when thunk returns normally.

5.1.7.3. Serialization Procedures
procedure: (deserialize [serial-input-port]) => value
Reads a Scheme value from an external representation retrieved from serial-input-port. If
serial-input-port is absent the data is read from the current input port.
procedure: (serialize value [serial-output-port]) => undefined
Writes an external representation of value to serial-output-port. If serial-output-port is absent the datais written to the current output port.

5.2. Networking
Requires: (import networking)
The SISC Networking library provides a mechanism for creating and manipulating IP network protocols
as standard Scheme ports. SISC supports TCP, UDP, and Multicast UDP. Each is described in thesections that follow.
Each protocol provides one or more socket constructors. These functions produce a Socket handle,
which is represented in SISC as #<socket>. A socket handle is then used to obtain Scheme ports.
IP addresses and network hostnames are represented as strings in SISC. Unless otherwise noted, thenetwork library functions that require an address may take a network address as a string which may be
any of:•A hostname, to be resolved through the domain name system.

43

Chapter 5. I/O•

An IPv4 network address in the standard dotted quad format. (RFC-791)•

An IPv6 network address in colon separated hexadecimal form, and zero-shortened form. (RFC-2373)

IPv6 addresses must be supported by the underlying operating system. An error may be raised if theaddress is not supported. All IP port values must be exact integers in the proper range.

5.2.1. IP Addressing
Several utility functions are provided for manipulating IP addresses. These are described below.
procedure: (get-host-ip-by-name hostname) => string
Attempts to resolve a hostname provided as a string into an IP address in dotted-quad form. If the host cannot befound, #f is returned.
procedure: (get-host-name-by-ip ip-address) => string
Attempts a reverse lookup of the given dotted-quad address to determine a registered domain name. If unsuccessful,#f is returned.
procedure: (get-local-host) => string
Attempts to determine the Internet visible IP address of the local machine. If successful, this address is returned indotted-quad notation. #f is returned otherwise.

5.2.2. Socket Operations
Once obtained using a protocol specific constructor, a Socket Handle allows manipulation of common
socket options, the creation of Scheme input/output ports, and closing of the socket.
procedure: (socket? value) => #t/#f
Returns true if and only if the provided value is a socket.
procedure: (open-socket-input-port socket [encoding]) => input-port
Opens a character input port to the socket. If the optional encoding parameter is provided, the character port
created will use the specified encoding rather than the default.
procedure: (open-socket-output-port socket [encoding] [auto-flush]) => output-port
Opens a character output port to the socket. If provided, the boolean argument specifies whether the given port
should be set to auto-flush mode. If unspecified, the port does not auto-flush. If the optional encoding parameter isprovided, the character port created will use the specified encoding rather than the default.
procedure: (open-binary-socket-input-port socket) => binary-input-port
Opens a binary input port to the socket.
procedure: (open-binary-socket-output-port socket [auto-flush]) => binary-output-port
Opens a character output port to the socket. If provided, the boolean argument specifies whether the given port
should be set to auto-flush mode. If unspecified, the port does not auto-flush.
procedure: (close-socket socket) => unspecified

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Chapter 5. I/O
Closes an IP socket.

The port-obtaining functions above work on most sockets. An exception applies for TCP server sockets,
which are used only to obtain connected TCP sockets.

5.2.3. TCP
The most commonly used Internet protocol maps most favorably to Scheme’s input/output model.
Writing to an output port retrieved from a TCP socket writes the data to that socket. Reading from an
input port reads from the connected socket. One important note is that one can control the amount of datathat fills a TCP packet by using an output port that does not auto-flush. Data is written to the port until
one considers the packet complete, and then uses (flush-output-port port) to complete thepacket. Note also that this does not guarantee that one gets the desired packet size, but does allow one toconstruct reasonably sized packets.
TCP sockets are obtained one of two ways. Either one creates an outgoing connection to anotherlistening host and then subsequently obtains a socket handle, or one creates a listening socket and thenobtains a socket by waiting for an incoming connection on the specified port. In either case, the result is asocket handle with an available input and output port that can be obtained using a function in theprevious section.
procedure: (open-tcp-socket host port) => socket
Attempts to connect to the host at the given hostname or IP address encoded as a string, at the given TCP port
specified as an integer. An error is raised if the host cannot be found or the connection fails. If successful, a socket isreturned.
procedure: (open-tcp-listener port [interface-address]) => server-socket
Creates a TCP server socket, which may only be used with accept-tcp-socket, or closed. The server socket will listenon the integer port specified. If provided, the interface-address, a string specifies the address of a local interface tobind to. If not provided, the port is bound on all available interfaces. An error is raised if the socket cannot be boundand set listening.
procedure: (server-socket? value) => #t/#f
Returns true if and only if the provided value is a server socket.
procedure: (accept-tcp-socket server-socket) => socket
Accepts an incoming connection on the provided server-socket, and returns a TCP socket handle. This function will
block until an incoming connection is made, or, if set, the socket timeout is exceeded. If the latter happens, an error
will be raised.
procedure: (set-so-timeout! socket timeout) => undefined
Sets the socket timeout on a socket. The socket can be either a server socket or connected socket. In the former case,this value specifies the number of milliseconds that an accept-tcp-socket can wait before timing out. In the latter, thevalue specifies the number of milliseconds that can elapse during a read call before timing out.

45

Chapter 5. I/O

5.2.4. TLS and SSL
Sockets which are encrypted using the Secure Sockets Layer (SSL) or Transport Security Layer (TLS)
can be created an accepted as well. This functionality can heavily depend on the underlying support in
the JVM, including security settings, restrictions, installed certificates, etc. For information on the Java
Secure Socket Extension (JSSE) and its setup, refer to the JSSE documentation(http://java.sun.com/products/jsse/reference/docs/index.html) page at Sun.
The SSL functionality is built atop the sockets and server sockets functionality as in TCP networking
above. A separate constructor for sockets (open-ssl-socket) and listening sockets(open-ssl-listener) exist, and produce sockets which are used identically to TCP sockets aspreviously described.
procedure: (open-ssl-socket host port [auto-close]) => socket
Attempts to connect to the host at the given hostname or IP address encoded as a string, at the given TCP port
specified as an integer, and establish an SSL connection using the default cipher suites and protocols available. An
error is raised if the host cannot be found or the connection fails. If successful, a socket is returned.
procedure: (open-ssl-listener port [interface-address]) => server-socket
Creates an SSL TCP server socket, which may only be used with accept-tcp-socket, or closed. The server socket will
listen on the integer port specified. If provided, the interface-address, a string specifies the address of a local
interface to bind to. If not provided, the port is bound on all available interfaces. An error is raised if the socket
cannot be bound and set listening.
The cipher suites, protocols, and modes which this server socket will accept are set with the following functions.
procedure: (get-enabled-cipher-suites ssl-server-socket) => list
Returns a list of strings naming the cipher suites which are enabled for this server socket.
procedure: (set-enabled-cipher-suites! ssl-server-socket suite-list) => list
Accepts an ssl server socket and a list of strings naming the cipher suites which should be available for negotiation
with the remote end. The previous list is returned.
procedure: (get-enabled-protocols ssl-server-socket) => list
Returns a list of strings naming the security protocols which are enabled for this server socket.
procedure: (set-enabled-protocols! ssl-server-socket protocol-list) => list
Accepts an ssl server socket and a list of strings naming the protocols which should be available for negotiation withthe remote end. The previous list is returned.
procedure: (session-creation-permitted? ssl-server-socket) => boolean
Returns true if new SSL/TLS sessions can be created by the sockets obtained from this server socket.
procedure: (set-session-creation-permitted! ssl-server-socket boolean) => boolean
Sets whether new SSL/TLS sessions can be created by the sockets obtained from this server socket. The previous
value is returned.
procedure: (is-client-mode? ssl-server-socket) => boolean

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Chapter 5. I/O
Returns true if the SSL server socket will be in the rare client mode after accepting the connection, rather than the
more common server mode.
procedure: (set-client-mode! ssl-server-socket boolean) => boolean
Sets whether the SSL server socket will be in client mode when accepting new connections. The previousvalue isreturned.
procedure: (get-client-auth ssl-server-socket) => symbol or #f
Returns the necessity of the remote client to authenticate to this server socket. The returned values are either
needed, indicating the remote must authenticate, wanted, indicating the remote will be requiested to authenticate
but is not required to, or #f, indicating the remote will not be requested to authenticate.
procedure: (set-client-auth! ssl-server-socket client-auth-mode) => symbol or #f
Sets the client authentication requirements, as described in get-client-auth above. One of the above values mustbe specified. The previous value is returned.

5.2.5. UDP
UDP sockets can be obtained for both receive only and send/receive sessions.
Note: The behavior of the char-ready? function is somewhat more difficult to predict on a UDP
input port. The function will return #t only when a previous datagram contained more bytes thanwere requested by the read operation that received it.

procedure: (open-udp-listen-socket listen-port [interface-address] [datagram-size]) => udp
socketOpens a UDP socket that listens on listen-port (optionally bound to only the interface oninterface-address). If provided, datagram-size specifies the buffer size (in bytes) for receiving UDP
datagrams. Datagrams larger than datagram-size are truncated to that size. If unspecified, the default datagram
size is 1500 bytes.
procedure: (open-udp-socket remote-host remote-port) => udp socket
Opens a UDP socket for sending datagrams to the Internet host specified by remote-host, on port remote-port.

After obtaining a UDP socket, input and output ports can be obtained in the usual manner. It is an error
to attempt to obtain an output-port from a listening UDP socket, or an input port from a sending UDP
socket.
UDP input ports behave as ordinary input ports. When a datagram arrives as a result of any read
operation on the port, their entire contents are stored in a buffer of length datagram-size bytes.
Successive read operations return data from that buffer until it is exhausted, at which point a read
operation will cause the UDP socket to listen for another datagram.
UDP output ports should be treated with some care, however. If a UDP output port was obtained in
auto-flush mode, each write operation to the output port will cause a new datagram to be sent. Control
over the size of the datagram must be maintained by using a port that does not auto-flush, writing thedesired data, and flushing once the amount of data that the user wants to occupy a single UDP datagram47

Chapter 5. I/O
is reached. The behavior of constructing very large UDP packets is undefined. The packet may besilently dropped or (more likely) fragmented at the IP layer.

5.2.6. Multicast UDP
SISC provides support for IP multicast UDP datagrams as well. This allows a program to both send andreceive to an IP multicast group. Multicast UDPs are an extension of ordinary UDP. Thus all I/O
operations on a Multicast UDP socket are subject to the same semantics as an ordinary UDP socket.
Note: The Multicast UDP library requires that the underlying operating system’s IP networking stacksupport Multicast. The functions described here may produce an error if the operating system doesnot.

A program wishing to use multicast UDP sockets must first obtain a multicast socket for either listening
to a multicast group, or for both listening and sending to such a group.
procedure: (open-multicast-socket listen-port [interface-address] [datagram-size]) =>multicast udp socket
Opens a multicast UDP socket that listens on listen-port (optionally bound only to the interface addressed byinterface-address. If provided, datagram-size specifies the buffer size (in bytes) for receiving UDP
datagrams. Datagrams larger than datagram-size are truncated to that size. If unspecified, the default datagram
size is 1500 bytes.
procedure: (open-multicast-socket group port [interface-address] [datagram-size]) =>multicast udp socket
Opens a multicast UDP socket for sending datagrams to the specified multicast group, on the specified port. The
returned socket will also be capable of listening to that group on the same port (and optionally bound only to theinterface addressed by interface-address), though the socket will not initially be a member of the group. If
provided, datagram-size specifies the buffer size (in bytes) for receiving UDP datagrams. Datagrams larger thandatagram-size are truncated to that size. If unspecified, the default datagram size is 1500 bytes.

Once a sending socket has been obtained (the second form), an output-port can be obtained in the usual
manner, and datagrams can be immediately sent to the multicast group. To receive datagrams, sockets
returned from both forms must join a multicast group.
A multicast group is specified by a class D IP address and by a standard UDP port number. Class D IP
addresses are in the range 224.0.0.0 to 239.255.255.255, inclusive. The address 224.0.0.0 is reserved andshould not be used.
Groups are joined and left using the following functions:procedure: (join-multicast-group multicast-socket group) => undefined
Causes the given multicast socket to join the group specified by the Internet address in group. Once joined, read
operations on an obtained input-port will be able to receive datagrams destined to that group.
procedure: (leave-multicast-group multicast-socket group) => undefined
Causes the given multicast socket to leave the group specified by the Internet address in group. Read operations on
any input ports obtained from this socket will no longer receive datagrams from that group.

48Chapter 5. I/O
A single multicast socket can simultaneously listen to more than one multicast group. A socket can only
send to one group, however: the group it was constructed with.
Multicast packets are limited in extent by their time-to-live. Each time a multicast packet crosses arouter, its ttl is decremented. In this manner, one can send datagrams only to local networks orsubnetworks, as well as more grand scopes. The TTL of a socket is set using set-multicast-ttl!
procedure: (set-multicast-ttl! multicast-socket ttl) => undefined
Sets the multicast TTL of the given socket to ttl, an integer. All datagrams sent after this call will have their TTL
set to the new value.

Valid multicast TTLs are in the range 0 (restricted to the same host) to 255 (unlimited in scope).

5.3. User Defined Ports
Requires: (import custom-io)
SISC provides the ability to create new I/O port types from within Scheme. To add a new port type, one
invokes a custom port constructors described below, passing procedures (Scheme or otherwise) which
implement the operations required by that port. Each custom port constructor returns a Scheme port
which may be used with any of the SISC or R5RS I/O functions. Port implementors may also use an
associated port-local value to coordinate state for some specialized ports.
procedure: (set-port-local! custom-port value) => value
Sets the port-local value of the given custom port.
procedure: (port-local wrapper-stream) => value
Gets the port-local value of the given custom port.
procedure: (make-custom-character-input-port read read-string ready? close) =>character-input-port
Creates a character-input-port whose functionality is implemented by the four provided fundamental procedures.
The fundamental procedures required are as follows:(read port) => integer
Return a character as an integer value, or -1 if the end of stream has been reached. port is a reference to the custom
Scheme port which contains the procedure.
(read-string port mutable-string offset count) => integer
Reads up to count characters into the given mutable string at position offset. The number of actual characters
read are returned as an integer value, or -1 if the end of stream has been reached. port is a reference to the custom
Scheme port which contains the procedure.
(ready? port) => boolean
Returns a non-false value if one or more characters are available for reading, false otherwise.
(close port) => undefined
Closes the port.

49

Chapter 5. I/Oprocedure: (make-custom-binary-input-port read read-block available close) =>binary-input-port
Creates a binary-input-port whose functionality is implemented by the four provided fundamental procedures.
The fundamental procedures required are as follows:(read port) => integer
Return a byte as an integer value, or -1 if the end of stream has been reached. port is a reference to the custom
Scheme port which contains the procedure.
(read-block port buffer offset count) => integer
Reads up to count bytes into the given binary buffer at position offset. The number of actual bytes read are
returned as an integer value, or -1 if the end of stream has been reached. port is a reference to the custom Scheme
port which contains the procedure.
(available port) => integer
Returns a count of the number of bytes which are available for reading. This number need not be accurate.
(close port) => undefined
Closes the port.
procedure: (make-custom-character-output-port write write-string flush close) =>character-output-port
Creates a character-output-port whose functionality is implemented by the four provided fundamental procedures.
The fundamental procedures required are as follows:(write port byte) => undefined
Writes a character represented as an integer value. port is a reference to the custom Scheme port which contains theprocedure.
(write-string port string offset count) => void
Writes count characters from the given string at position offset. port is a reference to the custom Scheme port
which contains the procedure.
(flush port) => undefined
Flushes any unwritten characters to the stream.
(close port) => undefined
Closes the port.
procedure: (make-custom-binary-output-port write write-block flush close) =>binary-output-port
Creates a binary-output-port whose functionality is implemented by the four provided fundamental procedures.
The fundamental procedures required are as follows:
The fundamental procedures required are as follows:(write port byte) => undefined
Writes a byte represented as an integer value. port is a reference to the custom Scheme port which contains theprocedure.
(write-block port buffer offset count) => void50

Chapter 5. I/OWrites count bytes from the given binary buffer at position offset. port is a reference to the custom Scheme port
which contains the procedure.
(flush port) => undefined
Flushes any unwritten characters to the stream.
(close port) => undefined
Closes the port.

Finally, the underlying procedures which implement a custom port can be retrieved with the following
function:procedure: (custom-port-procedures custom-port) => list
Returns the procedures which implement a custom port as a list.

As an example, here are String Ports, implemented as user defined ports:
Example 5-1. User defined string ports(import custom-io);; String Input Ports(define (sio/read port)(let ([local (port-local port)])(let ([ptr (vector-ref local 1)])
(if (= (vector-ref local 2) ptr)
-1
(let ([c (char->integer(string-ref (vector-ref local 0) ptr))])(vector-set! local 1 (+ ptr 1))c)))))(define (sio/read-string local buffer offset length)(let ([local (port-local port)])(let ([str (vector-ref local 0)][strlen (vector-ref local 2)])(do ([i offset (+ i 1)][j (vector-ref local 1) (+ j 1)][c 0 (+ c 1)])((or (= i (+ offset length))(= j strlen))(vector-set! local 1 (+ ptr 1))c)(string-set! buffer i (string-ref str j))))))(define null (lambda args (void)))(define (open-input-string str)
(unless (string? str)
(error ’open-input-string "expected string, got ’~a’.~%" str))(let ([port (make-custom-character-input-port
sio/read sio/read-string null null)]); Use the port local value to store the string and a pointer51

Chapter 5. I/O; for the current position(set-port-local! port (vector str 0 (string-length str)))port));; String Output Ports(define (sio/write port char)(set-port-local! port
(cons char (port-local port))))(define (sio/write-string port string offset length)(set-port-local! port
(append (reverse (string->list (substring string offset (+ length offset))))(port-local port))))(define (open-output-string)(let ([port(make-custom-character-output-port
sio/write sio/write-string null null)])(set-port-local! port ’())
port))(define (get-output-string port)(flush-output-port port)(let ([str (apply string(reverse (port-local port)))])(set-port-local! port ’())
str))

5.4. Miscellaneous
5.4.1. Pretty-Printing
SISC includes a pretty-printer, a function that behaves like write, but introduces whitespace in order tomake the output of data more readable to humans.
procedure: (pretty-print value [output-port]) => unspecified
Pretty-prints the specified value, either to the specified output-port, or to the console if no output-port is specified.

5.4.2. Source Loading
The load procedure accepts URLs as well as ordinary file names. See Section 5.1.1 for details on whatkinds of URLs are supported.
The file name passed to load is resolved relative to the current-url parameter. During the executionof load, current-url is set to the loaded file, so that any invocations of load from the loaded file52

Chapter 5. I/O
resolve the given file name relative to the file currently being loaded. For example, lets assume we have a
web site that serves the following files:;;;contents of http://foo.com/bar/bar1.scm ;;;(load "bar2.scm");;;contents of http://foo.com/bar/bar2.scm ;;;(load "/baz/baz1.scm");;;contents of http://foo.com/baz/baz1.scm ;;;(load "../baz/baz2.scm");;;contents of http://foo.com/baz/baz2.scm ;;;(display "Hello")

Invoking(load "http://foo.com/bar/bar1.scm")

results in each file being loaded; with the last file in the chain, baz2.scm, displaying Hello.
The load function supports many types of files which contain executable code. load will attempt todetermine (primarily by the file’s extension) which type of file is being loaded to load that file in thecorrect manner. The file types currently supported are those described in Section 2.5.
When a pure source file is loaded, each s-expression is evaluated in sequence, exactly as if entered into
the REPL one s-expression at a time.

5.4.3. Location Tracking
SISC allows the location of input, i.e. the file name, line number, and column number, to be tracked whenreading from an input port.
procedure: (open-source-input-file url) => input-port
This procedure behaves the same as open-input-file, except that it also tracks the location of the input.
procedure: (input-port-location input-port) => list
Returns the current location information associated with input-port. The return value is an association list
containing the following keys: source-file, line-number, column-number. If no location information isavailable, #f is returned.

5.4.4. Locating Resources
SISC provides a mechanism for locating and subsequently loading named resources, such as Scheme
source files, Scheme data files, property files. The resources are located using the mechanism describedin Section 5.4.6. This allows Scheme programs to load resources in a portable, J2EE-compliant manner.
procedure: (find-resource string) => url
Locates the resource named by string on the Java class path. The resource location is returned as a URL suitablefor SISC I/O operations. If the resource cannot be found, #f is returned.
procedure: (find-resources string) => url-list

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Chapter 5. I/O
Locates the resource named by string on the Java class path. The resource locations are returned as a list of URLs
suitable for SISC I/O operations. If the resource cannot be found, an empty list is returned.

5.4.5. File Manipulation
Requires: (import file-manipulation)
The file-manipulation library provides access to a number of functions for reading and manipulating files
and their attributes. The file-manipulation library acts on filenames in the same manner as other Scheme
file related functions, e.g. it accepts file and directory names as strings, which are resolved relative to thecurrent URL.
The following functions act on both files and directories. With the exception of file-exists? andget-parent-url, the behavior when applying these to non-existant files or directories is undefined.
procedure: (file-delete! filename) => #t/#f
Attempts to remove the given file or directory. If successful, #t is returned.
procedure: (file-exists? filename) => #t/#f
Returns true if the given file or directory exists.
procedure: (file-is-directory? filename) => #t/#f
Returns true if the given string names an existing directory.
procedure: (file-is-file? filename) => #t/#f
Returns true if the given string names an existing file.
procedure: (file-last-modified filename) => integer
Returns the number of milliseconds since the Unix epoch (Jan 1, 1970) of the date the file or directory was lastmodified.
procedure: (file-rename! source-filename dest-filename) => #t/#f
Renames the given source file or directory to the destination. This can be used both to rename a file or directory or tomove a file/directory in the same filesystem. If successful, #t is returned.
procedure: (file-set-last-modified! filename unixtime) => #t/#f
Sets the last modified date of the given filename to the given integer (in number of milliseconds since the epoch).
Returns #t if successful.
procedure: (get-parent-url filename) => string
Given any URL, returns the URL of its parent. For filenames, as an example, the parent directory is returned.

The following functions operate only on files. Their behavior when applied to directories or non-existantfiles is undefined.
procedure: (file-is-readable? filename) => #t/#f
Returns #t if the file can be opened for reading.
procedure: (file-is-writeable? filename) => #t/#f54

Chapter 5. I/O
Returns #t if the file can be opened for writing.
procedure: (file-length filename) => integer
Returns the length, in bytes, of the given file.
procedure: (file-set-read-only! filename) => #t/#f
Sets the given file read-only. Returns #t if successful.

Finally, the following functions are specific to directories. Their behavior on files is undefined. The
behavior of directory-list is undefined on non-existant directories.
procedure: (directory-list directory) => list of strings
Retrieves the children of the given directory, as a list of strings. Each string names one child, and is a filename
relative to the given directory.
procedure: (make-directory! directoryname) => #t/#f
Attempts to creates the given directory. Returns #t if successful.
procedure: (make-directories! directoryname) => #t/#f
Attempts to creates the given directory and all non-existing parent directories. Returns #t if successful.

