A LISP interpreter in Awk
Roger Rohrbach
1592 Union St., #94
San Francisco, CA 94123
January 3, 1989

ABSTRACTThis note describes a simple interpreter for the LISP programming language, written in awk. It provides intrinsic versions of the basic functions on s-expressions, andmany others written in LISP. It is compatible with the commonly available version ofawk that is supplied with most UNIX systems. The interpreter serves to illustrate the useof awk for prototyping or implementing language translators, as well as providing a simple example of LISP implementation techniques.

Intrinsic functions.
The interpreter has thirteen built-in functions. These include the five elementary functions on sexpressions defined by McCarthy [1]; some conditional expression operators; an assignment operator, andsome functions to control the evaluation process.
The intrinsic functions are summarized below. Familiarity with existing LISP systems is assumed in
the descriptions of these functions.
(car l)Returns the first element of the list l. An error occurs if l is atomic.
(cdr l)Returns the remainder of the list l, i.e., the sublist containing the second through the last elements. Ifl has only one element, nil is returned. cdr is undefined on atoms.
(cons e l)Constructs a new list whose car is e and whose cdr is l.
(eq x y)Returns t if x and y are the same LISP object, i.e., are either atomic and have the same print name,or evaluate to the same list cell. Otherwise, nil is returned.
(atom s)Returns t if s is an atom, otherwise nil.
(set x y)
Assigns the value y to x and returns y. x must be atomic, and may not be a constant or name an
intrinsic function.
(eval s)
Evaluates s and returns the result.
(error s)
Halts evaluation and returns nil. The atom s is printed.
(quote s)
Returns s, unevaluated. The form

-2-´expr
is an abbreviation for(quote expr)(cond (p1 [e1]) ... (pN [eN]))
Evaluates each p from left to right. If any evaluate to a value other than nil, the corresponding e isevaluated and the result is returned. If there is no corresponding e, the value of the p itself isreturned instead. If no p has a non-null value, nil is returned.
(and e1 ... eN)
Evaluates each e and returns nil if any evaluate to nil. Otherwise the value of the last e is returned.
(or e1 ... eN)
Evaluates each e and returns the first whose value is non-null. If no such e is found, nil is returned.
(list e1 ... eN)
Constructs a new list with elements e1 ... eN. Equivalent to(cons e1 (cons ... (cons eN nil).
Lambda functions.
The following functions are written in LISP and are defined in the file walk.w. Most of these arecommonly supplied with LISP systems.
(cadr s)
(cddr s)(caar s)(cdar s)(cadar s)(caddr s)(cddar s)(cdadr s)
These correspond to various compositions of car and cdr, e.g.,(cadr s) → (car (cdr s)).
(null s)Equivalent to (eq s nil).
(not s)Same as null.
(ff s)
Returns the first atomic symbol in s.
(subst x y z)
Substitutes x for all occurrences of the atom y in z. x and z are arbitrary s-expressions.
(equal x y)
Returns t if x and y are the same s-expression, otherwise nil.
(append x y)
Creates a new list containing the elements of x and y, which must both be lists.
(member x y)
Returns t if x is an element of the list y, otherwise nil.
(pair x y)
Pairs each element of the lists x and y, and returns a list of the resulting pairs. The number of pairs in
the result will equal the length of the shorter of the two input lists.

-3-(assoc x y)
Association list selector function. y is a list of the form ((u1 v1) ... (uN vN)) where the u’s are
atomic. If x is one of these, the corresponding pair (u v) is returned, otherwise nil.
(sublis x y)
x is an association list. Substitutes the values in x for the keys in y.
(last l)Returns the last element of l.
(reverse l)
Returns a list that contains the elements in l, in reverse order.
(remove e l)
Returns a copy of l with all occurrences of the element e removed.
(succ x y)
Returns the element that immediately follows the atom x in the list y. If x does not occur in y or is thelast element, nil is returned.
(pred x y)
Returns the element that immediately precedes the atom x in the list y. If x does not occur in y or isthe first element, nil is returned.
(before x y)
Returns the list of elements occurring before y in x. If y does not occur in x or is the first element,
nil is returned.
(after x y)
Returns the list of elements occurring after y in x. If y does not occur in x or is the last element, nil
is returned.
(plist x)
Returns the property list for the atom x.
(get x i)
Returns the value stored on x’s property list under the indicator i.
(putprop x v i)
Stores the value v on x’s property list under the indicator i.
(remprop x i)
Remove the indicator i and any associated value from x’s property list.
(mapcar f l)Applies the function f to each element of l and returns the list of results.
(apply f args)
Calls f with the arguments args, e.g.,(apply ’cons ’(a (b)))
is equivalent to(cons ’a ’(b))
Syntactic conventions.
Atoms take the following forms:
Regular identifiers
Atoms matching the regular expression[_A-Za-z][-A-Za-z_0-9]*
The initial value of an identifier is nil.