5.4.6. Class Loading
Some SISC features require classes and other resources to be loaded. By default, SISC will use thecurrent thread’s class loader, or, if none is present, the system class loader. SISC maintains a list of classpath extensions onto which class and resource loading falls back. This list can be inspected and extended
using the following functions:parameter: (class-path-extension) => string-list
Retrieves the current class path extension. The class path extension is a list of strings representing URLs, typicallypointing to jar files or directories. It is used a fall back during class and resource loading.
parameter: (class-path-extension-append! class-path) => undefined
Appends a list of URLs to the current class path extension.
The elements of the class-path list are normalized using the current-url (see Section 5.1.1), thus permitting
the usage of relative URLs.

Note that the class path extension is part of the dynamic environment„ so each thread has its own setting,initially inherited from the parent thread. See Chapter 6 for more details on SISC’s threading semantics.

Notes
1. Handling of JAR files in URLS may be dependent on the SISC host language, as well as someuncommon protocols. FILE, HTTP and FTP should be expected to work with any host language.

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Chapter 6. Threads and Concurrency
Requires: (import threading)
SISC provides a comprehensive library for executing Scheme code in parallel in multiple concurrent
threads. This allows code for simple code for handling blocking I/O sources (such as network servers) orfor the ability to do parallel computation across multiple processors.
In addition, functions are provided to ensure mutually exclusive access to data (mutexes), to assign
priorities to Scheme threads, and for inter-thread signaling and synchronization.

6.1. Scheme Thread Semantics
Care has been taken to ensure that Scheme code executing concurrently in two or more threads does not
result in unpredictable behavior. Assuming the executing threads do not share data, executing code inmultiple threads should behave just as executing the code in a single thread.
All threads executing in the system share some resources. The top-level and symbolic environments areshared by all threads. If a thread makes a change to these environments, the change will be visible in allother threads. Unless changed by the thread, all threads share the same console input and output ports.
As threads originate from a thunk created in the primordial thread or a child thread, the lexical
environment captured by the thunk may include some lexical variables from the parent thread. Thesevariables will be visible by both the parent and child threads. The lexical environments created in an
executing thread are visible by that thread only (unless that thread spawns a child thread whose thunkbinds it’s parent’s lexicals).
Some resources can be shared but may also be distinct from thread to thread. These resources are
inherited from the parent thread, but may be changed by the child or parent without affecting the other.
The dynamic environment (including the console input and output port, parameters, etc.) are inheritedfrom the parent, as is the dynamic-wind stack.
When a thread begins, it is considered to be isolated from its parent in terms of the dynamic-wind stack.
If a parent spawns a thread in the during section of a dynamic-wind call, the spawned thread escapes the
restrictions of the dynamic-wind call. This is logical, as the parent thread may then exit from the
dynamic extent even as the child thread executes, or may remain there waiting for the child thread tofinish, in which case the parent has not left the dynamic extent of the call. In short, the dynamic-wind isprotecting only the parent thread.
It is possible for more than one thread to access the same memory location (be it a lexical variable or a
named variable in the top-level or another symbolic environment), it is also possible that interactions onshared variables can have unpredictable results. As in any multi-threaded language, unprotected access toshared variables can result in race conditions and other concurrency mishaps. If the programmer
anticipates concurrent access to a shared variable and if any thread is to write to the variable, sections ofcode that access the variable should use a protection mechanism from Section 6.5.
A thread can complete in one of two ways. If the thunk that contains the thread’s code exits, the thread
will terminate and the thread handle will contain the return value of the thunk. This completion conditionis called a clean exit. If during the execution of the thunk’s body an error is raised and is not

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Chapter 6. Threads and Concurrency
subsequently caught, the thread will terminate and the thread handle will trap the error. The error will beraised to any caller that attempts to retrieve the return value of the thread.
It is perfectly legal for a thunk to both capture and invoke continuations, even continuations created byother threads. When applying a continuation captured outside of the thread, the resources of theexecuting thread are used, though the thread may be accessing lexical environments created by otherthreads.
Once created, a thread can be in one of four states: ready, running, finished, or finished-with-error. The
first two states indicate a newly created thread and a running thread, repectively. The last two represent
the end stages of a thread, finished indicating a thread that has exited cleanly, and finished-with-error
indicating exit with an error.
There are no guarantees that a Scheme thread will ever exit. It is perfectly valid for a thread to executeindefinitely. Furthermore, the SISC environment will not exit until all threads have completed, either
cleanly or with a failure, unless all remaining threads are so-called daemon threads. Daemon threads are
threads that may run indefinitely but will be forcibly terminated if no non-daemon threads (of which theprimordial thread is one) are still running. Termination of a daemon thread when no non-daemon threads
exist is the only instance where a thread can be forcibly terminated. There is no guaranteed thread stopor destroy operation.

6.2. Basic Thread Operations
This section describes the basic, low-level operations on threads, including how to create a thread, how
to start it, how to wait for it to terminate, and how to retrieve it’s result. A thread is managed in Schemeby its thread handle, an opaque value that is used to identify the thread. A thread handle is present as anargument to most of the thread library functions.
Threads are created with thread/new. This function takes as its sole argument a thunk. The body of the
thunk is the code that the thread will execute when started.
procedure: (thread? value) => #t/#f
Returns true if and only if the provided value is a thread handle.
procedure: (thread/new thunk) => thread-handle
Creates a new thread handle whose code is defined in the provided thunk.

Once a thread-handle is created, the thread is in the ready state, and can be started at any time by callingthread/start. At this point one can also set various thread parameters, such as thread priority anddaemon status.
procedure: (thread/start thread-handle) => undefinedStarts the thread identified by thread-handle. The thread must be in the ready state. It is an error to start a threadin any other state.
procedure: (thread/daemon! thread-handle boolean) => undefined
Sets the daemon status of a thread. It is an error to change the status of a thread that is not in the ’ready state.
procedure: (thread/daemon? thread-handle) => #t/#f

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Chapter 6. Threads and Concurrency
Returns #t if the given thread is a daemon thread.

It is common pattern to create a thread and immediately start it. The convenience method
thread/spawn encapsulates this operation.
procedure: (thread/spawn thunk) => thread-handle
Creates a new thread handle whose code is defined in the provided thunk, and starts the thread.

Once started, the thread will be in the running state. The body of the thunk is now being evaluated inparallel to the parent thread. The thread will remain in the running state until it completes and enters oneof the two finished states. The state can be read using thread/state.
procedure: (thread/state thread-handle) => symbol
Returns the state of the thread identified by thread-handle. The state is one of ’ready, ’running, ’finished,or ’finished-with-error.

The parent thread may continue executing its own code, or may attempt to join the child thread. To join
another thread is to wait until the other thread has completed. The parent thread can join a child using
thread/join. The parent can wait indefinitely or may specify a timeout, after which the thread/joincommand will return with #f.
procedure: (thread/join thread-handle [timeout]) => boolean
Attempts to join with the indicated thread. If the thread terminates, thread/join will return a non-false value. If atimeout is specified thread/join will only wait timeout milliseconds for the thread to complete. If the thread does
not terminate before the timeout, #f will be returned.

It is possible to join on an already completed thread. In such a case the join will immediately return #t.
The behavior of a join on a thread in the ready state is unspecified, and may cause an error. Finally, it ispossible that a join may return #f, even if no timeout is specified. Though unlikely, programmers whowish to wait indefinitely for a thread to complete should check the return value of thread/join andrepeat the join until #t is returned.
If enabled (using the permitInterrupts configuration parameter, see Section 2.4.2), running threads may
be interrupted at both the host language level and when executing Scheme code with thethread/interrupt function. If disabled (the default), only a host-language interrupt signal can be sent.
procedure: (thread/interrupt thread-handle) => undefined
Sends an interrupt signal to the given thread. This will cause an error to be raised from Scheme code, and a threadinterrupt in the host language.

When invoked, a signal is sent to the running thread which will cause an error to be raised from somepoint in its execution. If not caught, the thread will terminate in the finished-with-error state, and theraised error will be rethrown if thread/result is called. The error-continuation of the error throwninside the thread, when invoked with no arguments, will restart the computation exactly where it left off.
A thread may not properly resume if its code calls back into Scheme using any of the mechanisms
described in Section 8.1.

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Chapter 6. Threads and Concurrency

Final Continuation of a Thread
If a thread is interrupted and later its computation is resumed by calling theerror-continuation, the thread that hosts the resumed computation will exit when
the computation terminates. As a consequence, if a computation that ran in a
separate thread is resumed in the primordial thread (that usually hosts the REPL),
the primordial thread will terminate as soon as the computation completes. This
will circumvent the REPL entirely and cause SISC to exit if no non-daemon
threads remain. In general, interrupted threads should be resumed in a newly
created thread to avoid this scenario.

It is important to note that thread/interrupt does not guarantee the termination of a thread. A thread
may still capture the error at the scheme level with a with/fc, or catch the interrupt signal if executing
in the host language. In either case, the running code is not required to rethrow the error.
Once a thread has completed, the parent thread may wish to retrieve the result of the thread’s thunk, be it
an error or a valid result. This can be done with the thread/result function.
procedure: (thread/result thread-handle) => value
Returns the return value from a completed thread. If the thread completed with error, that error is raised from thiscall.

An error will be raised if an attempt is made to retrieve the result of a thread before that thread hascompleted.

6.3. High-level Functions
In addition to the basic thread operations, some high level syntax is provided to simplify some generalcase thread use.
procedure: (parallel thunk1 thunk2 [thunks] ...) => multiple values
Executes each thunk in its own thread in parallel. The call to parallel blocks until all the threads have finished. If all
threads completed without error, the results of each thunk are returned as multiple values. If any thread raised anerror, that error is raised from the call to parallel. The error is raised only after all other thunks have also completed.
If more than one thunk raises an error, it is undefined which error will be raised to the caller.

6.4. Thread Scheduling
All Scheme threads created in SISC are preemptive and managed by a scheduler. It is possible for a
program to manage the priorities of threads in order to give execution preference to higher priority
threads. It is also possible for threads to give up their execution time to other blocked threads.
The priority of a thread is represented by an integer. The range of priorities and the default priority of athread is unspecified and may be platform specific. Larger integers represent higher priorities thensmaller integers. If a higher or lower priority thread is desired, the recommended procedure is to get thecurrent priority of a thread and increment or decrement it. Though unspecified, it is possible that an error
will be raised if a priority level outside the platform specific range is selected.

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Chapter 6. Threads and Concurrency
Though not guaranteed, the behavior of the scheduler when two threads, one with higher priority thananother are both runnable but only one processor is available to run a thread, is that the higher priority
thread will be selected. If only equal priority threads are available to be run, the scheduler can choose
any thread to run. No guarantees are made about latency or fairness.
Thread priorities can be set by the parent thread or the thread itself. The behavior of a child threadattempting to set the parent’s priority, or a sibling’s priority is undefined.
procedure: (thread/priority thread-handle) => integer
Retrieves the current priority of the given thread.
procedure: (thread/priority! thread-handle new-priority) => undefined
Attempts to set the given thread’s priority to the integer new-priority .

In addition to setting priority levels, a program may wish to yield its execution time temporarily to otherthreads. Performing a yield allows the scheduler to select a thread to run on the processor of the threadthat just yielded control. It is possible that the yielding thread may be selected again, or another thread
may be chosen.
procedure: (thread/yield) => undefined
Causes the currently executing thread to yield to other threads.

Finally, a thread (including the main thread) may sleep for a specified amount of time, allowing otherthreads to execute in the mean time. If no other threads are running, sleeping effectively pauses the
program for the given time period.
procedure: (sleep milliseconds) => undefined
Causes the currently executing thread to sleep for the given number of milliseconds, specified as an exact integer.

6.5. Synchronization Primitives
SISC provides an implementation of mutexes and condition variables designed to support a wide range ofsynchronization models, including the monitor paradigm previously found in SISC. The implementation
provides both a mutex and condition-variable first-class value for concurrency protection and inter-threadcommunication. Mutexes provide mutual exclusion locking, while condition variables faciliate
synchronization of threads with the change in state of monitored data. SISC’s mutexes are reentrant, thatis, if a thread locks a mutex and then attempts to lock it again, the lock is granted immediately- it does
not block. Correspondingly, the mutex is not unlocked until the same number of unlock operations asprevious lock operations are performed by the owning thread.
SISC’s synchronization primitives map very closely to SRFI-18, which is supported by SISC. In fact, it isrecommended that programmers write to SRFI-18 if at all possible, as they will gain maximum
portability with little or no loss in efficiency on SISC.
Throughout the following sections, a thread is often said to block because of some circumstance. While athread is blocked on some resource, other threads are allowed to execute freely, at the discretion of thescheduler.
Several functions exist for performing low level operations such as creating mutexes and condition
variables. All require the mutex or condition variable as the first parameter. Mutexes are represented in

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Chapter 6. Threads and Concurrency
Scheme as opaque values displayed as #<mutex> while condition variables are represented as#<condition-variable>. It is important to understand that all the synchronization operations dependon the same mutex and/or condition variable being shared between any threads using the functionality.

6.5.1. Mutex Operations
In order to protect a segment of Scheme code from concurrent access, one can create a mutex that isshared by all threads that may access the segment. When entering the contested region of code (thecritical section), a thread would call mutex/lock. Upon exiting the region, mutex/unlock is called.
procedure: (mutex? value) => #t/#f
Returns true if and only if the provided value is a mutex.
procedure: (mutex/new) => mutex
Creates and returns a new mutex object.
procedure: (mutex-of value) => mutex
Returns a mutex that is uniquely associated to the given value. Subsequent (or concurrent) calls to this function are
guaranteed to return the same mutex if given the same value.
procedure: (mutex/lock! mutex) => undefined
Attempts to acquire the lock on the given mutex. Returns only when the lock has been successfully acquired.
procedure: (mutex/unlock! mutex) => undefined
Releases the lock on the given mutex. The behavior when unlocking a mutex when the running thread does not havethe lock is undefined, and may raise an error.

The semantics of mutex/lock! ensure that only one thread can execute beyond the lock call at any one
time. The first thread that reaches the call acquires the lock on the mutex. Any later threads will block at
the call to mutex/lock! until the thread that owns the lock releases the lock with mutex/unlock!.

6.5.2. Condition Variable Operations
A condition variable allows one thread to sleep until another wakes it. A common situation is for one
thread to check the status of a variable, and sleep if the condition is not met. While the thread sleeps, one
or more separate threads may execute and satisfy the condition (by changing the state of the variable)
and then notify the sleeping thread via the condition variable. The thread then awakes, checks the statevariable, and proceeds if the condition is met. If not, it sleeps again. This construct allows forcooperation between multiple threads on a computation.
To wait on a condition variable, the mutex/unlock! function is again used with a condition variable as
an additional argument. applied to a monitor. This will cause the thread to unlock the given mutex, thenblock until notified by another thread. When it is notified, or the provided timeout expires, the lock is not
reacquired.
procedure: (condvar? value) => #t/#f
Returns true if the provided value is a condition variable.
procedure: (condvar/new) => condition variable61

Chapter 6. Threads and Concurrency
Creates a new condition variable.
procedure: (mutex/unlock! mutex condvar [timeout]) => #t/#f
Causes the thread to sleep until notified on the provided condition variable by another thread. This call will not
return until notified, unless the optional timeout is specified and timeout milliseconds have elapsed without anotification. Before blocking, the lock on mutex is released. If the timeout is reached before notification, #f isreturned, otherwise #t is returned.

Another thread may wake a single waiting thread with the condvar/notify operation. When called,
one thread waiting on the condition variable is woken. If no threads are waiting this call has no effect. If
more than one thread is waiting, exactly one will be woken. Which is woken is unspecified. If a thread
wishes to wake all threads waiting on a given monitor, it may use the condvar/notify-all function.
procedure: (condvar/notify condvar) => undefined
Wakes exactly one thread waiting on the condition variable, if any such threads exist. If the notifying thread holds
the lock on condvar , the waiting thread will not proceed until the notifying thread releases the lock.
procedure: (condvar/notify-all condvar) => undefined
Wakes all threads waiting on the condition variable, if any waiting threads exist. If the notifying thread holds thelock on condvar , the waiting thread will not procede until the notifying thread releases the lock.

6.5.3. High-level Concurrency
In addition to the low-level operations on mutexes and condition variables, two library functions areprovided to greatly ease the construction and readability of thread-safe code.
procedure: (mutex/synchronize mutex thunk) => value
Protects execution of thunk as a critical section by holding the mutex’s lock during evaluation of the thunk. The
result of the thunk’s evaluation becomes the result of the mutex/synchronize expression.
procedure: (mutex/synchronize-unsafe mutex thunk) => value
Behaves exactly as mutex/synchronize without automatic unlocking when an error is raised or a continuation
escapes when executing thunk.
mutex/synchronize locks the mutex while the thunk provided is being executed. The lock is

automatically released when the expression has completed. Also, if an error is raised or a continuation is
invoked that escapes the call to mutex/synchronize, the lock is automatically released.
The added safety provided by mutex/synchronize may slow the execution of code that repeatedly
calls a critical section. If the programmer is absolutely sure that no error can be raised and that nocontinuations will be applied to escape the call, mutex/synchronize-unsafe may be used. It provides
no safety guarantees in those situations. If an error is raised or an escaping continuation invoked, themutex will remain locked which could cause a deadlock if another thread attempts to acquire the lock.

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Chapter 7. Types and Objects
7.1. Type System
Requires: (import type-system)
SISC’s extensible type system provides programmatic access to the type information of values andprovides a core set of type testing and comparison procedures. The type system is extensible in two
ways. Firstly any new native types are recognised automatically. Secondly, hooks are provided for
Scheme-level extensions of the type-system.
By convention, type names start with < and end with >, with normal Scheme identifier naming
conventions applying for everything in-between, i.e. all lower-case with words separated by dashes. For
example, <foo-bar-baz>. This convention helps to visually distinguish type names from names ofprocedures and top-level data bindings.

7.1.1. Core Procedures and Predicatesprocedure: (type-of value) => type
Returns the type of value. There is no standard representation for types, leaving type extensions free to choose arepresentation that suits them most.
The procedure is equipped with an extension hook, type-of-hook. See Section 7.1.3 for more details on hooks.
The default implementation of the hook type-of returns a type based on the Java type of the internal representationof value.
(type-of 1) ;=> #<scheme sisc.data.Quantity>(type-of (lambda (x) x)) ;=> #<scheme sisc.data.Closure>procedure: (type<= type1 type2) => #t/#f
Returns #t if type1 is a sub-type of type2.
The predicate is equipped with an extension hook, type<=-hook. See Section 7.1.3 for more details on hooks. The
default implementation of the hook determines sub-typing based on the inheritance relationship of the Java types
representing native types.
(type<= (type-of ’a) (type-of ’b)) ;=> #t(type<= (type-of ’b) (type-of ’a)) ;=> #t(type<= (type-of 1) (type-of ’a)) ;=> #f(type<= (type-of 1) <number>) ;=> #t(type<= (type-of ’a) <symbol>) ;=> #t(type<= <number;> <symbol>) ;=> #f(type<= <symbol;> <number>) ;=> #f(type<= <number> <value>) ;=> #t(type<= <symbol> <value>) ;=> #t63

Chapter 7. Types and Objects
procedure: (compare-types type1 type2 type3) => ’equal,’more-specific,’less-specific
Determines the relationship of type1 and type2 with respect to type3. type3 must be a sub-type of type1 andtype2. type1 and type2 are first compared using type<=. If that comparison indicates that the types are disjoint(i.e. type1 is not sub-type of type2, type2 is not a sub-type of type1 and the types are not equal) then additional
information from type3 is taken into account for the comparison.
The predicate is equipped with an extension hook, compare-types-hook that is invoked in the case thecomparison of type1 with type2 using type<= finds the two types to be disjoint. See Section 7.1.3 for more details
on hooks. The default implementation of the hook returns an error(compare-types <number> <value> <number>);=> ’more-specific(compare-types <value> <number> <number>);=> ’less-specific(compare-types <number> <number> <number>);=> ’equal

7.1.2. Derived Procedures and Predicates
The type system’s derived procedures and predicates are implemented in terms of the core proceduresand predicates.
procedure: (instance-of? value type) => #t/#f
Determines whether value is an instance of type.
The predicate obtains value’s type using type-of and then compares it to type using type<=.
(instance-of? 1 <number>) ;=> #t(instance-of? ’a <symbol>) ;=> #t(instance-of? 1 <symbol>) ;=> #f(instance-of? 1 <value>) ;=> #tprocedure: (type= type1 type2) => #t/#f
Determines whether two types are equal by comparing them using type<=.
(type= (type-of ’a) (type-of ’b)) ;=> #t(type= (type-of 1) (type-of ’a)) ;=> #f(type= (type-of 1) <number>) ;=> #t(type= (type-of ’a) <symbol>) ;=> #t(type= <number;> <symbol>) ;=> #fprocedure: (types<= type-list1 type-list2) => #t/#f
Determines whether all of the types in type-list1 are sub-types of the the corresponding (by position) types in
type-list2.

64Chapter 7. Types and Objects
A pair-wise comparison of the elements in the two lists using type<= is performed until a test returns #f, in whichcase #f is returned, or one (or both) of the lists has been exhausted, in which case #t is returned.
(types<= (list <number> <symbol>)(list <value> <value>)) ;=> #t(types<= (list <number> <symbol>)(list <number> <number>)) ;=> #fprocedure: (instances-of? value-list type-list) => #t/#f
Determines whether all of the values in value-list are instances of the the corresponding (by position) types in
type-list.
A pair-wise comparison of the elements in the two lists using instance-of? is performed until a test returns #f, inwhich case #f is returned, or one (or both) of the lists has been exhausted, in which case #t is returned.
(instances-of? (list 1 ’a)(list <number> <symbol>))(instances-of? (list 1 ’a)(list <number> <number>));=> #t;=> #fprocedure: (types= type-list1 type-list2) => #t/#f
Determines whether all of the types in type-list1 are equal to the the corresponding (by position) types in
type-list2.
A pair-wise comparison of the elements in the two lists using type= is performed until a difference is found, inwhich ase #f is returned, or one (or both) of the lists has been exhausted, in which case #t is returned.
(types= (list <number> <symbol>)(list <number> <symbol>))(types= (list <number> <symbol>)(list <number> <number>));=> #t;=> #f

7.1.3. Hooks
Hooks are the main mechanism by which Scheme code can extend the default type system. The core typesystem procedures type-of, type<= and compare-types all provide such hooks, called
type-of-hook, type-<=-hook, and compare-types-hook respectively.
Extension takes place by installing labelled handler procedures on the hook, which is done by invoking
the hook procedure. Installing a handler procedure with a label of an already installed procedure replaces
the latter with the former.
The handler procedures are called with a next procedure as the first argument and all the arguments ofthe call to the hook-providing procedures as the remaining arguments. Typically a handler procedure first
determines whether it is applicable, i.e. is capable of performing the requested comparison etc. If not it
calls the next handler procedure, which invokes the next hook or, if no further hooks exist, the default
implementation of the hooked procedure.

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Chapter 7. Types and Objects
Example 7-1. Hook Installation
This example shows how SISC’s record type module adds record types to the type system by installing
handler procedure on type-of-hook and type<=-hook.
(type-of-hook ’record(lambda (next o)(if (record? o)(record-type o)(next o))))(type<=-hook ’record(lambda (next x y)(cond [(record-type? x)(if (record-type? y)(eq? x y)(type<= <record> y))][(record-type? y) #f][else (next x y)])))

7.1.4. Standard Types
The type system pre-defines bindings for the native types corresponding to all the data types defined in
R5RS: <eof>, <symbol>, <list>, <procedure>, <number>, <boolean>, <char>, <string>,<vector>, <input-port>, <output-port>. One notable exception is that pairs and null are
combined into a <list> type.
The type system also defines a <value> type that is the base type of all SISC values, i.e. all SISC values
are instances of <value> and all types are sub-types of <value>.
The representations of other native types can be obtained using
procedure: (make-type symbol) => type
Constructs a type representing a built-in type. The symbol must denote a Java class that is a sub-class of
sisc.data.Value, the base of the SISC value type hierarchy.
(define <record> (make-type ’|sisc.modules.record.Record|))(type<= <record> <value>) ;=> #t

7.2. Generic Procedures
Requires: (import generic-procedures)
Generic procedures are procedures that select and execute methods based on the types of the arguments.
Methods have a type signature, which generic procedures use for method selection, and contain aprocedure which is invoked by generic procedures when the method has been selected for execution.

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Generic procedures have several advantages over ordinary procedures:•

It is not necessary to come up with unique names for procedures that perform the same operation ondifferent types of objects. This avoids cluttering the name space. All these procedures can be defined
separately but yet be part of the same, single generic procedure.

•

The functionality of a generic procedure can be extended incrementally through code located indifferent places. This avoids "spaghetti code" where adding a new type of objects requires changes toexisting pieces of code in several locations.

•

Code using generic procedures has a high degree of polymorphism without having to resort to ugly
and hard-to-maintain test-type-and-dispatch branching.

Generic procedures make extensive use of SISCs type system. See Section 7.1.
The use of generic procedures proceeds through three stages:
1. definition of the generic procedure
2. adding of methods to the generic procedure
3. adding of methods to the generic procedure
The adding of methods can be interleaved with invocation, i.e. methods can be added to genericprocedures while they are in use.

7.2.1. Defining Generic Procedures
There are one procedure and two special forms for defining generic procedures. Typical usage will
employ one of the special forms.
procedure: (make-generic-procedure generic-procedure ...) => generic-procedure
Creates a generic procedure. If generic-procedure parameters are specified, then their method lists are merged,in effect combining the generic procedures into one. For more details on generic procedure combination see
Section 7.2.5.1.
(define pretty-print1 (make-generic-procedure))(define pretty-print2 (make-generic-procedure));=> <procedure>(define pretty-print (make-generic-procedure pretty-print1
pretty-print2))

syntax: (define-generic name generic-procedure ...) => void
Creates a binding for name to a new generic procedure.
This form is equivalent to (define name (make-generic-procedure generic-procedure ...)).
(define-generic pretty-print1)(define-generic pretty-print2)(define-generic pretty-print pretty-print1 pretty-print2)

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syntax: (define-generics form ...) => void
where form is of the form name or (name generic-procedure ...)Creates bindings for several new generic procedures.
The form expands into several define-generic forms.
(define-generic pretty-print1)(define-generic pretty-print2)(define-generics
foo(pretty-print pretty-print1 pretty-print2)
bar)

7.2.2. Defining Methods
Methods can be define and subsequently added to generic procedures, or the two operations can be
combined, which is the typical usage.
There is one procedure and one special form to create methods:
procedure: (make-method procedure type-list rest?) => method
Creates a new method containing procedure whose type signature is type-list. If rest? is #t then theprocedure can take rest arguments.
Generic procedures always invoke method procedures with a special next: argument as the first parameter (seeSection 7.2.5.3), followed by all the arguments of the generic procedure invocation. Hence procedure needs toaccept (length type-list)+1 arguments.
(make-method (lambda (next x y) (next x y))(list <number> <number>)#f);=> <method>(make-method (lambda (next x . rest) (apply + x rest))(list <number>)#t);=> <method>

syntax: (method signature . body) => method
where signature is of the form ([(next: next)] (type param) ... [ . rest])
and body can contain anything that is valid inside the body of a lambda.
Creates a method.
This form is similar to a lambda form, except that all parameters must be typed. The form expands into an
invocation of the make-method procedure.
The first parameter name in the method’s signature can be the special next: parameter. See Section 7.2.5.3.
(method ((next: next)(<number> x)(<number> y)) (next x y));=> <method>(method ((<number> x) . rest) (apply + x rest));=> <method>68

Chapter 7. Types and Objects

There are two procedures to add methods to a generic procedure:
procedure: (add-method generic-procedure method) => void
Adds method to generic-procedure. Any existing method with the same signature as method is removed.
Method addition is thread-safe.
(define-generic m)(add-method m (method ((next: next)(<number> x)(<number> y)) (next x y)))(add-method m (method ((<number> x) . rest) (apply + x rest)))procedure: (add-methods generic-procedure method-list) => void
Adds all methods in method-list to generic-procedure. Any existing method with the same signature as oneof the methods in method-list is are removed. When several methods in method-list have the same signature,
only the last of these methods is added.
Method addition is thread-safe. Calling add-methods instead of add-method when adding several methods to a
generic procedure is more efficient.
(define-generic m)(add-methods m (list (method ((next: next)(<number> x)(<number> y)) (next x y))(method ((<number> x) . rest) (apply + x rest))))

The creation of methods and adding them to generic procedures can be combined using one of two
special forms:
syntax: (define-method (generic-procedure . signature) . body) => void
Creates a method and adds it to generic-procedure.
This form is equivalent to (add-method generic-procedure (method signature . body)(define-generic m)(define-method (m (next: next)(<number> x)(<number> y)) (next x y)))(define-method (m (<number> x) . rest) (apply + x rest))

syntax: (define-methods generic-procedure (signature . body) ...) => void
Creates several methods and adds them to generic-procedure.
This form is equivalent to (add-methods generic-procedure (list (method signature . body) ...)
(define-generic m)(define-methods m[((next: next)(<number> x)(<number> y)) (next x y)][((<number> x) . rest) (apply + x rest)])69

Chapter 7. Types and Objects
The list of methods contained in a generic procedure can be obtained as follows:
procedure: (generic-procedure-methods generic-procedure) => method-list
Returns the list of methods currently associated with generic-procedure.
(define-generic m)(define-methods m[((next: next)(<number> x)(<number> y)) (next x y)][((<number> x) . rest) (apply + x rest)])(generic-procedure-methods m) ;=> (<method> <method>)

7.2.3. Invoking Generic Procedures
Generic procedures are invoked like ordinary procedures. Upon invocation, generic procedures computea list of applicable methods, ordered by their specificity, based on the types of the parameters supplied in
the invocation. If the resulting list is empty an error is raised. Otherwise the first (i.e. most specific)
method is invoked. The remaining methods come into play when a method invokes the "next best
matching method". See Section 7.2.5.3 for details on the method selection algorithms.
The logic by which generic procedures select methods for invocation is made accessible to theprogrammer through the following procedures:procedure: (applicable-methods generic-procedure type-list) => method-list
Returns all methods of generic-procedure that are applicable, as determined by method-applicable? toparameters of the types specified in type-list. The methods are returned ordered by their specificity, determinedby pair-wise comparison using compare-methods.
(define-generic m)(define-methods m[((next: next)(<number> x)(<number> y)) (next x y)][((<number> x) . rest) (apply + x rest)])(applicable-methods m (list <number> <number>));=> (<method> <method>)

procedure: (method-applicable? method type-list) => #t/#f
Determines whether method is applicable to arguments of the types specified in type-list.
The rules for determining method applicability are defined in Section 7.2.5.3.
(method-applicable? (method ((next: next)(<number> x)(<number> y)) (next x y))(list <number>));=> #f(method-applicable? (method ((<number> x) . rest) (apply + x rest))(list <number>));=> #tprocedure: (compare-methods method method type-list) => ’equal,’more-specific,’less-specific70

Chapter 7. Types and Objects
Determines the relationship of two methods by comparing their type signatures against each other and using the
supplied type-list for disambiguation. Both methods must be applicable to type-list, as determined bymethod-applicable?.
The comparison algorithm is described in Section 7.2.5.3.
(compare-methods (method ((next: next)(<number> x)(<number> y)) (next x y))(method ((<number> x) . rest) (apply + x rest))(list <number> <number>));=> ’more-specific

Calling a generic will dispatch on the argument types as described above. This dispatch can change if
new methods are added to a generic procedure. On occasion the programmer may wish to fix thedispatch of a particular generic function, either to guarantee a specific function is called for a given partof a Scheme program, or to improve performance by avoiding the type dispatch at each call. SISC
provides a syntactic form which allows the programmer to bind/rebind a generic procedure to a new
lexical variable which is the monomorphized variant of the function call.
syntax: (let-monomorphic bindings expressions ...) => void
where bindings are of the form ((generic type...) ...)or (((binding generic) type...) ...)In the former binding form, the generic procedure specified by generic is rebound lexically with the same name,and monomorphized to the method which is applicable for the given types. In the latter form, the generic is reboundlexically to the new name specified by binding . Both forms may be used in a given call to let-monomorphic.
The bindings are made as if by let, i.e. no assumptions can be made as to the order in which they are bound. The
expressions are evaluated as in let as well, in order using an implicit begin.
(let-monomorphic ([foo-generic <number> <string>][(bar bar-generic) <char>])(foo-generic 3 "four")(bar #\x))7.2.4. Procedures on Methods
Methods are instances of the abstract data type <method>, which has range of procedures:procedure: (method? value) => #t/#f
Returns #t if value is a method, #f otherwise.
(method? (method ((<number> x) . rest) (apply + x rest)));=> #t(method? (lambda (x) x));=> #fprocedure: (method-procedure method) => procedure
Returns method ’s body as a procedure. Note that a method’s procedure always takes a "next method" procedure as
the first argument. See Section 7.2.5.3.

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Chapter 7. Types and Objects((method-procedure (method ((<number> x) . rest) (apply + x rest)))#f 1 2 3);=> 6

procedure: (method-types method) => type-list
Returns method ’s type signature, i.e. the types of the declared mandatory parameters.
(method-types (method ((next: next)(<number> x)(<number> y) . rest) (next x y)));=> (<number> <number>)procedure: (method-rest? method) => #t/#f
Returns #t if method has a rest parameter, #f otherwise.
(method-rest? (method ((<number> x) . rest) (apply + x rest)));=> #t(method-rest? (method ((next: next)(<number> x)(<number> y) . rest) (next x y)));=> #fprocedure: (method-arity method) => number
Returns the number of mandatory arguments of method . Note that the special next: parameter is not counted.
(method-arity (method ((<number> x) . rest) (apply + x rest)));=> 1(method-arity (method ((next: next)(<number> x)(<number> y) . rest) (next x y)));=> 2

procedure: (method= method method) => #t/#f
Returns #t if the two methods have identical signatures, i.e. have equal parameter types (as determined by types=)and rest parameter flag. #f is returned otherwise.
(method= (method ((next: next)(<number> x)(<number> y) . rest) (next x y))(method ((<number> a)(<number> b) . rest) (+ x y)));=> #t(method= (method ((<number> x)(<number> y) . rest) (+ x y))(method ((<number> a)(<number> b)) (+ x y)));=> #f(method= (method ((<number> x)(<value> y)) (+ x y))(method ((<number> a)(<number> b)) (+ x y)));=> #f72

Chapter 7. Types and Objects

7.2.5. Miscellaneous
7.2.5.1. Generic Procedure Combination
Generic procedure combination merges the method lists of multiple generic procedures. The typicalscenario for using this features is when several modules have defined generic procedure (and procedures
using these generic procedures) that perform identical operations but on different data types. Genericprocedure combination extends to coverage of the individual generic procedures and the dependent
procedures to the combined set of data types. Furthermore, the coverage of the dependent procedures is
implicitly extended to the combined set of data types.
The following example illustrates how generic procedure combination can be used to combine thefunctionality of two p-append procedures defined independently by two modules. It also shows how
generic procedure combination implicitly extends the coverage of the p-reverse-append andp-repeat procedures defined by the modules.
(import* misc compose)(module foo(p-append p-reverse-append)(define (p-reverse-append . args)(apply p-append (reverse args)))(define-generic p-append)(define-methods p-append[((<list> x) . rest)(apply append x rest)][((<vector> x) . rest)(list->vector (apply append(vector->list x)(map vector->list rest)))]))(module bar(p-append p-repeat)(define (p-repeat n x)
(let loop ([res ’()][nn])
(if (= n 0)
(apply p-append res)(loop (cons x res) (- n 1)))))(define-generic p-append)(define-methods p-append[((<string> x) . rest)(apply string-append x rest)][((<symbol> x) . rest)(string->symbol (apply string-append(symbol->string x)(map symbol->string rest)))]))(import* foo (p-append1 p-append) p-reverse-append)(import* bar (p-append2 p-append) p-repeat)(define-generic p-append p-append1 p-append2)(define-method (p-append (<procedure> x) . rest)(apply compose x rest))

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Chapter 7. Types and Objects(p-append ’(a b)) ;=> ’(a b)(p-append ’(a b) ’(c d) ’(e f)) ;=> ’(a b c d e f)
(p-append ’#(a b)) ;=> ’#(a b)(p-append ’#(a b) ’#(c d) ’#(e f)) ;=> ’#(a b c d e f)
(p-append "ab") ;=> "ab"(p-append "ab" "cd" "ef") ;=> "abcdef"(p-append ’ab) ;=> ’ab(p-append ’ab ’cd ’ef) ;=> ’abcdef((p-append car cdr cdr cdr) ’(1 2 3 4)) ;=> 4(p-reverse-append "ab" "cd" "ef") ;=> "efcdab"(p-repeat 3 ’(a b)) ;=> ’(a b a b a b)((p-reverse-append cdr cdr cdr car) ’(1 2 3 4))((p-repeat 3 cdr) ’(1 2 3 4)) ;=> (4);=> 4

7.2.5.2. Scoping Rules
Generic procedures are lexically scoped, but their methods are not. Hence defining methods in a local
scope is generally a bad idea. One exception are module definitions. It is perfectly safe for modules todefine private (i.e. not exported) generic procedures and add methods to them without interfering withother modules. However, care must be taken when generic procedures are imported or exported methods are added to generic procedures when the module gets defined rather then when it gets imported.
The following example illustrates the scoping rules.
(define-generic m)(define-method (m (<value> v)) v)(m 1) ;=> 1(let ([x 1])(define-method (m (<number> v)) (+ x v))(m 1)) ;=> 2(m 1) ;=> 2(module foo(m)(define-generic m)(define-method (m (<value> v)) v))(import foo)(m 1) ;=> 1(module bar()(import foo)(define-method (m (<number> v)) (+ 1 v)))
(m 1) ;=> 2

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7.2.5.3. Method Selection
When generic procedures are invoked they select the most specific applicable method and call it, with theremaining applicable methods being made available to the invoked method via the next:.
Method applicability is determined on the basis of the types of the parameters passed in the invocation of
the generic procedure. A method is applicable to a list of parameter types if and only if the following
conditions are met:
• If the method accepts rest arguments then the length of the list of parameter types must be equal or
greater than the method arity (as returned by method-arity).
• If the method does not accepts rest arguments then the length of the list of parameter types must be
equal to the method arity (as returned by method-arity).
• All the types in the method’s type signature (as returned by method-types) must be super-types of
the corresponding parameter types. This comparison is performed using the types<= procedure.
This algorithm is encapsulated by the method-applicable? procedure.
Method specificity is an ordering relation on applicable methods with respect to a specific list ofparameter types. Informally, the relative specificity of two methods is determined by performing aleft-to-right comparison of the type signatures of the two methods and the parameter types using
compare-types, returning the result of the type comparison at the point of the first discernabledifference.
More formally, the relative specificity of two applicable methods is computed by a triple-wisecomparison on successive elements of the method signatures (as returned by method-types) and actual
parameter types, using compare-types, such that•

If we run out of elements in both method signatures then•If both or neither method return rest arguments (as determined by method-rest?) then themethods are of equal specificity.

•

If the first method takes rest arguments (as determined by method-rest?) then the first method isless specific than the second.

•

If the second method takes rest arguments (as determined by method-rest?) then the first method
is more specific than the second.

•If we run out of elements in the first method’s signature only then the first method is less specific than
the second.

•If we run out of elements in the second method’s signature only then the first method is more specificthan the second.

•

If compare-types returns ’equal we proceed to the next triple.

•

If compare-types returns ’less-specific then the first method is less specific than the second.

•

If compare-types returns ’more-specific then the first method is more specific than the second.

This algorithm is encapsulated by the compare-methods procedure.
The method form and derived forms (i.e. define-method and define-methods) permit the
specification of a special first parameter to the method invocation. When a generic procedure invokes amethod, this parameter is bound to a procedure that when called will invoke the "next best matching"method. This is the next method in the list of applicable methods returned by applicable-methods
when it was called by the generic procedure upon invocation.

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If no "next best matching" method exists, i.e. the current method is the last in the list, then the next
parameter is #f. This allows methods to invoke the next best matching method selectively depending onwhether it is present. This is an important feature since the dynamic nature of method selection makes it
impossible to determine at the time of writing the method whether there is going to be a next best
matching method.
The next best matching method must be invoked with arguments to which the current method isapplicable.
The following example illustrates the method selection algorithm, and use of the next: parameter:(define-generic m)(define-methods m[((next: next) (<number> x) (<value> y) (<number> z) . rest)(cons ’a (if next (apply next x y z rest) ’()))][((next: next) (<number> x) (<value> y) (<number> z))(cons ’b (if next (next x y z) ’()))][((next: next) (<number> x) (<number> y) . rest)(cons ’c (if next (apply next x y rest) ’()))][((next: next) (<number> x) (<number> y) (<value> z))(cons ’d (if next (next x y z) ’()))])(m 1 1 1) ;=> ’(d c b a)(m 1 1) ;=> ’(c)(m 1 ’x 2) ;=> ’(b a)(m 1 1 ’x) ;=> ’(d c)(m 1 ’x ’x) ;=> error

7.3. Object System
Requires: (import oo)
Programming in the SISC object system usually entailse the definition of generic procedures, so typicallyone also has to (import generic-procedures) .
The key features of the object system are:•class-based, with a restricted form of multiple inheritance•instance variables (aka slots) are accessed and modified via generic procedures•

generic procedures implement all behaviour; there is no separate notion of methods•

introspection API•

complete integration into SISC’s extensible type system

The examples in this section follow a few naming conventions:Classes are types in the SISC type system and therefore class names follow the naming convention for type names (seeGeneric procedures whose sole purpose it is to access slots of objects have names starting with : and otherwise follow
Generic procedures whose sole purpose it is to modify slots of objects, are named after the corresponding accessor pro

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7.3.1. Classes
Classes have a name, a list of direct superclasses, and a list of direct slot descriptions. All classes are
instances of the type <class> and are themselves types in SISC’s extensible type system. All classes are
direct or indirect sub-classes of the class <object>, except for <object> itself, which has no super-classes.
Classes are created as follows:
procedure: (make-class symbol class-list slot-list [guid-symbol]) => classCreates a class named symbol with the classes in class-list as its direct super-classes. See Section 7.3.4 for
restrictions on super-classes. When no super-classes are specified, the superclass is <object>.
slot-list is a list of slot names (symbols).

Slots are inherited by sub-classes. For details on slot inheritance see Section 7.3.4.
If guid-symbol is specified then the new class is non-generative: if guid-symbol is already bound to a class thenthat class is modified, instead of a new class being created. Non-generative classes are serialised specially such thatdeserialising them also performs this check. By contrast, deserialisation of ordinary, generative classes and theirinstances results in duplicate types being created, which is usually not desirable.
(define-generics :x :y :y!)(define <foo> (make-class ’<foo> ’() ’()))(define <bar> (make-class ’<bar> ’() ’()))(define <baz> (make-class ’<baz> (list <foo> <bar>)’(x y)))

syntax: (define-class name-and-supers slot-def ...) => void
where name-and-supers is of the form (class-name super-class ...)and slot-def is of the form ( slot-name [accessor [modifier ]] )Binds class-name to a newly created class.
This form expands into a definition with call to make-class on the right hand side.
slot-name names a slot. accessor must be a generic procedure. An accessor method for the slot will be added toit. modifier must be a generic procedure. A modifier method for the slot will be added to it.
(define-generics :x :y :y!)(define-class (<foo>))(define-class (<bar>))(define-class (<baz> <foo> <bar>)(x :x)(y :y :y!))syntax: (define-nongenerative-class name-and-supers guid slot-def ...) => void
This is the same as define-class, except that the resulting class is non-generative with guid , a symbol, as theunique identifier. The significance of this is explained in make-class.

One can test whether a particular value is a class:procedure: (class? value) => #t/#f77

Chapter 7. Types and Objects
Returns #t if value is a class, #f otherwise.
(class? (make-class ’<foo> ’() ’()))(class? (lambda (x) x)) ;=> #f(define-class (<foo>))(class? <foo>) ;=> #t;=> #t

The following procedures provide access to the various elements of the <class> abstract data type:procedure: (class-name class) => symbol
Returns the name of the class class.
(class-name (make-class ’<foo> ’() ’()))(define-class (<foo>))(class-name <foo>) ;=> ’<foo>;=> ’<foo>procedure: (class-direct-superclasses class) => class-list
Returns the list of direct super-classes of the class class.
(define-class (<foo>))(define-class (<bar>))(define-class (<baz> <foo> <bar>))(map class-name (class-direct-super-classes <foo>)(map class-name (class-direct-super-classes <baz>);=> ’());=> ’(<foo> <bar>))procedure: (class-direct-slots class) => slot-list
Returns the list of descriptions of the direct slots of the class class.
Slot descriptions are created by make-class for each slot definitions. The procedures operating on slot descriptions
are documented in Section 7.3.2.
(class-direct-slots (make-class ’<foo> ’() ’()))(define-generics :x :y :y!)(define-class (<baz>)(x :x)(y :y :y!))(class-direct-slots <baz> ;=> (<slot> <slot>));=> ’()

7.3.2. Slots
Slot descriptions are instances of the abstract data type <slot>. They are created implicitly when classes
are created; it is not possible to create them directly.
procedure: (slot? value) => #t/#f
Returns #t if value is a slot description, #f otherwise.

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Chapter 7. Types and Objects(define-class (<baz>) (x) (y))(map slot? (class-direct-slots <baz>));=> ’(#t #t)procedure: (slot-name slot) => symbol
Returns the name of the slot described by slot. This is the name given to the slot when its class was created.
(define-class (<baz>) (x) (y))(map slot-name (class-direct-slots <baz>));=> ’(x y)procedure: (slot-class slot) => class
Returns the class to which the slot description slot belongs.
(define-class (<baz>) (x) (y))(eq? (slot-class (car (class-direct-slots <baz>))) <baz>);=> #tSlot definitions can produce procedures that allow access and modification of slots. Since slots can be
defined without accessors and modifiers, this may be the only way to access/modify a particular slot.
procedure: (slot-accessor slot) => procedure
Returns a procedure than when applied to an instance of slot’s class, will return the slot’s value on that instance.
(define-class (<baz>) (x))(define baz (make <baz>))((slot-accessor (car (class-direct-slots <baz>))) baz) ;=> 1

procedure: (slot-modifier slot) => procedure
Returns a procedure than when applied to an instance of slot’s class and a value, will set the slot’s value on that
instance to the supplied value.
(define-generics :x :x!)(define-class (<baz>) (x :x))(define baz (make <baz>))(:x baz) ;=> 1((slot-modifier (car (class-direct-slots <baz>))) baz 2)(:x baz) ;=> 2

In addition to procedures for slot access and modification, slot definitions can also produce equivalentmethods. These are suitable for adding to generic procedures and can, for instance, be used to achieve thesame effect as specifying generic procedures for slot access/modification at class creation time.
procedure: (slot-accessor-method slot) => method
This is the same as slot-accessor, except it returns a method instead of a procedure.
procedure: (slot-modifier-method slot) => method
This is the same as slot-modifier, except it returns a method instead of a procedure.

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7.3.3. Instantiation
Class instantiation is a two-stage process. First a new object is created whose type is that of theinstantiated class. Then the initialize generic procedure is called with the newly created instance andadditional arguments. initialize serves the same purpose as constructors in other object systems.
All the phases of class instantiation are carried out by a single procedure:
procedure: (make class value ...) => instance
Creates a new object that is an instance of class. The instance and values are then passed as parameters to a call tothe initialize generic procedure. The result of calling make is the instance.
(define-generics :x :x!)(define-class (<baz>)(x :x :x!))(define-method (initialize (<baz> b) (<number> x)) (:x! b x))(define baz (make <baz> 2))(:x baz) ;=> 2

By default the initialize contains a no-op method for initialising objects of type <object> with nofurther arguments. Since all objects are instances of <object> all classes can be instantiated by calling(make class).
(define-generics :x :x!)(define-class (<baz>)(x :x :x!))(define baz (make <baz>))(:x baz) ;=> #f

7.3.4. Inheritance
Inheritance is a form of sub-typing. A class is a sub-type of all its direct and indirect superclasses.
Method selection in generic procedures (and hence also slot access/modification) is based on arelationship called class precedence; a total order of classes based on the partial orders established by the
direct super-classes:
procedure: (class-precedence-list class) => class-list
Returns the total order of class and all direct and indirect super-classes, as determined by the partial orders
obtained from calling class-direct-superclasses.
The ordering of classes returned by class-direct-superclasses is considered "weak" whereas the ordering of
the class itself to its direct super-classes is considered strong. The significance of this is that when computing theclass precedence list weak orderings are re-arranged if that is the only way to obtain a total order. By contrast, strong
orderings are never rearranged.
(define-class (<foo>))(define-class (<bar> <foo>))(define-class (<baz> <foo>))(define-class (<boo> <bar> <baz>))(define-class (<goo>))80

Chapter 7. Types and Objects(define-class (<moo1> <bar> <baz> <goo>))(map class-name (class-precedence-list <moo1>));=> (<moo1> <bar> <baz> <goo> <foo> <object>)(define-class (<moo2> <bar> <baz> <foo> <goo>))(map class-name (class-precedence-list <moo2>));=> (<moo1> <bar> <baz> <foo> <goo> <object>)(define-class (<moo3> <baz> <bar> <boo>))(map class-name (class-precedence-list <moo3>));=> (<moo3> <boo> <baz> <bar> <foo> <object>)

For any two classes in the class precedence list that have direct slots, one must be a sub-class of the other.
In effect this enforces single inheritance of slots while still giving control over the order of classes in theclass-precedence list.
Slots from a superclass are only ever inherited once, regardless of the number of paths in the inheritancegraph to that class.
If a sub-class defines a slot with the same name as one of its super-classes, instances of the resulting classends up with two slots of the same name. If the slots are define with different accessors and modifiers
then this does not cause any problems at all. If they are not then an invocation of the accessor/modifier
will acess/modify the slot of the sub-class. Note that it is still possible to access/modify the slot of thesuperclass by using procedures/methods obtained from the slot descriptor. See Section 7.3.2.
The following example illustrates the rules governing slot inheritance.
;;ordinary slot access and modification(define-generics:x :x! :y :y! :z :z!
:xb :xb! :yb :yb! :zb :zb!)(define-class (<foo>)(x :x :x!)(y :y :y!)(z :z))(define f (make <foo>))(:x f) ;=> #f(:y f) ;=> #f(:z f) ;=> #f(:x! f 2)(:y! f 2)(:x f) ;=> 2(:y f) ;=> 2;;overloading slots in sub-class(define-class (<bar> <foo>)(x :x :x!)(y :yb :yb!)(zb :zb :zb!))(define b (make <bar>))(:x b) ;=> #f(:y b) ;=> #f81

Chapter 7. Types and Objects(:z b) ;=> #f(:yb b) ;=> #f(:zb b) ;=> #f(:y! b 3)(:yb! b 4)(:y b) ;=> 3(:yb b) ;=> 4;;accessing a fully shadowed slot
(define :foo-x(cdr (assq ’x (map (lambda (x)(cons (slot-name x) (slot-accessor x)))(class-direct-slots <foo>)))))(:foo-x b) ;=> 1;;inheritance restriction on slots(define-class (<boo> <baz>)(yb :yb :yb!))(define-class (<goo>))(define-class (<moo> <bar> <boo> <goo>));=> error

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SISC can be used as a scripting language for Java, or Java may be used to provide functionality to
Scheme. Such activity is collectively termed ’bridging’. In SISC bridging is accomplished by a Java API
for executing Scheme code and evaluating Scheme expressions, and a module that provides
Scheme-level access to Java objects and implementation of Java interfaces in Scheme.

8.1. Calling Scheme from Java
Calling programmer-defined Scheme code from Java is a simple process, but requires some initialization
before proceeding. Depending on the use case of SISC, the initialization may be largely automatic. Any
Scheme call first requires four resources:
1. An Application Context (AppContext), which encapsulates the entirety of a single application’s
interactions with SISC. This is initialized with ...
2. A Heap, which provides the environments and base code to execute programs using ...
3. A Dynamic Environment, which contains thread specific values such as the current I/O ports, and
4. An Interpreter, which provides the engine and API for actually evaluating Scheme code.
Each resource is described in the sections below. The gateway for interacting with these resources is
primarily the sisc.interpreter.Context utility class.

8.1.1. Application Contexts
The Application Context, represented by the class sisc.interpreter.AppContext, holds references
to the top-level environment, global configuration properties, and other resources which are unique to asingle usage of SISC. This should not be confused with multiple threads or invocations into the same
application.
An AppContext is created simply using the default constructor, or a constructor which takes a Java
Properties class with overrides for global Scheme properties as described throughout this manual.
The application context is the token used by most of the API for calling Scheme from Java code, thoughmany of the API calls can either infer the AppContext from the currently executing thread, or defer to adefault context.
A default application context may be necessary when Scheme code is called through callbacks or otherJava mechanisms in code which is not aware of SISC or the Scheme code which is being called. The
default application context can be set explicitly after creating the context, but is also set implicitly by
SISC if there is not already a default context when one is needed. To set the default application context,
use the following method from Context:
public void setDefaultAppContext(sisc.interpreter.AppContext ctx);
Set the default AppContext to the instance provided.

public sisc.interpreter.AppContext getDefaultAppContext();

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Retrieves the current default AppContext, possibly creating and initializing one using the default heap if no default
was set.

Remember that an application context is nearly always useless until initialized with a heap, as describedin the next section.

8.1.2. The SISC Heap
The heap is a serialization of all the pre-compiled code which makes up both the base Scheme language
provided by SISC, and many of its libraries. Distributions of SISC come with a prepackaged heap which
is sufficient for most usages, and customizing a heap should be viewed as a last resort, as precompiled
libraries usually solve the same problem.
A heap is a randomly accessed structure which must be loaded into an application context. The heap canbe automatically located if it is in the directory pointed to by the SISC_HOME environment variable, thecurrent directory, or on the Java classpath. In the last case however, it will be loaded into memory
entirely, rather than read in as needed. The automatic heap location involves calling addDefaultHeap
on the AppContext, and is used when an application context is created implicitly.
public void addDefaultHeap();
Uses findHeap, openHeap, and addHeap to find and register a default heap with this application context

The URL that is produced through this discovery can be obtained using the findHeap method in
AppContext:
public static java.net.URL findHeap(String heapName);
Searches for the heap with the given name, or if null, sisc.shp in several candidate locations. If a heap is found, a
URL pointing to it is returned, otherwise null is returned.

One of the locations searched is the classpath, by searching for a heap anchor, and loading the heap filefrom the same package. To do this, the heap must be on the classpath in the same location as
HeapAnchor.class. A utility script, build-heapjar.scm, executed as an SRFI-22 script, is includedin the scheme-src directory of the full distribution. It is invoked from the directory containing theheap, and will by default emit sisc-heap.jar into the current directory. If that jar file is on theclasspath, findHeap will locate it automatically.
Once located, the heap is opened and registered with the AppContext, allowing the context to then be
used for Scheme evaluation. This is done with the openHeap and addHeap methods:
public static sisc.ser.SeekableInputStream openHeap(java.net.URL heapURL);
Opens the heap pointed to by the given URL, returning an appropriate random access input stream suitable forpassing to addHeap.
public boolean addHeap(sisc.ser.SeekableInputStream heap);
Registers the given heap stream with the application context. Returns true if the registration is successful, false
otherwise.

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8.1.3. The Dynamic Environment
The dynamic environment, represented by the sisc.env.DynamicEnvironment class, is thedatastructure which stores dynamic variables. That is, variables whose values are not scoped lexicallyand are associated with threads rather than code. This includes such values as the current input andoutput ports, Scheme parameters, and the class path.
Each interpreter contains a reference to its current dynamic environment, and the dynamic environment
cannot be shared between two threads, or by more than one application context called in the same thread.
They should be shared across call boundaries in a single thread and single application, but must be
created anew for external calls and cross application calls, and cloned for new threads.
It should seem obvious, then, that maintaining the correct dynamic environment in all call situations canbe tricky. Fortunately, the supported API calls in Context detect and do the Right Thing in mostsituations.

8.1.4. The Interpreter
The Interpreter class contains the engine for evaluating Scheme code, and API methods fortriggering that evaluation. Each thread of execution must have its own Interpreter instance.
Interpreters are obtained before one or more calls into Scheme, and using methods on the Context
helper class depending on whether the call is an Internal or External call.
A Scheme application can execute in multiple threads. Each thread must have its own dynamic
environment, containing entities such as the current input and output ports. Dynamic environments are
represented by instances of the sisc.env.DynamicEnvironment class.
Internal calls need to create fresh interpreters in order to preserve and subsequently restore the state of
the surrounding Scheme execution. Thus a single thread may be home to several interpreters.

Warning
In any case, the programmer must ensure that all calls to an interpreter are madeby the same thread. If another thread wishes to execute Scheme code, it must
follow the API below to obtain a different interpreter.

8.1.4.1. External Calls
An external call is a call made from Java to Scheme with no preceding call from Scheme to Java, in otherwords, the call is entirely external to the Scheme environment. For example, this may occur as a result ofa timer expiration or a thread created by Java. In this case, the application context must be specified, anda new dynamic environment (containing thread specific information such as the current input and output
ports) must be created. The preferred method for an external call is called a managed external call, anduses the visitor pattern with one of the following methods in the Context class:public static Object execute(sisc.interpreter.AppContext ctx, sisc.interpreter.SchemeCaller caller) throws
sisc.interpreter.SchemeException;

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Creates an Interpreter context for the application context if provided, or the default application context if ctx is null,and calls execute in the caller with the new Interpreter. When the caller returns, the Interpreter is freed, and thereturn value of the caller is returned.
public static Object execute(sisc.interpreter.SchemeCaller caller) throws sisc.interpreterSchemeException;
Creates an Interpreter context for the default application context, loading the default heap if necessary, and calls theexecute method of the caller object with the new Interpreter (that is, it is equivalent to execute(null,caller)). This simple interface is the one you should use if you do not have a particular reason to use a non-default
application context.

The visitor instances implement sisc.interpreter.SchemeCaller and are responsible for
implementing the following method:
public Object execute(sisc.interpreter.Interpreter r) throws sisc.interpreter.SchemeException;
Utilizes the given Interpreter to make Java to Scheme calls. Any Object may be returned.

As an alternative to this managed external call, a call from Java to Scheme can be made by using theenter method of Context to obtain an interpreter, then making several calls to the interpreter, thenreleasing it using exit. This has some weaknesses, however. Because this pair of calls cannot enforce
that all intervening calls to the Interpreter run in the same thread, subtle issues can arrise if the Interpretercontext is saved as a reference in a program and used by competing threads. Other, more subtle issues
exist, such as the association of a thread handle (as retrievable using SRFI-18) in Scheme with the Java
thread which is making the call. For this reason, the managed external call form is preferred wheneverpossible.
public sisc.interpreter.Interpreter enter(sisc.interpreter.AppContext ctx, sisc.env.DynamicEnvironment ctx);
Obtains an instance of Interpreter for the provided application context if provided, the current or default otherwise. If
provided, the given dynamic environment is used rather than the environment selected automatically for the type ofcall.

The dynamic environment optional argument in the enter method may be specified if one wants to use a
different mechanism for finding applications and dynamic environments. For instance, threads createdfrom Scheme should probably execute within the application that created them and using a dynamic
environment that is cloned from the dynamic environment present when the thread is started. The enter
method can therefore be used as general mechanism for obtaining a new interpreter that uses a specificapplication and dynamic environment.
When the Interpreter is no longer by the current thread needed it must be released using:public void exit();
Release the resources of the current interpreter.

8.1.4.2. Internal Calls
Internal calls are calls to Scheme made from Java, from a previous call into Java from Scheme. In otherwords, the call to Scheme is made internally from other Scheme code. One can determine if a call isinternal in the following manner:
Interpreter current = Context.currentInterpreter();if (current == null) { ...make external call...}else { ...make internal call... }86

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This case is more complex, as it requires maintaining the correct dynamic environment and Interpreter
instance to preserve return context. When making an internal call, one typically wants to make the call in
an interpreter that shares the same application context and dynamic enviornment as the calling interpreter.
Fortunately, the details are managed by the Context helper class. The same calling mechanisms areused as in an external call, but the application context is ommited as a parameter to the functions.

8.1.4.3. The Interpreter API
No matter which mechanism is used, the Interpreter is eventually used to call Scheme code, using
any of the following methods:•

public Value eval(String expr) throws sisc.interpreter.SchemeException, java.io.IOException;
Evaluates expressions read from a string, returning the result of the evaluation of the last expression.

•

public Value eval(InputStream stream, String sourceId) throws sisc.interpreter.SchemeException,
java.io.IOException;
Evaluates expressions read from an input stream, returning the result of the evaluation of the last expression. The
sourceId identifies the source of the stream for display purposes.

•

public Value eval(Value val) throws sisc.interpreter.SchemeException;
This is the same as calling (eval val) in Scheme.

•

public Value eval(Procedure proc, Value[] args) throws sisc.interpreter.SchemeException;
This is the same as calling (proc arg ...) in Scheme. This is the most efficient type of call, as it requires no
parsing or compilation.

Several such calls can be made consecutively on the same interpreter. A SchemeException may bethrown by any of these methods if an error occurs in the Scheme code.

8.1.5. Miscellaneous Features
8.1.5.1. Error HandlingInterpreter.eval() throws a sisc.interpreter.SchemeException when an evaluation causes
an exception that is not caught inside the evaluation. When making internal calls the exception can bepropagated to the calling interpreter in one of four ways:•by throwing a RuntimeException - this will be reported as"Error in "prim-name:description".

•by calling Module.throwPrimException("description") - this will be reported as "Error inprim-name: description".

•by calling throwNestedPrimException("description", schemeException) - this will bereported as "Error in prim-name: description\nnested-description".

•by calling throwNestedPrimException(schemeException) - this will be reported as "Errorin prim-name: exception during nested call\nnested-description".

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Scheme code can throw Java exceptions (see Section 8.2.9). The sisc.modules.s2j.Util classcontains a static method javaException(schemeException) that extracts the Java exception from a
SchemeException, or, if no Java exception is present, returns the SchemeException.

8.1.5.2. Continuations
Continuations do not cross the Scheme/Java boundary. In the embedded call scenario invoking acontinuation inside the embedded call will not discard the computation of the caller. The embedded callwill return when the continuation returns. If the continuation contains the read-eval-print loop the return
will never happen. Similarly, capturing a continuation inside a call (embedded or external) will only
capture the continuation to point where the call was made.
Capturing and invoking a continuation within the same call works correctly.

8.1.5.3. Scheme I/O
Scheme ports are thin wrappers around the Java I/O hierarchy, adding some functionality needed by
Scheme. As such, it is trivial to obtain Java compatible I/O objects from Scheme ports. For information
on obtaining Scheme and Java compatible I/O objects in Scheme, see the Java I/O module described in
Section 5.1.6.
Scheme ports are encapsulated in the SchemeBinaryInputPort, SchemeBinaryOutputPort,
SchemeCharacterInputPort, SchemeCharacterOutputPort classes in the sisc.data package.
An instance of a port class contains an accessor which returns the relevant Java I/O type, as described foreach class below.
public class sisc.data.SchemeBinaryInputPort {public java.io.InputStream getInputStream();}Return a Java InputStream for accessing this Scheme port.
public class sisc.data.SchemeBinaryOutputPort {public java.io.OutputStream getOutputStream();}Return a Java OutputStream for accessing this Scheme port.
public class sisc.data.SchemeCharacterInputPort {public java.io.Reader getReader();}

Return a Java Reader for accessing this Scheme port.
public class sisc.data.SchemeCharacterOutputPort {public java.io.Writer getWriter();}

Return a Java Writer for accessing this Scheme port.

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8.1.6. Quick Reference
The tables below cover the most common use cases for calling Scheme from Java, and provide simple
pseudocode examples.
Table 8-1. Typical Java to Scheme, External Calls
Situation

Code
Default Application Context

Heap can be located automatically...

No action required.

...or, with a custom heap

AppContext ctx=new AppContext();
SeekableInputStream myHeap=AppContext.openHeap(myctx.addHeap(myHeap);
Context.setDefaultAppContext(ctx);

Then, making the external call.

Context.execute(mySchemeCaller);

Custom Application Context
Creating the context.
AppContext ctx=new AppContext(myProperties);

Heap can be located automatically ...

ctx.addDefaultHeap();... or, with a custom heap

SeekableInputStream myHeap=AppContext.openHeap(myctx.addHeap(myHeap);

Then, making the external call.
Context.execute(ctx, mySchemeCaller);

Table 8-2. Typical Java to Scheme, Internal Calls
Situation
Making the internal call.

CodeContext.execute(mySchemeCaller);

8.2. Calling Java from Scheme
Requires: (import s2j)
The High-Level S2J API allows Scheme code to instantiate Java classes, call methods on Java objects,
access/modify fields of Java objects and implement Java interfaces that delegate to Scheme code.

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8.2.1. Classes
Java classes are types in SISC’s extensible type system (see Section 7.1). They are made accessible to
Scheme code by one of the following procedures / special forms:
procedure: (java-class symbol) => jclass
Returns the Java class of name symbol, which can also be the name of a primitive type or an array type.
(java-class ’|java.lang.String|) ;=> <jclass>(define <java.io.object-input-string/get-field**>(java-class ’|java.io.ObjectInputStream$GetField[][]|))

syntax: (define-java-class scheme-name [java-name]) => void
Binds scheme-name to the Java class named by java-name, or, if no such parameter is supplied, by the mangled
scheme-name.
(define-java-class <jstring> |java.lang.String|)(define-java-class <java.io.object-input-string/get-field**>)syntax: (define-java-classes form ...) => void
where form is of the form scheme-name or (scheme-name java-name )Creates bindings for several Java classes.
The form expands into several define-java-class forms.
(define-java-classes(<jstring> |java.lang.String|)<java.io.object-input-string/get-field**>)Mangling of class names allows classes to be identified more schemely, e.g.
<java.io.object-input-stream/get-field**> corresponds to the Java type
java.io.ObjectInputStream.GetField[][] (note that GetField is a nested class). More formally,
mangling of class names checks for the presence of angle brackets (<>) around the scheme-name. Ifthey are present they are stripped. All the identifiers between the dots (.) are passed through field namemangling (see Section 8.2.3). The character following the last dot is upcased. A slash (/) is treated as anested class indicator; it is replaced with dollar ($ and the character following it is upcased. Trailing stars(*) characters are replaced with pairs of brackets ([]).
There are predicates for determining whether a Scheme value is a Java class or interface. All Java
interfaces are also Java classes.
procedure: (java-class? value) => #t/#f
Returns #t if value is a Java class, #f otherwise.
(java-class? (java-class ’|java.lang.String|) ;=> #t(define <java-io-object-input-string/get-field**>(java-class ’|java.io.ObjectInputStream$GetField[][]|))(java-class? <java.io.object-input-string/get-field**>) ;=> #t90

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procedure: (java-interface? value) => #t/#f
Returns #t if value is a Java interface, #f otherwise.
(java-interface? (java-class ’|java.util.Map|)) ;=> #t(java-interface? (java-class ’|java.lang.String|) ;=> #f

Java classes are serializable by the SISC runtime.

8.2.2. Methods
Java methods are made accessible to Scheme code as procedures that can invoke any method of a given
name on any Java object. Method selection is performed based on the types of the arguments to theprocedure call. Static Java methods can be invoked by passing an instance of the appropriate class or an
appropriately typed null object (see Section 8.2.4) as the first argument to the procedures.
procedure: (generic-java-method symbol) => procedure
Returns a procedure that when invoked with a Java object as the first argument and Java values as the remainingarguments, invokes the best matching named method named symbol on the Java object and returns the result.
(generic-java-method ’|getURL|) ;=> <jmethod>(define empty-list? (generic-java-method ’|isEmptyList|))syntax: (define-generic-java-method scheme-name [java-name]) => void
Binds scheme-name to the generic Java method named by java-name, or, if no such parameter is supplied, by the
mangled scheme-name.
(define-generic-java-method get-url |getURL|)(define-generic-java-method empty-list?)

syntax: (define-generic-java-methods form ...) => void
where form is of the form scheme-name or (scheme-name java-name )Creates bindings for several generic Java methods.
The form expands into several define-generic-java-method forms.
(define-generic-java-methods(get-url |getURL|)
empty-list?)

Method name mangling allows methods to be identified more schemely, e.g. empty-list? corresponds
to the Java method name isEmptyList. More formally, mangling of method names removes trailingexclamation marks (!) and replaces trailing question marks (?) with a leading is-. The result of thismangling is passed through field mangling (see Section 8.2.3).

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Generic Java methods are serializable by the SISC runtime.

8.2.3. Fields
Fields are made accessible to Scheme code as procedures that can get / set any field of a given name on
any Java object. If several fields of the same name are present in the object due to the object class’s
inheritance chain, the most specific field, i.e. the one bottommost in the inheritance hierarchy, is selected.
Static Java fields can be accessed / modified by passing an instance of the appropriate class or an
appropriately typed null object (see Section 8.2.4) as the first argument to the procedures.
Generic Java field accessors, i.e. procedures that allow Scheme code to obtain the value of a Java field,
can be defined as follows:
procedure: (generic-java-field-accessor symbol) => procedure
Returns a procedure that when invoked with a Java object as the first argument, retrieves the value of the Java field
named symbol in the Java object.
(generic-java-field-accessor ’|currentURL|) ;=> <jfield>(define :current-input-port (generic-java-field-accessor ’|currentInputPort|))syntax: (define-generic-java-field-accessor scheme-name [java-name]) => void
Binds scheme-name to the generic Java field accessor for fields named java-name, or, if no such parameter is
supplied, the mangled scheme-name.
(define-generic-java-field-accessor :current-url |currentURL|)(define-generic-java-field-accessor :current-input-port)

syntax: (define-generic-java-field-accessors form ...) => void
where form is of the form scheme-name or (scheme-name java-name )Creates bindings for several generic Java field accessors.
The form expands into several define-generic-java-field-accessor forms.
(define-generic-java-field-accessor(:current-url |currentURL|):current-input-port)

Generic Java field modifiers, i.e. procedures that allow Scheme code to set the value of a Java field, canbe defined as follows:
procedure: (generic-java-field-modifier symbol) => procedure
Returns a procedure that when invoked with a Java object as the first argument and a Java value as the second
argument, sets the value of the Java field named symbol in the Java object to that value.
(generic-java-field-modifier ’|currentURL|) ;=> <jfield>(define :current-input-port! (generic-java-field-modifier ’|currentInputPort|))92

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syntax: (define-generic-java-field-modifier scheme-name [java-name]) => void
Binds scheme-name to the generic Java field modifier for fields named java-name, or, if no such parameter is
supplied, the mangled scheme-name.
(define-generic-java-field-modifier :current-url! |currentURL|)(define-generic-java-field-modifier :current-input-port!)

syntax: (define-generic-java-field-modifiers form ...) => void
where form is of the form scheme-name or (scheme-name java-name )Creates bindings for several generic Java field modifiers.
The form expands into several define-generic-java-field-modifier forms.
(define-generic-java-field-modifier(:current-url! |currentURL|):current-input-port!)

The mangling of field names allows fields to be identified more schemely. By convention field accessors
should be named with a leading colon (:) followed by the field name, and field modifiers with a leadingcolon (:) followed by the field name followed by an exclamation mark (!), e.g. :foo-bar and:foo-bar! are the names of the accessor and modifier for Java fields named fooBar. Mangling of fieldnames upcases any character following a dash (-) and removes all characters that are not legal as part ofJava identifiers.
Generic Java field accessors and modifiers are serializable by the SISC runtime.

8.2.4. Instances
Scheme code can instantiate Java classes with a call to the following procedure:
procedure: (java-new jclass jobject ...) => jobject
Selects a constructor of jclass based on the types of the jobjects and calls it, returning the newly created object.
(define-java-class <java.util.linked-hash-set>)(java-new <java.util.linked-hash-set> (->jint 100));=> <jobject>

There is a predicate for determining whether a value is a Java object:
procedure: (java-object? value) => #t/#f
Returns #t if value is a Java object, #f otherwise.
Note that, unlike in Java, instances of primitive Java types are considered to be Java objects.
(define-java-class <java.util.linked-hash-set>)(define hs (java-new <java.util.linked-hash-set> (->jint 100)))(java-object? hs) ;=> #t(java-object? (->jint 100)) ;=> #t93

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Unlike in Java, null objects are typed. Typed null objects play a key role in invoking static methods anaccessing / modifying static fields.
procedure: (java-null jclass) => jnull
Returns a Java null object of type jclass.
(define-java-class <java.util.linked-hash-set>)(java-null <java.util.linked-hash-set>) ;=> <jnull>

There is a predicate for determining whether a value is a Java null. All nulls are also Java objects.
procedure: (java-null? value) => #t/#f
Returns #t if value is a Java null object, #f otherwise.
(define-java-class <java.util.linked-hash-set>)(java-null? (java-null <java.util.linked-hash-set>));=> #tFor convenience, jnull is bound to the typed null object obtained by calling java-null on
java.lang.Object.
Any invocation of a Java method, or access to a Java fields that returns a Java null does so typed based on
the declared return / field type.
Comparison of Java objects using eqv? compares the objects using Java’s == comparison. equal?, on
the other hand, compares the objects using Java’s equals method. eq? uses pointer equality on the
Scheme objects representing the Java objects and is therefore not generally useful. Applying eq?, eqv?
or equal? to a mixture of Java objects and other Scheme values returns #f.
Java objects are only serializable by the SISC runtime if they support Java serialization. Java nulls arealways serializable.

8.2.5. Arrays
Scheme code can create Java arrays with a call to the following procedure:
procedure: (java-array-new jclass size) => jarray
Creates an array of component type jclass with dimensions size, which can be a number (for asingle-dimensional array), or a vector / list of numbers (for multi-dimensional arrays).
(java-array-new <jint> 2) ;=> <jarray>(define-java-class <java.lang.string>)(java-array-new <java.lang.string> ’#(2 2 2));=> <jarray>

There is a predicate for determining whether a value is a Java array. All Java arrays are also Java objects.
procedure: (java-array? value) => #t/#f94

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Returns #t if value is a Java array, #f otherwise.
(java-array? (java-array-new <jint> 2));=> #tElements of arrays are accessed and modified with:procedure: (java-array-ref jarray index) => jobject
Returns the element at index index of jarray . index can be a number, for indexing into the first dimension of thearray, or vector / list of numbers for multi-dimensional indexing.
(define a (->jarray (map ->jint (iota 10)) <jint>))(java-array-ref a 1) ;=> <java int 1>(java-array-ref a ’(1)) ;=> <java int 1>

procedure: (java-array-set! jarray index jobject) => void
Sets the element at index index of jarray to jobject. index can be a number, for indexing into the first
dimension of the array, or vector / list of numbers for multi-dimensional indexing.
(define a (->jarray (map ->jint (iota 10)) <jint>))(java-array-set! a 1 (->jint 2))(java-array-ref a 1) ;=> <java int 2>(define a (java-array-new <jint> ’#(2 2 2)))
(java-array-set! a ’#(1 1 1) (->jint 1))

The length of a Java array can be determined withprocedure: (java-array-length jarray) => number
Returns the length of jarray .
(define a (->jarray (map ->jint (iota 10)) <jint>))(java-array-length a) ;=> 10(define a (java-array-new <jint> ’#(2 3 4)))
(java-array-length a) ;=> 2

Scheme vectors and lists can be converted to Java array and vice versa.
procedure: (->list jarray) => list
Creates a list containing the elements of jarray .
(define a (->jarray (map ->jint (iota 5)) <jint>))(map ->number (->list a)) ;=> ’(0 1 2 3 4)

procedure: (->vector jarray) => vector
Creates a vector containing the elements of jarray .
(define a (->jarray (map ->jint (iota 5)) <jint>))95

Chapter 8. Java Interaction(map ->number (vector->list (->vector a)));=> ’(0 1 2 3 4)

procedure: (->jarray list-or-vector jclass) => jarray
Creates a one-dimensional array of type jclass and fills it with the values obtained from the Scheme vector or list.
(define a (->jarray (map ->jint (iota 5)) <jint>))

8.2.6. Proxies
Scheme code cannot create sub-classes of existing Java classes. It is, however, possible to create classes
implementing existing Java interfaces. These classes are called proxies. Calling a method on a proxyinvokes a user-definable Scheme procedure, based on the name of the method, passing the proxy object
and the parameters of the method invocation as arguments. The result of the invocation is returned as theresult of the method call.
syntax: (define-java-proxy signature interfaces method ...) => void
where signature is of the form (name param ...),
interfaces is of the form (interface ...), andmethod is of the form (define method-name procedure), or (define ( method-name method-arg ...) . body)Creates a proxy generator procedure and binds it to name. A proxy class is created that implements all theinterfaces. When the generator is invoked, an instance of the proxy class is returned that delegates all method
invocations to the Scheme procedures in the method definition list, based on the names of the methods.
The first kind of definition form defines procedure to be the method handler for the java method named
method-name. method-name undergoes name mangling as described in Section 8.2.2. Note that procedure isinside the lexical scope of the generator procedure, so params are accessible inside it.
The second kind of definition form is equivalent to the following first-type form: (define method-name (lambda(method-arg ...) . body)).
If a method is invoked on a proxy for which no method handler exists and error is returned to the caller.
(define-java-classes<java.util.comparator><java.util.arrays><java.lang.object>)(define-java-proxy (comparator fn)(<java.util.comparator>)(define (.compare p obj1 obj2)(let ([x (java-unwrap obj1)][y (java-unwrap obj2)])(->jint (cond [(fn x y) -1][(fn y x) +1][else 0])))))(define-generic-java-method sort)(define-java-class <java.lang.object>)(define (list-sort fn l)(let ([a (->jarray (map java-wrap l) <java.lang.object>)])(sort (java-null <java.util.arrays>) a (comparator fn))(map java-unwrap (->list a))))(list-sort < ’(3 4 2 1)) ;=> ’(1 2 3 4)96

Chapter 8. Java Interaction(list-sort string<? ’("foo" "bar" "baz"));=> ’("bar" "baz" "foo")

8.2.7. Types and Conversions
For convenience, all the primitive Java types, i.e. void, boolean, double, float, long, int, short,
byte, char, are predefined and bound to <jvoid>, <jboolean>, <jdouble>, <jfloat>, <jlong>,<jint>, <jshort>, <jbyte>, <jchar>, respectively.
When calling Java methods, invoking Java constructors, accessing or modifying Java fields, no automaticconversion is performed between ordinary Scheme values and Java values. Instead explicit conversion ofarguments and results is required. Automatic conversion is not performed for the following reasons:• For some Scheme types, such as numbers, the mapping to Java types is one-to-many, e.g. a Scheme
number could be converted to a byte, short, int, etc. This causes ambiguities when automaticconversion of parameters is attempted.
• Some Java types have several corresponding Scheme types, e.g. a Java array could be represented as
Scheme list or vector - this causes ambiguities when automatic conversion of results is attempted.
• Conversion carries an overhead that can be significant. For instance, Java strings have to be copied "byvalue" to Scheme strings since the former are immutable and the latter aren’t. In a chained-callscenario, i.e. where the results of one method invocation are passed as arguments to another, theconversion is unnecessary and a wasted effort.
• Conversion breaks the object identity relationship. In a chained-call scenario, the identities of theobjects passed to the second call are different from the ones returned by the first. This causes problems
if the called Java code relies on the object identity being preserved.
• Conversion conflicts with generic procedures. The method selection mechanism employed by genericprocedures relies on objects having exactly one type. Automatic conversion effectively gives objects
more than one type - their original type and the type of the objects they can be converted to. While itwould be technically possible to devise a method selection algorithm that accommodates this, thealgorithm would impose a substantial overhead on generic procedure invocation and also make itsignificantly harder for users to predict which method will be selected when invoking a genericprocedure with a particular set of arguments.

Conversion functions are provided for converting instances of primitive Java types to instances ofstandard Scheme types:
procedure: (->boolean jboolean) => #t/#f
procedure: (->character jchar) => character
procedure: (->number jbyte/jshort/jint/jlong/jfloat/jdouble) => number

Conversion functions also exists for the opposite direction, i.e. converting instances of standard Scheme
types to instances of primitive Java types
procedure: (->jboolean boolean) => jboolean
procedure: (->jchar character) => jchar

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Chapter 8. Java Interaction
procedure: (->jbyte number) => jbyte
procedure: (->jshort number) => jshortprocedure: (->jint number) => jint
procedure: (->jlong number) => jlong
procedure: (->jfloat number) => jfloat
procedure: (->jdouble number) => jdouble

Finally, there are conversion functions for converting between Java strings and Scheme strings andsymbols:
procedure: (->string jstring) => string
procedure: (->symbol jstring) => symbol
procedure: (->jstring string/symbol) => jstring

Scheme values are not Java objects and hence cannot be passed as arguments in Java method orconstructor invocations or when setting Java fields. However, all Scheme values are internally
represented by instances of classes in the SISC runtime. S2J provides a mechanism to get hold of thisinternal representation as an S2J Java object. The converse operation is also supported - a Java instanceobtained via a Java method or constructor invocation or field access in S2J can be turned into a Scheme
value if it is an instance of an appropriate SISC runtime class. These two operations are called"wrapping" and "unwrapping" respectively because conceptually the scheme object is wrapped to makeit appear like a Java object and the wrapper is removed in order to recover the original Scheme object.
procedure: (java-wrap value) => jobject
Returns the Java object that represents the Scheme value in SISC’s runtime.
procedure: (java-unwrap jobject) => value
Returns the Scheme value represented by the jobject. If jobject is not an object representing a Scheme value in
SISC’s runtime and error is thrown.
(define-java-class <java.lang.object>)(define a (java-array-new <java.lang.object> ’#(1)))(java-array-set! a ’#(0) (java-wrap ’foo))(java-unwrap (java-array-ref a ’#(0))) ;=> ’foo

Wrapping and unwrapping allows Scheme values to be used in generic (i.e. not type-specific) Java
operations, such as those of the Java collection API. It is also frequently used in connection with proxies
when Scheme objects are passed back and forth through layers of Java to a Scheme-implemented proxythat manipulates them. Finally, wrapping and unwrapping permit SISC Scheme code to interface to the
SISC runtime.

8.2.8. Multi-threading
In Java each object is a potential thread synchronization point. Therefore Scheme code needs to be able
to synchronize on Java objects in order for it to interoperate properly with Java in a multi-threaded98Chapter 8. Java Interaction
application. This is accomplished by the following procedure:
procedure: (java-synchronized jobject thunk) => value
Runs thunk in a block synchronized on jobject, returning the result returned by thunk. This is the equivalent tosynchronized (jobject) { return thunk(); } in Java.
It is illegal for thunk to invoke continuations that escape thunk, or for code outside thunk to invoke a continuation
captured inside thunk.
(define-java-class <java.lang.object>)(define mtx (java-new <java.lang.object>))(define v 0)
(define (inc-v)(java-synchronized mtx (lambda () (set! v (+ v 1)) v)))(define (dec-v)(java-synchronized mtx (lambda () (set! v (- v 1)) v)))
(import threading)(begin (parallel inc-v dec-v inc-v inc-v dec-v dec-v) v) ;=> 0

8.2.9. Exception HandlingJava exceptions are propagated to scheme and can be caught like any other exception, e.g. with with/fc
as defined in Section 3.4.1. The s2j module exports augmented versions of the print-stack-trace
and print-exception functions that handle Java exceptions. For example(define-generic-java-method char-at)(with/fc (lambda (m e) (print-exception (make-exception m e)))(lambda () (char-at (->jstring "foo") (->jint 3))))will catch the IndexOutOfBoundsException, print its stack trace and return #f.
In Scheme, Java exceptions can be thrown by raising an error containing the Java exception as themessage, e.g.
(define-java-class <java.util.no-such-element-exception>)(error (java-new <java.util.no-such-element-exception>))or(throw (make-error (java-new <java.util.no-such-element-exception>)))If this occurs inside a proxy method (see Section 8.2.6), the exception is propagated to the invoking Javacode.

8.2.10. Access Permissions
Invoking [define-]java-class[es], java-new or any of the procedures defined with[define-]generic-java-{method,field-accessor,field-modifier}[s] causes S2J toperform reflection on the named Java class(es), the class passed as the first argument, or the classcorresponding to the type first argument passed to the other procedures, respectively. This process99Chapter 8. Java Interaction
collects information about all the constructors, methods and fields of the class and its
superclasses/interfaces.
The only class members processed during this automatic reflection are public ones declared in publicclasses. This almost exactly mimics the visibility rules in Java for code residing in packages other thanthe one the member is residing in. It is also in line with the default permissions granted to the Java
reflection API. There is one rare case where this rule is more restrictive than Java’s: public members of
package-protected classes are not visible even when accessed via a public sub-class.
Depending on the security settings, the Java reflection API is in fact capable of granting access to any
members of any class. However, using this in the automatic reflection performed by S2J would constitutea significant departure from normal Java behaviour and result in unpredictable results to the user. For
instance, undocumented private methods would be invoked in preference to documented public methods
if the formers type signature provided a better match.
Automatic reflection ignores security exceptions thrown by the Java reflection API, i.e. the class inquestion will appear to have no constructors, methods and fields. This is designed to cope with situations
where the default security settings have been altered in a way that prevents access to members of some(or even all) classes.
In some applications the reflection API permissions depend on the context of the invocation. For
instance, in applets it is usually possible to access class member information as part of the initialisation
but not after that. Since [define-]java-class[es] triggers automatic reflection, it can be used tocontrol when automatic reflection for specific classes takes place.

8.2.11. Common Usage
This section provides a summary of all the commonly used S2J features, correlating them with thecorresponding Java code. It makes use of some functions from the srfi-1, srfi-26 and misc modules(require-library ’sisc/libs/srfi)(import* srfi-1 fold)(import* srfi-26 cut cute)(import* misc compose)

Table 8-3. Common S2J Usage
Java

Schemecreate bindings for classes, methods and fields

n/a(define-java-classes <foo.bar-baz> <foo.bar-boo>)(define-generic-java-methods get-bar get-baz set(define-generic-java-field-accessors :bar :baz)(define-generic-java-field-modifiers :bar! :baz!)instantiate class100

Chapter 8. Java Interaction
Java

Schemefoo.BarBaz fooObj = new foo.BarBaz(a, b,(definec);foo-obj (java-new <foo.bar-baz> a b c))

invoke method on instanceObject res = fooObj.barBaz(a, b, c)(define res (bar-baz foo-obj a b c))

invoke method on class
Object res = foo.Bar.baz(a, b, c)(define res (baz (java-null <foo.bar>) a b c))access instance fieldObject res = fooObj.bar;(define res (:bar foo-obj))

access class fieldObject res = foo.Bar.baz;(define res (:bar (java-null <foo.bar>)))modify instance field
fooObj.bar = val;(:bar! foo-obj val)

modify class field
foo.Bar.baz = val;(:bar! (java-null <foo.bar>) val)

chained field access
Object res = fooObj.bar.baz.boo(define res (fold (cut <> <>) foo-obj (list :baror(define res ((compose :boo :baz :bar) foo-obj))

This works equally well for bean fields.
chained field modification
fooObj.bar.baz.boo = moo;(:boo! (fold (cut <> <>) foo-obj (list :bar :baz)or(:boo! ((compose :baz :bar) foo-obj) moo)This works equally well for bean fields.
accessing several fields
a = fooObj.bar; b = fooObj.baz;c = fooObj.boo;(apply (lambda (a b c) ...)
(map (cute <> foo-obj) (list :bar :baz :boo)))This works equally well for bean fields.
modifying several fields

101Chapter 8. Java Interaction
Java

SchemefooObj.bar = a; fooObj.baz = b;
fooObj.boo = c;(for-each (cute <> foo-obj <>)(list :bar! :baz! :boo!)(list a b c))Thisworks equally well for bean fields.
creating an array
int[][] ar = new int[2][2];(define ar (java-array-new <jint> ’(2 2)))This works equally well for bean fields.
accessing an array element
int res = ar[1][1];(define res (java-array-ref ar ’(1 1)))

modifying an array elementar[1][1] = val;(java-array-set! ar ’(1 1) val)iterating over an arrayfor(int i=0; i<ar.length; i++) ar[i].fooBar(a,b);(for-each (cute foo-bar <> a b) (->list ar))

implementing interfaces(define-java-proxy (foo x y)
public class Foo implements Bar, Baz { (<bar> <baz>)private int x;
private int y;(define (bar-method p z)
public Foo(int x, int y) {(->jint (+ x y (->number z))))this.x = x;this.y = y;}(define (baz-method p z)
public int barMethod(int z) {(->jint (+ x y (- (->number z))))))
return x+y+z;}... (define foo-obj (foo 1 2))
public int bazMethod(int z) {return x+y-z;} } ...
Foo fooObj = new Foo(1, 2);

8.3. Java Reflection Interface
Requires: (import s2j)
The S2J Reflection API lets Scheme code access all the core functions of the Java reflection API. It
underpins the High Level S2J Interface (see Section 8.2). Normal interaction with Java from Scheme
does not require knowledge of this API, just like normal use of Java does not require knowledge of theJava reflection API.

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Chapter 8. Java Interaction

8.3.1. Classes
These functions access attributes and members of classes.
procedure: (java-class-name jclass) => symbol
Returns the name of jclass.
procedure: (java-class-flags jclass) => list of symbols
Returns the modifiers of jclass, for example public static final.
procedure: (java-class-declaring-class jclass) => jclass
Returns the Java class in which jclass was declared, or null if it was declared at the top level.
procedure: (java-class-declared-superclasses jclass) => list of jclass
Returns the direct superclasses of jclass.
Normally this is the class’ superclass followed by all of its interfaces in the order they were specified in the classdeclaration. There are a number of exceptions which ensure that the result is consistent with the precedence order
employed by Java for method lookup on overloaded method. Interfaces and classes that directly inherit fromjava.lang.Object are all given java.lang.Object as the last element in their superclass list. For primitive andarray types the direct superclass or superclasses reflect the widening conversions performed by Java. For example,<jint>’s superclass is <jlong> and <java.util.array-list[][]> ’s superclasses are:• <java.util.abstract-list[][]>• <java.util.list[][]>• <java.util.random-access[][]>• <java.lang.cloneable[][]>• <java.io.serializable[][]>

Note that this behavior is different from the corresponding method in the Java reflection API.
procedure: (java-class-declared-classes jclass) => list of jclasses/#f
Returns all the classes declared by jclass, or #f if access to this information is prohibited.
procedure: (java-class-declared-constructors jclass) => list of jconstructor/#f
Returns all the constructors declared by jclass, or #f if access to this information is prohibited.
procedure: (java-class-declared-methods jclass) => list of jmethods/#f
Returns all the methods declared by jclass, or #f if access to this information is prohibited.
procedure: (java-class-declared-fields jclass) => list of jfields/#f
Returns all the fields declared by jclass, or #f if access to this information is prohibited.
procedure: (java-class-precedence-list jclass) => list of jclasses
Returns the total order of jclass and all direct and indirect superclasses, as determined by the partial orders
obtained from calling java-class-declared-superclasses.
The class precedence list is important when comparing types using the type system’s compare-types procedure,
which is used by the generic procedure method selection algorithm (see compare-methods in Section 7.2.3). Since
generic Java methods and field accessors/mutators are implemented in terms of generic procedures they are allaffected by the class precedence list.

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Chapter 8. Java Interaction

8.3.2. Constructors
procedure: (java-constructor? value) => #t/#f
Determines whether value is a Java constructor.
procedure: (java-constructor-name jconstructor) => symbol
Returns the name of jconstructor .
procedure: (java-constructor-flags jconstructor) => list of symbols
Returns the modifiers of jconstructor , such as public static final.
procedure: (java-constructor-declaring-class jconstructor) => jclass
Returns the Java class in which jconstructor was declared.
procedure: (java-constructor-parameter-types jconstructor) => list of jclasses
Returns the declared types of the parameters of jconstructor .
procedure: (java-constructor-procedure jconstructor) => procedure
Returns a procedure that when called invokes the constructor with the passed arguments, returning the newly createdobjected.
procedure: (java-constructor-method jconstructor) => method
Returns a method suitable for adding to generic procedures that, when called invokes the underlying Java constructor
with the passed arguments. The resulting newly created object is returned.

8.3.3. Methods
procedure: (java-method? value) => #t/#f
Determines whether value is a Java method.
procedure: (java-method-name jmethod) => symbol
Returns the name of jmethod .
procedure: (java-method-flags jmethod) => list of symbols
Returns the modifiers of jmethod , such as public static final.
procedure: (java-method-declaring-class jmethod) => jclass
Returns the Java class in which jmethod was declared.
procedure: (java-method-parameter-types jmethod) => list of jclasses
Returns the declared types of the parameters of jmethod .
procedure: (java-method-procedure jmethod) => procedure
Returns a procedure that when called invokes the method with the passed arguments, returning the newly createdobjected.
procedure: (java-method-method jmethod) => method
Returns a method suitable for adding to generic procedures that, when called invokes the underlying Java method on
the object passed as the first argument, and with the remaining arguments passed as parameters. The result of the104

Chapter 8. Java Interaction
method invocation is returned. Static methods can be invoked by passing a typed null object as the first parameter tothe generic procedure.

8.3.4. Fields
procedure: (java-field? value) => #t/#f
Determines whether value is a Java field.
procedure: (java-field-name jfield) => symbol
Returns the name of jfield .
procedure: (java-field-flags jfield) => list of symbols
Returns the modifiers of jfield , such as public static final.
procedure: (java-field-declaring-class jfield) => jclass
Returns the Java class in which jfield was declared.
procedure: (java-field-type jfield) => jclass
Returns the declared type of jfield .
procedure: (java-field-accessor-procedure jfield) => procedure
procedure: (java-field-modifier-procedure jfield) => procedure
Returns a procedure that when called returns or sets (respectively) the value of the field on the object specified by the
first parameter to the invocation. Static fields can be accessed/modified by passing a null object.
procedure: (java-field-accessor-method jfield) => method
procedure: (java-field-modifier-method jfield) => method
Returns a method suitable for adding to generic procedures that, when called returns/sets the value of the field on theobject specified by the first argument to the generic procedure invocation. Static fields can be accessed/modified bypassing a typed null object as the first parameter to the generic procedure.

8.3.5. Arrays
procedure: (java-array-class jclass dimensions) => jclass
Returns a class representing the array type that has jclass as the component type and dimensions as the numberof dimensions. For example, the following expressions are equivalent:(java-array-class <jint> 2)(java-class ’|int[][]|)The list of direct superclasses returned by java-class-declared-superclasses for an array class is
consistent with the widening conversion performed by Java, e.g. the direct superclasses ofjava.util.ArrayList[][] are:• java.util.AbstractList[][]• java.util.List[][]• java.util.RandomAccess[][]105

Chapter 8. Java Interaction• java.lang.Cloneable[][]• java.io.Serializable[][]This is different from what the Java reflection APIs return.

8.3.6. Proxies
procedure: (java-proxy-class jinterface) => jclassCreates a Java class that implements the specified interfaces. The class can be instantiated with an invocation handler,
such as the one returned by java-proxy-dispatcher below, that delegates method invocation to Scheme code.
procedure: (java-proxy-dispatcher alist) => invocation-handler
Creates an invocation handler suitable for use in the instantiation of a proxy (see java-proxy-class above). The
keys in alist are Java method names and the values are Scheme procedures.
When a method is invoked on a proxy, the procedure matching the method’s name is invoked with the proxy object
and the parameters of the method invocation as arguments. The result of the invocation is returned as the result of themethod call. If alist does not contain a binding for the method name, an error is signalled.

106Chapter 9. Additional Libraries
9.1. Optional SISC Libraries
The optional SISC libraries are modules whose definition is included in the full SISC distribution, but not
the lite distribution.

9.1.1. Definitions
Requires: (import misc)
In addition to the standard R5RS definition syntaxes, SISC provides an additional value definition andsyntax definition form.
First, define-values, which allows more than one binding to be created at once, given themultiple-value return of its body.
syntax: (define-values (binding binding ... ) expression) => undefined
Evaluates the expression in the body, which must return the same number of values as there are binding names. Eachvalue is then bound (in an undefined order) to each binding name.
define-values behaves like define in terms of which environment the bindings are created. If thedefine-values statement is at the top-level then bindings are created in the top-level environment. If

the statement is in a lexical environment, then it behaves just as an internal define.
Second, define-simple-syntax provides a shorthand for syntax definition when the syntactic form’s
appearance is similar to a function.
syntax: (define-simple-syntax (name vars ... ) body) => undefined
Creates a syntactic form with the given name, and any number of listed syntactic variables, which expands to thegiven body (with instances of the syntactic variables hygienically expanded).

Here is an example usage of define-simple-syntax to define the when macro:(define-simple-syntax (when condition body ...)
(if condition(begin body ...)))

9.1.2. Bitwise Logical Operations
Requires: (import logicops)
In addition to the R5RS set of procedures that deal with numbers, SISC provides operators for performing
bitwise logic operations on exact integers.

107Chapter 9. Additional Libraries
procedure: (logand integer [integer] ...) => integer
Performs the logical AND of all the provided arguments.
procedure: (logor integer [integer] ...) => integer
Performs the logical OR of all the provided arguments.
procedure: (logxor integer [integer] ...) => integer
Performs the logical exclusive-OR of all the provided arguments.
procedure: (lognot integer) => integer
Performs the logical NOT of the provided integer.
procedure: (logcount integer) => integer
Returns the count of the number of 1 bits in the representation of a given positive integer, or 0 bits in a negative
integer.

In addition, two operators are provided to perform arithmetic shifts on any integer (these operators do nothave the range limitation the previous logical functions do). The shift operators return a newly generated
number formed by shifting the provided number left or right by the given number of bits.
procedure: (ashl integer bits) => integer
Arithmetically shifts integer left by bits bits.
procedure: (ashr integer bits) => integer
Arithmetically shifts integer right by bits bits.

Mathematically, if r is the number, and s is the number of bits, ashl calculates:r x 2swhile ashr calculatesr / 2sin the integer domain. Both ashl and ashr operate on exact integers and produce only exact integers.

9.1.3. Records
Requires: (import record)
SISC provides a native implementation of record types as defined in SRFI-9. See
http://srfi.schemers.org/srfi-9/ for details. In addition to the define-record-type syntax provided by
SRFI-9, a more compact (but less flexible) define-struct syntax is offered.
syntax: (define-struct name (field ...)) => void
Defines a SRFI-9 record type as follows:(define-record-type (make-name field ...)
name?
(field name-field set-name-field !)108

Chapter 9. Additional Libraries...)

i.e. naming conventions are used to determine the names of the record type constructor, predicate, field access andfield modifier procedures.

Records are eq? and eqv? if and only if they are identical. Records are equal? if and only if they are
instances of the same record type and all their fields are equal?.
It is also possible to define non-generative record types, using
define-nongenerative-record-type and define-nongenerative-struct. Non-generative
record types are associated with a user-specified guid. If an attempt is made to define a record type with a
guid that is already bound to an existing record type then the existing record type is modified, instead ofa new record type being created. Non-generative record types are serialised specially such thatdeserialising them also performs this check. By contract, deserialisation of ordinary, generative record
types and their instances results in duplicate types being created, which is usually not desirable.
syntax: (define-nongenerative-record-type name guid (constructor-name field...) (predicate ...) (field-spec ...)) => void
This is the same as define-record-type, except that the resulting record type is non-generative with guid , asymbol, as the unique identifier.

syntax: (define-nongenerative-struct name guid (field ...)) => void
This is the same as define-struct, except that the resulting struct is non-generative with guid , a symbol, as theunique identifier.

9.1.4. Hash Tables
Requires: (import hashtable)
Hash tables store mappings of keys to values. Hence they are similar to association lists, except that hashtables allow retrieval, addition and modification in constant time whereas association lists typically
perform these operations in linear time based on the number of elements.

9.1.4.1. Creation and Introspection
Hash tables are a distinct data type. They can be created empty or filled with the contents of an
association list. The converse, creating an association list from a hash table, is also supported.
procedure: (make-hashtable [equivalence-predicate] [hash-function] [thread-safe?][weak?]) => hashtable
Creates a hash table. The first optional argument supplies the equivalence test procedure that the hashtable should
use for comparison of keys. This must be a function accepting two arguments and returning a boolean. It defaults toequal?.
The second optional argument supplies the hash function, which must accept one argument and return a numericvalue. For the equivalence predicates eq?, eqv?, equal?, string=?, string-ci=? it defaults to hash-by-eq,
hash-by-eqv, hash-by-equal, hash-by-string=, hash-by-string-ci=? respectively, andhash-by-equal otherwise.

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Chapter 9. Additional Libraries
The third optional argument determines whether operations on the hash table should be made thread-safe. The
default is #t. Thread synchronization (see Chapter 6) is required if there are potentially several threads operatingconcurrently on the hash table and one of these threads performs a structural modification (i.e. adds or removes anentry; merely changing the value of an entry is not a structural modification). Failure to enforce proper threadsynchronization has unpredicatable results.
The fourth optional argument determines whether the keys in the hash table are held with weak references, allowing
them to be garbage collected, and automatically removed from the hashtable when they are not referenced from
elsewhere. The default is #t.
For reasons of disambiguation, the hash function argument can only be supplied if the preceeding equivalence
predicate was also supplied, and the weakness argument can only be supplied if the preceeding thread-safetyargument was also supplied.
The equivalence and hash function must produce stable results for the keys in a hash table.
The effects of invoking an escaping continuation inside the equivalence predicate or hash function, or invoking acontinuation captured inside the equivalence predicate or hash function after that function has returned, areunspecified.
procedure: (alist->hashtable alist [equivalence-predicate] [hash-function][thread-safe?] [weak?]) => hashtable
Creates a hashtable and initializes it with the keys and values found in alist. alist must be a list of pairs, with the
car of each pair representing a key and the cdr representing its associated value. The optional arguments are thesame as for make-hashtable.
If there are multiple pairs which contain the same key (with respect to chosen equivalence test) then the resulting
hash table will associate the key with the value of the last such pair.
procedure: (hashtable? value) => #t/#f
Returns #t if value is a hash table, #f otherwise.
procedure: (hashtable/equivalence-function hashtable) => procedure
Returns the equivalence predicate of hashtable.
procedure: (hashtable/hash-function hashtable) => procedure
Returns the hash function of hashtable.
procedure: (hashtable/thread-safe? hashtable) => #t/#f
Returns #t if hashtable is thread safe, #f otherwise.
procedure: (hashtable/weak? hashtable) => #t/#f
Returns #t if the keys in hashtable are held by weak references, #f otherwise.
procedure: (hashtable/size hashtable) => number
Returns the number of key/value pairs stored in hashtable.
procedure: (hashtable->alist hashtable) => alist
Returns an association list comprising the elements of hashtable. The list contains pairs whose cars are they keys
found in hashtable and whose cdrs contain the associated values.

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9.1.4.2. Hash Functions
Several hash functions that return results consistent with common equivalence predicates are predefined.
procedure: (hash-by-eq value) => number
procedure: (hash-by-eqv value) => number
procedure: (hash-by-equal value) => number
procedure: (hash-by-string= string) => number
procedure: (hash-by-string-ci= string) => number
These procedures return a hash code of their argument that is consistent with eq?, eqv?, equal?, string=?,string-ci=?, respectively.

9.1.4.3. Access and Modification
All hash table access operations follow a similar pattern. They return the value that was associated withthe the given key at the time the operation was invoked. If no binding for the key existed, an optionallysupplied value is returned that defaults to #f. This allows the programmer to associate keys with #f values
and distinguish this case from not having any association for a key.
procedure: (hashtable/put! hashtable key val [nobinding]) => value
Associates key with val in hashtable. Returns the previous association of key or nobinding , which defaults to#f, if key has no previous association.
procedure: (hashtable/get hashtable key [nobinding]) => value
Returns the value associated with key in hashtable, or nobinding , which defaults to #f, if key has no
association.
procedure: (hashtable/get! hashtable key thunk [unsafe?]) => value
Returns the value associated with key in hashtable. If key has no association then thunk is called and the result
is associated with key in hashtable and also returned. The unsafe?, which defaults to #t, indicates whether
thunk may invoke escaping continuations or raise errors. Setting unsafe? to #f results in more efficient execution
but may cause deadlocks if thunk is unsafe. See also mutex/synchronize-unsafe in Section 6.5.3.
When hashtable is thread-safe this operation is atomic.
procedure: (hashtable/contains? hashtable key) => #t/#f
Returns the #t if hashtable contains an entry for key , #f otherwise.
procedure: (hashtable/remove! hashtable key [nobinding]) => value
Removes the association of key in hashtable. Returns the associated value of key or nobinding , which defaults
to #f, if key has no association.

9.1.4.4. Bulk Operations
Bulk operations are operations that apply to all elements of a hash table.
procedure: (hashtable/clear! hashtable) => void111

Chapter 9. Additional Libraries
Removes all elements from hashtable.
procedure: (hashtable/keys hashtable) => list
Returns the keys contained in hashtable.
procedure: (hashtable/for-each proc hashtable) => void
Applies proc to each element of hashtable. proc is called with two parameters - the key and the value of theelement.
procedure: (hashtable/map proc hashtable) => list
Applies proc to each element of hashtable. proc is called with two parameters - the key and the value of theelement. The results of calling proc are returned as a list.

9.1.5. Binary Buffers
Requires: (import buffers)
Binary buffers provide an opaque container for a fixed amount of binary data. The binary buffer libraryprovides a number of functions for creating and accessing those buffers. The buffer is very similar to avector, in that it is a randomly accessable, zero-based structure. But as a tradeoff for space efficiency,
binary buffers are only capable of storing bytes. The bytes are stored as 8-bit, unsigned fixed integers (ofthe range 0-255).
procedure: (buffer? value) => #t/#f
Returns true if and only if the provided argument is a binary buffer.
procedure: (make-buffer size [fill-value]) => buffer
Creates a new buffer capable of storing size bytes. size must be a fixed non-negative integer. If provided, the valueof all bytes in the buffer is initialized to fill-value. If not provided, the contents of the buffer is unspecified.
procedure: (buffer [value] ...) => buffer
Creates a new buffer whose size is equal to the number of arguments given and whose contents are the bytes given asarguments.
procedure: (buffer-length buffer) => fixed integer
Returns the capacity of the given buffer.
procedure: (buffer-ref buffer index) => fixed integer
Returns the byte at offset index in the specified buffer. It is an error if index is out of range.
procedure: (buffer-set! buffer index new-value) => undefined
Sets the byte at offset index of the specified buffer to the given fixed integer new-value. It is an error if index isout of range.
procedure: (buffer-copy! source-buffer source-offset dest-buffer dest-offset [count]) =>undefined
Copies count bytes starting from index source-offset in the source buffer to successive bytes starting at index
dest-offset in the destination buffer. If count is unspecified, it is assumed to be the length of the source buffer. It

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is an error to copy more bytes from the source buffer than are available, or to copy more bytes into the destinationbuffer than its capacity allows.

Buffers are serializable (can exist in loadable libraries or a SISC heap), but are not representable in ans-expression. For this reason, they bear the printed representation of #<buffer>.

9.1.6. Procedure Properties
Requires: (import procedure-properties)
SISC allows key/value bindings to be associated with procedures. This has a number of applications. For
instance, generic procedures store their methods in a procedure property.
Keys must be symbols. Values are any valid Scheme value. All operations are thread-safe.
procedure: (procedure-property proc symbol [nobinding]) => value
Returns the value associated with the property symbol of procedure proc, or nobinding , which defaults to #f, if
the property is not set.
procedure: (set-procedure-property! proc symbol val [nobinding]) => value
Sets the property symbol of procedure proc to the value val. Returns the previous value of the property or
nobinding , which defaults to #f, if the property was unset.
procedure: (procedure-property! proc symbol thunk [unsafe?]) => value
Returns the value associated with the property symbol of procedure proc. If the property is unset then thunk iscalled and the property is set to the result, which is also returned. The unsafe?, which defaults to #t, indicates
whether thunk may invoke escaping continuations or raise errors. Setting unsafe? to #f results in more efficient
execution but may cause deadlocks if thunk is unsafe. See also mutex/synchronize-unsafe in Section 6.5.3.

9.1.7. Loadable Scheme Libraries
Requires: (import libraries) 1
Scheme code can be packaged into libraries that can have dependencies on other libraries and can beloaded as required. Libraries are identified by a name that follows Java package file naming conventions,i.e. using path-style names typically containing domain, organisation name, project name and libraryname. For instance, if company Foo produces a library Baz for project Bar and that library contains three
files, the file structure might look as follows:
com/foo/bar/baz.scm
com/foo/bar/baz/baz1.scm
com/foo/bar/baz/baz2.scm
com/foo/bar/baz/baz3.scm

This library can be made accessible from SISC by adding the base directory or a jar file containing thesefiles to the Java class path. Libraries are loaded by the following procedure.
procedure: (require-library name) => undefined
Checks whether the library identified by name (a string), has already been loaded and, if not, loads it. An error israised if the library cannot be found.

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Libraries are loaded using the load procedure from a resource located by the find-resource procedure. The
name of the resource is derived from the name of the library by appending ".scc", ".sce" and, if that does not
succeed, ".scm".

Note that require-library only loads a single file. The definition of dependencies on other libraries
and the loading of other files therefore needs to happen within that file. For instance, the filecom/foo/bar/baz.scm from the above example might contain the following:(require-library ’com/foo/bar/boo)(load "baz/baz1.scm")(load "baz/baz2.scm")(load "baz/baz3.scm")

It is possible to programmatically check whether a particular library exists and whether it has been
loaded:procedure: (library-exists? name) => #t/#f
Returns #t if the library identified by name (a string) exists, #f otherwise.
procedure: (library-loaded? name) => #t/#f
Returns #t if the library identified by name (a string) has been loaded, #f otherwise.

9.1.8. Operating System Interface
Requires: (import os)
The operating system interface currently contains functions for spawning external processes on the hostoperating system, obtaining input/output ports to the resulting process, and monitoring their status.
Two procedures exist for spawning processes:
procedure: (spawn-process program/commandline [arglist]) => process
Spawns a process, returning a process handle. If the optional argument list is provided, then the first argument is thebinary to run with those arguments. If omitted, the first argument is tokenized as a commandline and used to spawn
the process.
procedure: (spawn-process-with-environment program arglist environment[working-directory]) => processprocedure: (spawn-process/env program arglist environment [working-directory]) => process
Spawns a process named by program with the arguments given in arglist, in the given environment. The
environment is an association list of strings to strings. The key in the association list is an environment variable
name, and the corresponding value is the value to assign to that environment variable. If the environment parameteris #f, the environment variables of the current SISC instance are used.

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The optional parameter working-directory specifies the directory which will be set as the current directorywhen the process is spawned. If ommited, the value of the current-directory parameter (i.e. the current
directory of the running Scheme program) is used instead.
procedure: (process? value) => #t/#f
Returns #t if the given value is a process handle.

Once started, a process will run in parallel to the current Scheme program according to the usual
scheduling of the host platform. The process handle obtained can be used to obtain the input, output, anderror streams of the process using the following functions:procedure: (get-process-stdout process) => binary-input-port
Returns a binary input port which will read bytes which the given process has written to its standard output stream.
procedure: (get-process-stderr process) => binary-input-port
Returns a binary input port which will read bytes which the given process has written to its standard error stream.
procedure: (get-process-stdin process) => binary-output-port
Returns a binary output port which when written to will send bytes to the given process’ standard input stream.

Finally, functions are provided to check the status of a spawned process, and to wait for a process tocomplete:
procedure: (process-terminated? process) => integer or #f
Checks to see if the given process has terminated, and returns the process’ return code if so. If the process is stillrunning, #f is returned.
procedure: (wait-for-process process) => integer or #f
Waits for the given process to terminate, and returns the process’ return code if so. The wait operation may beinterrupted by other code, in which #f is returned.

9.2. Third-Party Libraries
SISC provides hooks for accessing a number of third-party Scheme libraries.

Warning
This functionality has not undergone much testing.

9.2.1. SRFIs
The Scheme Requests For Implementation (SRFI) process aims to coordinate libraries and otheradditions to the Scheme language between different Scheme implementations. For details see
http://srfi.schemers.org/ which describes the process and contains a list of all available SRFIs.

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9.2.1.1. SRFI Modules
In SISC each SRFI is encapsulated in a module. See Chapter 10 for details of SISC’s module system. The
definitions for SRFI modules are not included in the standard SISC heap build and hence must be loaded
separately from various compiled library files in the sisc-lib.jar jar file in the root directory of the
SISC binary distribution. As long as this jar file is on the classpath, which is the case by default, any
SRFI’s module definition may be loaded with the expression (require-library’sisc/libs/srfi/srfi-n), where n is the SRFI’s number. For example:(require-library ’sisc/libs/srfi/srfi-9)

All SRFI’s may be loaded at once by requiring sisc/libs/srfi.

9.2.1.2. Using SRFIs
SISC currently supports SRFIs 0, 1, 2, 5, 6, 7, 8, 9, 11, 13, 14, 16, 18, 19, 22, 23, 25, 26, 27, 28, 29, 30,
31, 34, 35, 37, 38, 39, 40, 42, 43, 45, 48, 51, 54, 55, 59, 60, 61, 62, 66, 67, 69 and 78. Once the SRFI
module definitions have been loaded as described above, a SRFI n can be imported using(import srfi-n)e.g.
(import srfi-1)(xcons 1 2) ;=> (2 . 1)

SRFI modules, like all modules in SISC, can be imported/used by other modules. Doing so does notpollute the top-level environment with the definitions exported by the module, i.e. any code outside theimporting module remains unaffected.
If, however, an SRFI is to be imported into the top-level, one can use the require-extension
mechanism (see Section 10.2.1).

9.2.1.3. SRFI Extensions
Some SRFIs have built-in extension points that Scheme implementations can use to augment a SRFI’s
functionality. It is also the case that some SRFIs would benefit from slightly extended APIs.
This section documents the SRFI extensions implemented by SISC.
9.2.1.3.1. SRFI 69 (Basic hash tables)
The make-hash-table function takes two additional optional arguments: thread-safe? and weak?.
See make-hashtable in Section 9.1.4.1 for details.
The basic hash table API is extended in a separate module:
Requires: (import srfi-69-ext)

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procedure: (hash-table-thread-safe? hashtable) => #t/#f
Returns #t if hashtable is thread safe, #f otherwise.
procedure: (hash-table-weak? hashtable) => #t/#f
Returns #t if the keys in hashtable are held by weak references, #f otherwise.
procedure: (hash-table-ref! hashtable key thunk) => value
Returns the value associated with key in hashtable. If key has no association then thunk is called and the result
is associated with key in hashtable and also returned.
When hashtable is thread-safe this operation is atomic.
procedure: (hash-table-ref! hashtable key default) => value
Returns the value associated with key in hashtable. If key has no association then default is associated with
key in hashtable and also returned.
When hashtable is thread-safe this operation is atomic.

9.2.2. SLIBThe SLIB portable scheme library provides compatibility and utility functions for standard Scheme
implementations. It is supported by many Schemes, including SISC.

9.2.2.1. Downloading and Installation
The latest version of SLIB is available from http://swissnet.ai.mit.edu/~jaffer/SLIB.html as both a zip fileand RPM. The site also hosts an online version of the SLIB manual.
Download SLIB and install it in a convenient location. The RPM will by default be installed in/usr/share/slib/. Do not worry when you see some errors about missing programs such asmzscheme and scheme48 when installing the RPM - these happen because SLIB tries to auto-configure
itself for various Schemes that you may not have installed on your system.

9.2.2.2. Environment
Using SLIB in SISC requires two Java system properties to be set:•sisc.home. This should (but does not actually have to) point to the location where you have installed
SISC. If you are using one of the scripts from the binary SISC distribution in order to run SISC thenthis property will automatically be set to the value of the SISC_HOME environment variable.

•

sisc.slib. This must point to the location where you installed SLIB. Other Schemes supporting SLIB
tend to use an environment variable SCHEME_LIBRARY_PATH, so it is advisable to define that (if it
is not already defined) and run Java with a -Dsisc.slib=... option based on the environment
variable. If you are using the scripts from the binary $SISC; distribution in order to run SISC then you
can set the property by adding the -Dsisc.slib=... to the JAVAOPT environment variable.
Note: Note that the value of this property should be a fully qualified url, e.g.
file:///usr/share/slib

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You need to ensure that all potential users of SLIB have read permissions to files in the directories
referred to by the above system properties.

9.2.2.3. Building the Catalog
Make sure that the above system properties are set and that you have write permissions to the sisc.home
directory; often this means you need to be logged in as a privileged user.
Start SISC as you normally would. At the prompt type(require-library ’sisc/libs/slib)(require ’new-catalog)(exit)

The above should create a file slibcat in the sisc.home directory. It is a good idea to check that this has
indeed happened.

9.2.2.4. Using SLIB
Make sure the above system properties are set. Start SISC as you normally would. At the prompt load the
SISC SLIB as described above, i.e.
(require-library ’sisc/libs/slib)

You can now load SLIB modules using require, e.g.
(require ’tsort)(tsort ’((shirt tie belt)(tie jacket)(belt jacket)(watch)(pants shoes belt)(undershorts pants shoes)(socks shoes))eq?)loads the topological sorting module and invokes one of the procedures defined by it.
Please refer to the SLIB manual for further details of what modules are available. Note however that, aswith most other Schemes supported by SLIB, there will be some modules that are not available or do notwork in SISC.

9.3. Creating Libraries
Requires: (import libraries)

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SISC allows the creation of compiled libraries which contain compiled scheme code. These libraries canthen be executed into a running SISC session in order to extend the functionality without processing or
possessing the original source. Such libraries can be loaded using load as would any ordinary Scheme
source file.
Compiled code files (.scc) are created using the compile-file function, which takes a Scheme source
file and a target output file, and processes the Scheme source through the various expansion andcompilation phases, and then serializes the resulting SISC microexpressions to the given target file. The
resulting file may then be loaded with load as any ordinary Scheme file would, or can be placed in thelibrary path and resolved using require-library.
procedure: (compile-file source-file target-file) => undefinedCompiles the Scheme source present in source-file, writing the resulting micro-expressions into target-file,
suitable for loading.
As a side effect, the micro-expressions are also evaluated, i.e. in effect compile-file compiles and evaluates thesource-file. The latter is necessary because the compilation of an expression may depend on the results ofevaluating a previous expression, e.g. as is typically the case for libraries that depend on other libraries.
.sll Deprecation: Scheme Loadable Libraries (.sll files) were deprecated in version 1.9. This wasdue to unresolvable incompatibilities in the engine’s closure representation and the .sll functionality.

Notes
1. This module gets imported by default.

119Chapter 10. Modules and Libraries
10.1. Modules
Modules provide an additional level of scoping control, allowing symbolic and syntactic bindings to bebundled in a named or anonymous package. The package can then be imported into any scope, making
the bindings contained in the module visible in only that scope.
SISC’s modules are provided by the portable syntax-case macro expander by R. Kent Dybvig and Oscar
Waddell. A comprehensive explanation of the provided module system is best found in the Chez Scheme
Users Guide (http://www.scheme.com/csug.html), specifically Section 9.3, Modules(http://www.scheme.com/csug/syntax.html#g2187). What follows is an informal introduction to thatmodule system.

10.1.1. Overview
The basic unit of modularization in SISC is a module. A typical module definition has this appearance:(module foo(bar baz)(import boo1)(import boo2)(include "file.scm")(define (bar x) ...)
(define-syntax baz ...)
(define (something-else ...) ...)
(do-something)(do-something-else))

A module definition consists of a name (foo), a list of exports (bar and baz) and a body. Expressions
which can appear in the body of a module are the same as those which can appear in a lambda body. The
import form imports bindings from a named module (in this case boo1 and boo2) into the current
lexical scope. The include form performs a textual inclusion of the source code found in the named file
(file.scm). In other words, it works as if the contents of the file had appeared literally in place of the
include statement.
All identifiers appearing in the export list must be defined or define-syntaxed in the body of themodule, or imported from another module.

10.1.2. Style
It is recommended to clearly separate modularization from actual code. The best way to accomplish thisis to•

List all imports in the module body rather than in included files•

Include all files directly from the module body, avoiding nested includes

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Place all definitions and expressions in included files, avoiding them in the module body

There are several reasons for this. First, it makes refactoring easier, as one can move relevant code frommodule to module merely by rewriting the module definitions, leaving the implementation codeunchanged. Second, it makes debugging easier, as one can load the implementation code directly into the
Scheme system to have access to all bindings, or load the module definition to view the finished,encapsulated exports. Finally, it stylistically separates interface (the modules) from implementation (theincluded Scheme source).

10.1.3. Modularizing Existing CodeSince module bodies are treated like the bodies of lambdas, the R5RS rules of how internal definitions
are treated apply to all the definitions in the module body (both ordinary and syntax), including all code
included from files. This is often a source of errors when moving code from the top-level into a module
because:•All definitions must appear before all expressions,•

The list of definitions is translated into letrec/letrec-syntax, which means it must be possible toevaluate each right-hand side without assigning or referring to the value of any of the variables being
defined.

This often necessitates re-arranging the code and the introduction of set! expressions. Here is an
example of a sequence of top-level definitions/expressions and how they need to be rewritten so that they
may appear in a module body:(define (foo) 1)(define bar (foo))(do-some-stuff)(define (baz) (bar))==>(define (foo) 1)(define bar)(define (baz) (bar))(set! bar (foo))(do-some-stuff)

The general strategy is to go through the list of expressions/definitions from top to bottom and build twolists - one of definitions and one of expressions - as follows:•

If a non-definition is encountered, append it to the expression list•

If a "naked" definition (i.e. a definition whose right-hand side is not a function) that refers to a binding
defined within the module is encountered, append an empty definition to the definition list and append
a set! with the right-hand side expression to the expression list•

Otherwise, i.e. for an ordinary definition, append it to the definition list

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The concatenation of the resulting definition list with the expression list makes a suitable module body.

10.1.4. Evaluation
Modules are lexically scoped. It is possible to define modules inside lambdas and inside other modules
and to export modules from modules. Example:(define (f c)(module foo(bar)(module bar(baz)(define (baz x y) (- x y))(display "defining baz\n")))(if (> c 0)(let ((a 1))(import foo)(let loop ((b c))(import bar)(if (> b 0) (loop (baz b a)) (f (- c 1)))))))The expressions in a module body get executed at the time and in the context of module definition. So, in
the above example, the body of bar containing the display statement is executed once for every call to frather than once for every iteration of the inner loop containing the import of the bar module.
There are quite a few more things you can do with modules. For instance one can define anonymous
modules, which are a short cut for defining a named module and then importing it, import selectedbindings from a module and renaming them rather then importing all bindings as is etc etc. For moredetails again refer to the Chez Scheme user manual.

10.2. Libraries
Libraries provide a means of encapsulating code that can be shared by many, independently developed
applications.
Libraries are simply bundles of Scheme code, usually precompiled, which are packaged so that they may
be resolved relative to a library path. Libraries are typically compiled using the meachanism from
Section 9.3. Loading the resulting library makes the library available to the loading code. To create acompiled library from a module, compile a source file which contains any necessary require-librarycalls, followed by the module definition. When loaded, this will cause the necessary libraries to beloaded, and then define the module into the environment. For example, the source file may resemble:(require-library ’sisc/libs/srfi/srfi-1)(require-library ’com/foo/lib2)(module lib3
(a-function)(import srfi-1)(import com/foo/lib2)

122Chapter 10. Modules and Libraries(define (a-function)(do-something (another-function)))(define (another-function)(something-else))

Libraries should not depend on any top-level definitions outside the standard SISC top-level, except thedefinition of other library modules. Otherwise it is not possible to use the libraries portably.
Libraries can be packaged with supporting code (e.g. ordinary Java code and native modules) and otherlibraries into jar files. A typical structure for such a jar file would becom/foo/lib1.scc
com/foo/lib1/Class1.classcom/foo/lib1/Class2.classcom/foo/lib2.scc
com/foo/lib2/Class1.classcom/foo/lib2/Class2.classcom/foo/lib3.scc
com/foo/lib3/Class1.classcom/foo/lib3/Class2.class

It is usually a good idea to name a module after the path names in the jar, for example
com/foo/lib{1,2,3} in the above example.

10.2.1. require-extension
SISC supports SRFI-55 (http://srfi.schemers.org/srfi-55/srfi-55.html) for loading libraries and extensions
as well. SRFI-55 provides require-extension, which in SISC simultaneously loads a library, thenimports its module definition into the current interaction environment. This may be more convenient thanthe combination of require-library and import, when one is loading dependent top-level libraries
for a program. It is less flexible, though, since you cannot import into a lexical scope.
SRFI-55 is supported in the initial SISC environment, no require-library or import is needed touse require-extension.
At the time of this writing, SISC supports two extension identifier schemes, the srfi scheme as requiredby SRFI-55 itself, and a SISC specific lib scheme for loading a SISC library. Some examples:
Example 10-1. Loading and importing with require-extension; Load and import SRFI 1(require-extension (srfi 1)); Load and import SISC library com/foo/lib1(require-extension (lib com/foo/lib1)); Load and import SRFIs 13 and 14,; and SISC libraries com/foo/lib2 and com/foo/lib3(require-extension (srfi 13 14) (lib com/foo/lib2 com/foo/lib3))

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SISC modules loaded using the lib extension scheme must use the full path and file as the module
name. For example, in the above example, com/foo/lib1’s module name must be com/foo/lib1.

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Chapter 11. Extensibility
Occasionally functionality may be desired that is not easily accomplished at the Scheme level. A new
first-class type may be desired, or efficient access to a Java library. SISC provides a simple API for suchmodifications.

11.1. Adding Types
A Scheme value is represented in SISC as a subclass of the abstract Java class sisc.data.Value.

11.1.1. External Representation of Values
In order to be able to display the value in the Scheme system, all Values must implement the display
method:public void display(sisc.io.ValueWriter writer);Uses the various output methods of sisc.io.ValueWriter to construct an external representation of the given Valuesuitable for output from the display Scheme function.

If the programmer desires the Value to have a different representation when written with write, thewrite method must be overridden. If it is not, the output of display is used for write as well.
public void write(sisc.io.ValueWriter writer);Uses the various output methods of sisc.io.ValueWriter to construct an external representation of the given Valuesuitable for output from the write Scheme function. If not implemented, the output constructed the by the display
method is used as output from write.

Finally, if the external representation of a new Value is likely to be long, the programmer should
implement the synopsis method, which generates a summary representation of the value.
public String synopsis(int limit);
Returns approximately limit characters from the printable representation of this value as if returned by write.
This method is used for displaying the value in error messages where the entire representation may be superfluous.

The sisc.io.ValueWriter type that is passed as an argument to both display and write contains a
number of methods to generate the representation. First there are several methods for appending Java
Strings, characters, and SISC Values. In each, the called ValueWriter is returned, to allow for easy
chaining of calls.
public sisc.io.ValueWriter append(char c);
Appends a single character to the external representation.
public sisc.io.ValueWriter append(String s);
Appends the contents of a Java String to the external representation.
public sisc.io.ValueWriter append(sisc.data.Value v);
Appends the contents of a Scheme value to the external representation. The value is converted to an external
representation using the print style with which the ValueWriter was constructed (for example, display or write).

125Chapter 11. Extensibility
In addition to the above append methods, the programmer may wish to force display or write rather
than use the same method as the ValueWriter. To do this, one can call the display or write methods
on the ValueWriter.
public void display(sisc.data.Value v);
Appends the external representation of the given value as returned by display.
public void write(sisc.data.Value v);
Appends the external representation of the given value as returned by write.

11.1.2. Equality
If the one wants a Value to be comparable for any more than pointer equality, or for the concept ofpointer equality to be less strict than actual pointer equality, one or more of the equality methods must be
overridden.
public boolean eq(Object other);
Returns true if another provided Java object is equal in the sense of eq? to this Value.
public boolean valueEqual(Value other);
Returns true if another provided Scheme value is equal in the sense of equal? to this Value.

11.1.3. Serializable Values
If the type that is being added will be serialized in a SISC heap, and it contains one or more member
variables, the Value must include a default constructor (a constructor with no arguments), and implement
the deserialize, serialize, and visit methods described in Section 11.3.

11.2. Adding Native Bindings
One can add native bindings to the Scheme environment by implementing a subclass of the abstract classsisc.nativefun.NativeLibrary. Such a subclass needs to implement four methods:
public String getLibraryName();Returns the name of this library. The name should also be acceptable for use in filenames.
public float getLibraryVersion();Returns the version of this library.
public sisc.data.Symbol[] getLibraryBindingNames();
Returns an array of the names of the bindings exported by this library. Each name is a Scheme symbol.
public Value getBindingValue(sisc.data.Symbol name);
Returns the value of a given binding exported by this library.

11.2.1. Native Procedures
It is possible to implement Scheme functions whose behavior is implemented natively in Java code.
Many of SISC’s procedures are implemented this way. Native procedures extend the126Chapter 11. Extensibility
sisc.nativefun.NativeProcedure abstract class. Working NativeProcedure subclasses must
implement the doApply method, as described below.
public Value doApply(sisc.interpreter.Interpreter interp);
Perform the necessary computations of the NativeProcedure, returning a Value as the result of the procedure call.
The arguments to the procedure can be found in the value rib array field, vlr, of the Interpreter passed as anargument. The number of arguments to the procedure can be found from the length of the array (vlr.length).

If the native procedure wishes to raise an error, it may do so by throwing any Java runtime exception(subclass of java.lang.Runtime). For a more descriptive error, one may raise a SISC error using anyof a number of error forms in sisc.util.Util. Consult the source of Util or inquire on thesisc-devel mailinglist for assistance.

11.2.2. Fixable Native Procedures
Often it is unnecessary to have access to the full Interpreter context to implement a native procedure. If
the arguments to the procedure are sufficient and the procedure is purely functional (causes no side
effects), it is recommended that the programmer create a fixable native procedure. These native
procedures may be inlined into generated code when enabled, allowing much faster execution. In
addition, the fixable native procedure interface is simpler to use.
The FixableProcedure abstract class consists of five methods which may or may not be subclassed.
These five methods correspond to the case of calling the procedure with no, one, two, three, and morethan three arguments respectively. Not overriding one of these methods will cause a call to the fixable
procedure to throw the invalid number of arguments error to the caller.
public Value apply();
Perform the necessary computations of the FixableNativeProcedure, returning a Value as the result of the procedurecall. No argument variant.
public Value apply(Value v1);
Perform the necessary computations of the FixableNativeProcedure, returning a Value as the result of the procedurecall. One argument variant.
public Value apply(Value v1, Value v2);
Perform the necessary computations of the FixableNativeProcedure, returning a Value as the result of the procedurecall. Two argument variant.
public Value apply(Value v1, Value v2, Value v3);
Perform the necessary computations of the FixableNativeProcedure, returning a Value as the result of the procedurecall. Three argument variant.
public Value apply(Value[] v);
Perform the necessary computations of the FixableNativeProcedure, returning a Value as the result of the procedurecall. More than three argument variant.

127

Chapter 11. Extensibility

11.2.3. Indexed Native Libraries
In the most common case, a Library is created to define several bindings, including procedures whose
implementations are in Java code. For this common case, a skeleton subclass of NativeLibrary,sisc.nativefun.IndexedLibraryAdapter is provided. The IndexedLibraryAdapter class provides
implementations for all four required NativeLibrary methods, and introduces a new abstract method
which must be implemented, called construct. In addition, the method define is provided.
In an indexed native library, each binding is associated with a Java int unique to that binding within thelibrary. The IndexedLibraryAdapter subclass should in its constructor call define for each binding
provided by the library, according to the contract of the method:
public void define(String name, int id);
Register the native binding with the given name, and assign it the given library-unique id.

In implementing the getBindingValue method of the NativeLibrary class, anIndexedLibraryAdapter will call the abstract method construct required by the its subclasses:
public sisc.data.Value construct(int id);
Return an instance of the indexed binding.

Most frequently, the bindings created in an indexed library are native procedures. In such a case, asecond class is created which subclasses sisc.nativefun.IndexedProcedure. IndexedProcedure is
subclass of NativeProcedure. An IndexedProcedure subclass’ constructor must call thesuperconstructor with an int, the unique id for that binding. That int is stored in the id field ofIndexedProcedure. A subclass can then use the id instance variable to dispatch to many native
procedures in the body of the doApply method required by native procedures.
So, typically, an IndexedNativeLibrary subclass is created whose construct method creates instancesof IndexedProcedure subclasses. The IndexedNativeLibrary subclass its itself nested in the
IndexedProcedure class which it is constructing. See the various indexed libraries in sisc.modules forconcrete examples.

11.3. Serialization
SISC provides an API for serializing the state of a running Interpreter. The SISC heap is a dump of thestate of an Interpreter with the necessary code to implement R5RS Scheme, for example. In order tofacilitate this serialization, SISC Expressions and Values can implement helper methods to define theserialization of the object. If the Expression or Value contains no internal state that need be serialized,
the serialization methods may be ignored. If not, the Expression or Value must contain a default (noargument) constructor, and implement the following three methods:
public void serialize(sisc.ser.Serializer serializer) throws java.io.IOException;
Serializes the contents of the Expression to the given Serialization context.
public void deserialize(sisc.ser.Deserializer deserializer) throws java.io.IOException;
Sets the state of the Expression to the serialized data read from deserializer.
public boolean visit(sisc.util.ExpressionVisitor visitor);

128

Chapter 11. Extensibility
When called, the Expression should call visitor.visit(n) on any nested Expressions.

The Serializer and Deserializer objects implement Java’s java.io.DataOutput andjava.io.DataInput interfaces, respectively. This means that you can use any of the write/readfunctions in those interfaces to serialize the state of your Expression or Value. In addition, a number ofmethods are provided that are helpful for this domain.
The ExpressionVisitor passed to visit contains only one method, visit, which bears the same contract
as the visit above. When called, an Expression would then call the ExpressionVisitor’s visit method
once for each nested Expression. This method is used during serialization and during printing to detect
cycles in data and code structures.

11.3.1. Deserializer methods
public BigInteger readBigInteger() throws java.io.IOException;
Reads a BigInteger from the stream.
public BigDecimal readBigDecimal() throws java.io.IOException;
Reads a BigDecimal from the stream.
public Class readClass() throws java.io.IOException;
Reads a Java Class object from the stream.
public Expression readExpression() throws java.io.IOException;
Reads a SISC Expression from the stream.
public Expression readInitializedExpression() throws java.io.IOException;
Reads a SISC Expression from the stream, fully initialized. This method should only be used if fields internal to theExpression returned must be available during deserialization.
public sisc.data.Expression[] readExpressionArray() throws java.io.IOException;
Reads an array of Expressions from the stream.
public sisc.data.Value[] readValueArray() throws java.io.IOException;
Reads an array of Values from the stream.
public SymbolicEnvironment readSymbolicEnvironment() throws java.io.IOException;
Reads a SISC Symbolic Environment from the stream.

11.3.2. Serializer methods
public void writeBigInteger(BigInteger bigint) throws java.io.IOException;
Writes a BigInteger to the stream.
public void writeBigDecimal(BigDecimal bigdecim) throws java.io.IOException;
Writes a BigDecimal to the stream.

129

Chapter 11. Extensibility
public void writeClass(Class clazz) throws java.io.IOException;
Writes a Java Class object to the stream.
public void writeExpression(Expression expr) throws java.io.IOException;
Writes a SISC Expression to the stream.
public void writeInitializedExpression(Expression expr) throws java.io.IOException;
Writes a SISC Expression to the stream. This method should only be used if fields internal to the Expression returned
must be available during deserialization.
public void writeExpressionArray(sisc.data.Expression[] ary) throws java.io.IOException;
Writes an array of Expressions to the stream.
public void writeValueArray(sisc.data.Value[] ary) throws java.io.IOException;
Writes an array of Values to the stream.
public void writeSymbolicEnvironment(SymbolicEnvironment e) throws java.io.IOException;
Writes a SISC Symbolic Environment to the stream.

130

Appendix A. Errata
This appendix describes where this manual and the implementation of SISC depart. This section should
ideally remain small or empty, as it is the goal of the system to conform to this document, not for thisdocument to describe the idiosyncrasies of the system.

131

Appendix B. R5RS Liberties and Violations
This section lists all ways in which SISC interprets the R5RS specification, where the standard is notparticular clear. Such interpretations may allow non-portable code to be written and executed on SISC.
Additionally, all known R5RS violations are listed. Actual violations are considered SISC bugs, and havea high priority for being fixed. Violations of the standard are written in bold text.
1. 2.3 - SISC allows identifiers to start with ’+’, ’-’, or ’.’ if they cannot be read as numbers.
2. 2.3 - SISC uses the reserved characters ’[’ and ’]’ as synonyms for ’(’ and ’)’ respectively.
3. 2.3 - SISC does not raise any warning or error when encountering the reserved characters "[]{}|",
and allows "{}|" to appear in identifiers.
4. 4.1.3 - SISC does not distinguish between () and the quoted empty list ’().
5. 6.2.3 - The standard desires that that operations such as sqrt try to provide exact results when given
exact arguments. While SISC meets the requirement for sqrt, it is not clear what other mathematical
functions should have this behavior. SISC takes no heroic efforts to meet this requirement.
6. 6.2.6 - SISC allows radixes other than those specified in the contract for number->string andstring->number. In particular, any radix up to 36 is allowed, and any unsupported value causes
SISC to revert to base 10.
7. 6.5 - SISC currently returns an environment from the scheme-report-environment whichcontains four bindings not specified in R5RS, bindings which are needed by R5RS syntactic
keywords.
8. 6.6.2 - SISC currently only warns when end-of-file is reached in read, rather than signaling anerror. The unterminated datum is discarded.

All of the liberties described above are implemented for the convenience of the programmer. If desired,strict R5RS syntax and semantics may be enabled with the strictR5RSCompliance configuration
parameter (see Section 2.4.2), causing SISC to raise errors in all R5RS situations that result in "an error",as well as respect the lexical syntax’s reserved characters. When in strict compliance mode, the above
mentioned deviations no longer apply and SISC is entirely R5RS compliant.

132

Appendix C. TroubleshootingThis appendix covers issues with running SISC in certain environments.

C.1. Kaffe
There is a known limitation with Kaffe, whose default stack size is too small for SISC to parses-expressions. To fix this, either edit the SISC startup scripts to pass -Xss32m, or set the JAVAOPT
environment variable to the same.

133

Appendix D. Backend Details
Note: This appendix is under development

This appendix describes details of SISC on particular backends. This is not intended to guideprogramming. The programmer should code according to the main body of this document. However, thissection still describes useful performance tips and limitations of SISC’s operation.

D.1. Limits
This appendix lays out the various limits in SISC running on a JVM backend. These limits are notspecifications for an expected set of limits on all platforms, but serve as a real-world guide.

D.1.1. Datastructure Limits
Table D-1. SISC Limits
Description

Limit

Fixed-point Exact Integers-231 < n < +231-1

AP Exact Integers-2(2 -1) < x < +2(2 -1)-1.

Inexacts (32Float)

See IEEE 754-1985 Floating Point Standard

Inexacts (64Float)

See IEEE 754-1985 Floating Point Standard

Inexact Mantissa (APFloat)same as AP Exact Integers

Inexact Exponent (APFloat)same as fixed-point exact integer

Max vector elements

Same as max fixed-point integer

Max string elements

Same as max fixed-point integer (?)

Representable characters

see Section 3.1.2

Maximum formal parameters

Same as max fixed-point integer

Maximum lexical depth

Same as max fixed-point integer

Maximum symbolic-environment bindings

Same as max fixed-point integer (?)

Addressable file size

min of 264-1 and operating system limit

32

32

Arbitrary precision integers aren’t quite arbitrary precision. SISC has a hard limit to the number of bits inan exact integer and thus to the range of representable numbers. Exact integers are stored as two’s
complement signed integers, with a bit limit (including the sign bit) of 232. This limits the range ofrepresentable exact integers to the numbers quoted above.
Likewise, arbitrary precision inexact numbers (when present) have a similar hard limit. The arbitrary
precision inexact is constructed with an arbitrary precision exact number with the limits described above
as the number’s mantissa, and an exponent whose range is equivalent to that of a fixed-point exact
integer. The inexact is then then mantissa*10exponent.

134Appendix D. Backend Details

D.1.2. Symbol UniquenessIn order to support compilation in multiple threads, on multiple machines, or in multiple times, generated
symbols for module bindings, lexical variables, etc. must be 4d-unique, that is they must be uniqueacross space and time. SISC attempts to balance this requirement with the space inefficiency ofgenerating symbols with very long names.
SISC’s unique symbols are generated by creating a number of the form current-time +(random-16-bit-natural*311040000000) + (counter*155520000000). The current time
variable is only updated when the value of counter reaches 65536. In this manner, two entities thatgenerate the same symbol will only generate a colliding symbol if they generate the symbol on the same
system millisecond, with the same counter value, and the same random number, or, if one entity happens
to generate the symbol with the same millisecond and does so (counter-1)*50 years, (counter-2)*100
years... in the future, and with the same random or (random-1)*100 years, (random-2)*200 years, etc in
the future. Only if all of these factors align will a colliding symbol be generated. This is not as unlikely
as say a Microsoft GUID or Java VMID number, but it should be sufficiently unlikely. The advantage
over other GUID algorithms is that the value produced by SISC’s is significantly smaller (and thus does
not bloat expanded code).

D.2. Performance and Efficiency considerations
D.2.1. MathThe SISC numeric library is most efficient when operating on fixed bitlength numbers. Exact numbers
are in their fixed bitlength mode if they are in the representable range for fixed exact integers, as
described in Table D-1. Fixed bitlength inexact numbers are only available in the 64Float and 32Float
libraries. For SISC on Java on the x86 32-bit architecture, the 64Float library is generally more efficientthan the 32Float library, while both are more efficient than the APFloat library.
Fixed bitlength exact integers are only used for whole numbers. Rational numbers use arbitrary precisioncomponents and thus are less efficient than whole fixed integers.
Arbitrary precision inexact numbers are progressively slower as the bitlength of the mantissa and thescale of the exponent increase. Using the precision constraints can prevent an unbounded increase in thescale of arbitrary precision inexacts which will very rapidly slow calculations.

D.2.2. Strings
At the time of this writing, the Scheme string type can be represented either as a character array, a native
string, or simultaneously as both. The character array representation allows efficient, constant time
modification of a mutable string (using string-set! for example). The native string representation
allows efficient output to ports, string comparison, and substring operations.
By default, SISC allows the Scheme string to contain both representations simultaneously, ensuring that
there is not a costly representation conversion necessary to perform certain operations. However, in thisdefault mode, strings may occupy twice the memory as a string in a single representation. If a program

135Appendix D. Backend Details
uses many strings or several very large strings, the programmer may wish to create strings that may only
be in one representation at any given time. SISC provides a parameter to control this behavior.
parameter: (compact-string-rep [boolean]) => #t/#f
This parameter, if set #t, will force strings to be represented either as a character array, or as a native string, butnever both. If false, simultaneous representations are possible.

D.2.3. Interrupts
Interrupts allow running Scheme code to be forcibly broken from another thread, causing the Scheme
code to raise an error. The interrupt signal handling code does add an appreciable overhead (usually
between 1-5% depending on the JVM) to execution. It can disabled using the sisc.permitInterrupts
system property.

136

Appendix E. GNU General Public License
E.1. Preamble
The licenses for most software are designed to take away your freedom to share and change it. By
contrast, the GNU General Public License is intended to guarantee your freedom to share and change
free software - to make sure the software is free for all its users. This General Public License applies tomost of the Free Software Foundation’s software and to any other program whose authors commit tousing it. (Some other Free Software Foundation software is covered by the GNU Library General Public
License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses aredesigned to make sure that you have the freedom to distribute copies of free software (and charge for thisservice if you wish), that you receive source code or can get it if you want it, that you can change thesoftware or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask
you to surrender the rights. These restrictions translate to certain responsibilities for you if you distributecopies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give therecipients all the rights that you have. You must make sure that they, too, receive or can get the sourcecode. And you must show them these terms so they know their rights.
We protect your rights with two steps:
1. copyright the software, and
2. offer you this license which gives you legal permission to copy, distribute and/or modify thesoftware.

Also, for each author’s protection and ours, we want to make certain that everyone understands that there
is no warranty for this free software. If the software is modified by someone else and passed on, we want
its recipients to know that what they have is not the original, so that any problems introduced by others
will not reflect on the original authors’ reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that
redistributors of a free program will individually obtain patent licenses, in effect making the program
proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone’s freeuse or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.

E.2. TERMS AND CONDITIONS FOR COPYING,

137

Appendix E. GNU General Public License

DISTRIBUTION AND MODIFICATION
E.2.1. Section 0This License applies to any program or other work which contains a notice placed by the copyrightholder saying it may be distributed under the terms of this General Public License. The "Program",
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Activities other than copying, distribution and modification are not covered by this License; they areoutside its scope. The act of running the Program is not restricted, and the output from the Program iscovered only if its contents constitute a work based on the Program (independent of having been madeby running the Program). Whether that is true depends on what the Program does.

E.2.2. Section 1
You may copy and distribute verbatim copies of the Program’s source code as you receive it, in any
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You may modify your copy or copies of the Program or any portion of it, thus forming a work based on
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2. You must cause any work that you distribute or publish, that in whole or in part contains or isderived from the Program or any part thereof, to be licensed as a whole at no charge to all third
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3. If the modified program normally reads commands interactively when run, you must cause it, whenstarted running for such interactive use in the most ordinary way, to print or display anannouncement including an appropriate copyright notice and a notice that there is no warranty (orelse, saying that you provide a warranty) and that users may redistribute the program under these
conditions, and telling the user how to view a copy of this License.
Exception:: If the Program itself is interactive but does not normally print such an announcement,
your work based on the Program is not required to print an announcement.)138

Appendix E. GNU General Public License

These requirements apply to the modified work as a whole. If identifiable sections of that work are notderived from the Program, and can be reasonably considered independent and separate works inthemselves, then this License, and its terms, do not apply to those sections when you distribute them as
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Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely byyou; rather, the intent is to exercise the right to control the distribution of derivative or collective works
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In addition, mere aggregation of another work not based on the Program with the Program (or with awork based on the Program) on a volume of a storage or distribution medium does not bring the otherwork under the scope of this License.

E.2.4. Section 3
You may copy and distribute the Program (or a work based on it, under Section 2 in object code or
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3. Accompany it with the information you received as to the offer to distribute corresponding sourcecode. (This alternative is allowed only for noncommercial distribution and only if you received theprogram in object code or executable form with such an offer, in accord with Subsection b above.)

The source code for a work means the preferred form of the work for making modifications to it. For an
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associated interface definition files, plus the scripts used to control compilation and installation of theexecutable. However, as a special exception, the source code distributed need not include anything that isnormally distributed (in either source or binary form) with the major components (compiler, kernel, andso on) of the operating system on which the executable runs, unless that component itself accompanies
the executable.
If distribution of executable or object code is made by offering access to copy from a designated place,then offering equivalent access to copy the source code from the same place counts as distribution of thesource code, even though third parties are not compelled to copy the source along with the object code.

139Appendix E. GNU General Public License

E.2.5. Section 4
You may not copy, modify, sublicense, or distribute the Program except as expressly provided under thisLicense. Any attempt otherwise to copy, modify, sublicense or distribute the Program is void, and willautomatically terminate your rights under this License. However, parties who have received copies, orrights, from you under this License will not have their licenses terminated so long as such parties remain
in full compliance.

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You are not required to accept this License, since you have not signed it. However, nothing else grants
you permission to modify or distribute the Program or its derivative works. These actions are prohibitedby law if you do not accept this License. Therefore, by modifying or distributing the Program (or any
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Each time you redistribute the Program (or any work based on the Program), the recipient automaticallyreceives a license from the original licensor to copy, distribute or modify the Program subject to theseterms and conditions. You may not impose any further restrictions on the recipients’ exercise of therights granted herein. You are not responsible for enforcing compliance by third parties to this License.

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If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (notlimited to patent issues), conditions are imposed on you (whether by court order, agreement orotherwise) that contradict the conditions of this License, they do not excuse you from the conditions of
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If any portion of this section is held invalid or unenforceable under any particular circumstance, thebalance of the section is intended to apply and the section as a whole is intended to apply in othercircumstances.
It is not the purpose of this section to induce you to infringe any patents or other property right claims or
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distribute software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of thisLicense.

140

Appendix E. GNU General Public License

E.2.9. Section 8
If the distribution and/or use of the Program is restricted in certain countries either by patents or bycopyrighted interfaces, the original copyright holder who places the Program under this License may add
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E.2.10. Section 9
The Free Software Foundation may publish revised and/or new versions of the General Public Licensefrom time to time. Such new versions will be similar in spirit to the present version, but may differ indetail to address new problems or concerns.
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If you wish to incorporate parts of the Program into other free programs whose distribution conditions
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E.2.12. NO WARRANTY Section 11
BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR
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E.2.13. Section 12
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILLANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FORDAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL

141

Appendix E. GNU General Public License
DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDINGBUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR
LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO
OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS
BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
END OF TERMS AND CONDITIONS

E.3. How to Apply These Terms to Your New Programs
If you develop a new program, and you want it to be of the greatest possible use to the public, the best
way to achieve this is to make it free software which everyone can redistribute and change under theseterms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source
file to most effectively convey the exclusion of warranty; and each file should have at least the"copyright" line and a pointer to where the full notice is found.
<one line to give the program’s name and a brief idea of what it does.> Copyright (C) <year> <name ofauthor>This program is free software; you can redistribute it and/or modify it under the terms of the GNU
General Public License as published by the Free Software Foundation; either version 2 of the License, or(at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULARPURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not,write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) year name of author Gnomovision comes with ABSOLUTELYNO WARRANTY; for details type ‘show w’. This is free software, and you are welcome to redistribute it
under certain conditions; type ‘show c’ for details.
The hypothetical commands ‘show w’ and ‘show c’ should show the appropriate parts of the General
Public License. Of course, the commands you use may be called something other than ‘show w’ and‘show c’; they could even be mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a"copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program ‘Gnomovision’ (which makes
passes at compilers) written by James Hacker.

142

Appendix E. GNU General Public License<signature of Ty Coon>, 1 April 1989 Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If
your program is a subroutine library, you may consider it more useful to permit linking proprietary
applications with the library. If this is what you want to do, use the GNU Library General Public License
instead of this License.

143

Index of Functions,Parameters, andSyntax->jint, 98->jlong, 98->joutput-stream, 41->jreader, 42->jshort, ??
->jstring, 98->jwriter, 42

Symbols<boolean>, 66<char>, 66<class>, 77<eof>, 66<input-port>, 66<jboolean>, 97<jbyte>, 97<jchar>, 97<jdouble>, 97<jfloat>, 97<jint>, 97<jlong>, 97<jshort>, 97<jvoid>, 97<list>, 66<method>, 71<number>, 66<object>, 77<output-port>, 66<procedure>, 66<slot>, 78<string>, 66<symbol>, 66<value>, 66<vector>, 66->binary-input-port, 41->binary-output-port, 41->boolean, 97->character, 97->character-input-port, 41->character-output-port, 41->jarray, 96->jboolean, 97->jbyte, 98->jchar, 97->jdouble, 98->jfloat, 98->jinput-stream, 41->list, 95->number, 97->string, 98->symbol, 98->vector, 95

Aaccept-tcp-socket, 45
add-method, 69
add-methods, 69
addDefaultHeap, 84
addHeap, 84
alist->hashtable, 110
append, 125applicable-methods, 70
apply, 127ashl, 108
ashr, 108Bbinary-input-port?, 40
binary-output-port?, 40box, 16
box?, 17
buffer, 112
buffer-copy!, 112
buffer-length, 112
buffer-ref, 112
buffer-set!, 112
buffer?, 112144

Index of Functions, Parameters, and Syntax

DC

define, 128
define-class, 77
define-generic, 67
define-generic-java-field-accessor, 92
define-generic-java-field-accessors, 92
define-generic-java-field-modifier, ??
define-generic-java-field-modifiers, 93
define-generic-java-method, 91
define-generic-java-methods, 91
define-generics, 68
define-java-class, 90
define-java-classes, 90
define-java-proxy, 96
define-macro, 20
define-method, 69
define-methods, 69
define-nongenerative-class, 77
define-nongenerative-record-type, 109
define-nongenerative-struct, 109
define-simple-syntax, 107
define-struct, 108
define-values, 107
defmacro, 20
deserialize, 43, 128
directory-list, 55
display, 125
doApply, 127

call-with-binary-input-file, 39
call-with-binary-output-file, 39
call-with-failure-continuation, 21
call-with-input-buffer, 40
call-with-input-file, 36
call-with-input-string, 38
call-with-output-buffer, 40
call-with-output-file, 37
call-with-output-string, 38
call-with-serial-input-file, 43
call-with-serial-input-port, 42
call-with-serial-output-file, 43
call-with-serial-output-port, 42
call/fc, 21
case-sensitive, ??
character-set, ??
circular?, 28class-direct-slots, 78
class-direct-superclasses, 78
class-name, 78
class-path-extension, 55
class-path-extension-append!, 55
class-precedence-list, 80
class?, 77
clear-breakpoint!, 33
close-socket, 44
compact-string-rep, 136
compare-methods, 70
compare-types, 64
compare-typess-hook, 65
compile-file, 119
compose, 29
condvar/new, 61
condvar/notify, 62
condvar/notify-all, 62
condvar?, 61
construct, 128
continue, 33
current-breakpoint-args, 33
current-breakpoint-continuation, 33
current-url, 34
custom-port-procedures, 51

E
emit-annotations, ??
emit-debugging-symbols, ??
enter, 86
eq, 126
error, 24error-location, 23
error-message, 23
error-parent-continuation, 23
error-parent-error, 23
eval, 87
exception-continuation, 25
exception-error, 25
exception?, 25
execute, 85
exit, 4, 86

145

Index of Functions, Parameters, and Syntax

FH
hash-by-eq, 111
hash-by-equal, 111
hash-by-eqv, 111
hash-by-string-ci=, 111
hash-by-string=, 111
hash-table-ref!, 117
hash-table-ref!/default, 117
hash-table-thread-safe?, 117
hash-table-weak?, 117
hashtable->alist, 110
hashtable/clear!, 111
hashtable/contains?, 111
hashtable/equivalence-function, 110
hashtable/for-each, 112
hashtable/get, 111
hashtable/get!, 111
hashtable/hash-function, 110
hashtable/keys, ??
hashtable/map, 112
hashtable/put!, 111
hashtable/remove!, 111
hashtable/size, 110
hashtable/thread-safe?, 110
hashtable/weak?, 110
hashtable?, 110

file-delete!, 54
file-hidden?, 54
file-is-directory?, 54
file-is-file?, 54
file-is-readable?, 54
file-is-writeable?, 54
file-last-modified, 54
file-length, ??
file-rename!, 54
file-set-last-modified!, 54
file-set-read-only!, 55
find-resource, 53
find-resources, 53
findHeap, 84
flush-output-port, 35

Ggeneric-java-field-accessor, 92
generic-java-field-modifier, 92
generic-java-method, 91
generic-procedure-methods, 70
get-client-auth, 47
get-enabled-cipher-suites, 46
get-enabled-protocols, 46
get-host-ip-by-name, 44
get-host-name-by-ip, 44

Iinitialize, 80
input-port-location, 53
input-port?, 37
instance-of?, 64
instances-of?, 65
iota, 29
is-client-mode?, 46

get-last-exception, 30
get-local-host, 44
get-output-buffer, 40
get-output-string, 38
get-parent-url, 54
get-process-stderr, 115
get-process-stdin, 115
get-process-stdout, 115
getBindingValue, 126
getInputStream, ??
getLibraryBindingNames, 126
getLibraryName, 126
getLibraryVersion, 126
getOutputStream, ??
getprop, 27
getReader, ??
getWriter, ??

J
java-array-class, 105
java-array-length, 95
java-array-new, 94
java-array-ref, 95
java-array-set!, 95
java-array?, 94
java-class, 90
java-class-declared-classes, 103146

Index of Functions, Parameters, and Syntax
java-class-declared-constructors, 103Ljava-class-declared-fields, 103

leave-multicast-group, 48
let-monomorphic, 71
library-exists?, 114
library-loaded?, 114
logand, 108
logcount, 108
lognot, 108logor, 108
logxor, 108

java-class-declared-methods, 103
java-class-declared-superclasses, 103
java-class-declaring-class, 103
java-class-flags, 103
java-class-name, 103
java-class-precedence-list, 103
java-class?, 90
java-constructor-declaring-class, 104
java-constructor-flags, 104
java-constructor-method, 104
java-constructor-name, ??
java-constructor-parameter-types, 104
java-constructor-procedure, 104
java-constructor?, 104
java-field-accessor-method, 105
java-field-accessor-procedure, 105
java-field-declaring-class, 105
java-field-flags, 105
java-field-modifier-method, 105
java-field-modifier-procedure, 105
java-field-name, 105
java-field-type, 105
java-field?, 105
java-interface?, ??
java-method-declaring-class, 104
java-method-flags, 104
java-method-method, 104
java-method-name, 104
java-method-parameter-types, 104
java-method-procedure, 104
java-method?, 104
java-new, 93
java-null, 94
java-null?, 94
java-object?, 93
java-proxy-class, 106
java-proxy-dispatcher, 106
java-synchronized, 99
java-unwrap, 98
java-wrap, 98
jnull, 94
join-multicast-group, 48

Mmake, 80
make-buffer, 112
make-child-environment, 28
make-class, 77
make-custom-binary-input-port, ??
make-custom-binary-output-port, 50
make-custom-character-input-port, 49
make-custom-character-output-port, 50
make-directories!, 55
make-directory!, 55
make-error, 22
make-exception, ??
make-generic-procedure, 67
make-hashtable, 109
make-method, 68
make-nested-error, 23
make-type, 66
max-float-precision, ??
max-stack-trace-depth, ??
method, 68
method-applicable?, 70
method-arity, 72
method-procedure, 71
method-rest?, 72
method-types, 72
method=, 72
method?, 71
min-float-precision, ??
mutex-of, 61
mutex/lock!, 61
mutex/new, 61
mutex/synchronize, 62
mutex/synchronize-unsafe, 62
mutex/unlock!, 61, 62

147

Index of Functions, Parameters, and Syntax
mutex?, 61

print-shared, ??
print-stack-trace, 31
procedure-property, 113
procedure-property!, 113
process-terminated?, 115
process?, ??
proper-list?, 16
putprop, 27

Nnormalize-url, 34

Oopen-binary-input-file, 39
open-binary-output-file, 39
open-binary-socket-input-port, 44
open-binary-socket-output-port, 44
open-buffered-binary-input-port, 35
open-buffered-binary-output-port, 35
open-buffered-character-input-port, 35
open-buffered-character-output-port, 35
open-character-input-port, 36
open-character-output-port, 36
open-input-buffer, 40
open-input-file, 36
open-input-string, 38
open-multicast-socket, 48
open-output-file, 36
open-serial-input-port, 42
open-serial-output-port, 42
open-socket-input-port, 44
open-socket-output-port, 44
open-source-input-file, 53
open-ssl-listener, ??
open-ssl-socket, 46
open-tcp-listener, 45
open-tcp-socket, 45
open-udp-listen-socket, 47
open-udp-socket, 47
openHeap, 84
output-port?, 37

P
parallel, 59
parent-environment, 28
peek-byte, 39
permissive-parsing, ??
permit-interrupts, ??
port-local, 49
pretty-print, 52
print-exception, 31

R
read-block, 39
read-byte, 39
read-string, 36
readBigDecimal, 129
readBigInteger, 129
readClass, 129
readExpression, 129
readExpressionArray, 129
readInitializedExpression, 129
readSymbolicEnvironment, 129
readValueArray, 129
repl-prompt, ??
require-library, 113

S
serial-input-port?, 42
serial-output-port?, 42
serialize, 43, 128
server-socket?, 45
session-creation-permitted, 46
set-box!, 16
set-breakpoint!, 33
set-client-auth!, 47
set-client-mode!, ??
set-current-breakpoint-args!, 33
set-enabled-cipher-suites!, 46
set-enabled-protocols!, 46
set-multicast-ttl!, 49
set-port-local!, 49
set-procedure-property!, 113
set-session-creation-permitted!, 46
set-so-timeout!, 45
sisc-initial-environment, 28
sleep, 60
slot-accessor, 79148

Index of Functions, Parameters, and Syntax
slot-accessor-method, 79
slot-class, 79
slot-modifier, 79
slot-modifier-method, 79
slot-name, 79
slot?, 78
socket?, 44
source-annotations, 31
spawn-process, 114
spawn-process-with-environment, 114
spawn-process/env, 114
stack-trace, 30
stack-trace-on-error, ??
strict-r5rs-compliance, ??
string->uninterned-symbol, 14
string-input-port?, 38
string-output-port?, 38
suppressed-stack-trace-source-kinds, 31
synopsis, 125
synopsis-length, ??

Tthread/daemon!, 57
thread/daemon?, 57
thread/interrupt, 58
thread/join, 58
thread/new, 57
thread/priority, 60
thread/priority!, 60
thread/result, 59
thread/spawn, 58
thread/start, 57
thread/state, 58
thread/yield, 60
thread?, 57
throw, ??
time, 29
trace, 32
trace-lambda, 32
trace-let, 32
type-<=-hook, 65
type-of, 63
type-of-hook, 65
types<=, 63, 64
types=, 64

Uunbox, 16
unless, 19
untrace, 32

VvalueEqual, 126
vector-length-prefixing, ??
visit, 128W
wait-for-process, 115
when, 19
with-binary-input-from-file, 39
with-binary-output-to-file, 39
with-character-set, 35
with-current-url, 34
with-failure-continuation, 21
with-input-from-buffer, 40
with-input-from-file, 37
with-input-from-port, 37
with-input-from-string, 38
with-output-to-buffer, 40
with-output-to-file, 37
with-output-to-port, 37
with-output-to-string, 38
with-serial-input-from-file, 43
with-serial-input-from-port, 43
with-serial-output-to-file, 43
with-serial-output-to-port, 43
with/fc, 21
write, 125
write-block, 40
write-byte, 39
write-string, 36
writeBigDecimal, 129
writeBigInteger, 129
writeClass, 130
writeExpression, 130
writeExpressionArray, 130
writeInitializedExpression, 130
writeSymbolicEnvironment, 130
writeValueArray, 130

149