-4-Integers
Atoms matching the regular expression [0-9][0-9]*. Integers are constants, i.e., evaluate tothemselves.
Weird atoms
Identifiers matching the regular expression ".*". Weird atoms are not constants.
A semicolon introduces a comment, which continues for the rest of the line.
Usage.
The command for running the interpreter iswalk [ files ]on BSD UNIX and derivative systems, or
awk -f walk [ files ]on UNIX System V. The file name − represents the standard input. This can be omitted if no other files arebeing read in, or if the interpreter is being run non-interactively.
Normally, the interpreter is used interactively, augmented with the functions defined in walk.w,and, perhaps, other files. The command line to use for this purpose iswalk walk.w [ other files ] p The interpreter will first read walk.w, printing the results of evaluating the function definitions therein.
Then it will read p. This file contains no LISP definitions; the interpreter recognizes it by name and prints
a prompt to signal the user that all the prerequisite files have been read and that the interpreter is waiting forinput. (This is the only way to get awk to do this; this can be hidden from the user with a shell programthat invokes the interpreter if desired.) Thereafter, it will evaluate expressions typed in by the user, printing a prompt after each one. Normally the prompt is −>; the first character of the prompt changes whenappropriate to an integer that represents the number of unmatched left parentheses read in so far.
The interpreter exits when it encounters the end of its last input file. If this file is the standard input,
the number of LISP objects created is reported.
Several files defining auxiliary functions are provided.
Implementation.
So that it can run on any UNIX system, The LISP interpreter has been written using the UNIX V7
version of awk, which predates the version described in The Awk Programming Language [2]. The only
complex data type provided by this language is the array. Data that in C might be stored in structures isrepresented, therefore, using multiple arrays, one for each field. For example, the C codep = allocate_cell();p->car = s;p->cdr = NIL;
can be approximated with:
p = ++cell;car[p] = s;
cdr[p] = nil;
Lists (using nested array references) and stacks are also simulated with arrays. The most important data
structures are explained in the program and in the following description.
As is usual for LISP implementations, the interpreter is constructed as a loop that reads an s-expression, evaluates it, and prints the result. The reader collects an s-expression, reading multiple input lines ifnecessary. Like the other two phases of the interpreter, this is a recursive procedure and in awk this must
be managed explicitly. When an s-expression is read, its internal representation in list structure is formed
using the stack read[]. Atoms and cons operators are pushed onto the read stack and periodically ‘reduced’-5-or replaced with list cells when a complete list has been read; the reader returns an atom or list on the topof the stack. The reader must be able to return an s-expression in the middle of an input line, so the entire
interpreter is enclosed in a loop that allows the current input line to be completely scanned before the next
input record is read. The general outline is:
BEGIN {
initialize interpreter
say hello if interactive}{
initialize reader variables
while (chars left on this line){
readif (have read an s-expression){evalprint}}
prompt if interactive}
END {say goodbye if interactiveexit}
The evaluator maintains two stacks, one for input and one for output. The result returned by thereader is copied onto the input stack (eval[]), and evaluated according to the usual LISP rules. Evaluated sexpressions are placed on the output stack, arg[]. When an intrinsic function that takes evaluated arguments appears on the top of the evaluation stack, its arguments are popped from the argument stack. Functions (like cond) that take unevaluated arguments are handled as special forms before their arguments havebeen pushed onto eval[]. The arguments are handled differently depending on the semantics of the function. Lambda (user-defined) functions are evaluated by temporarily binding the formal parameters in thefunction definition to the results of evaluating the actual arguments with which the function was called, andthen evaluating the body of the function. Temporary bindings only are kept on a special pushdown list (thealist). Atoms have a global value that is stored separately; this keeps the alist small.
The evaluation procedure is sketched below:

-6-atom?
lambdarestore previous environment (lambda function
body has been evaluated already)constant?
returnbound?
look up local value
otherwise
return global value
intrinsic function?
apply to already evaluated arguments
lambda function?
bind formal parameters to already evaluated arguments
evaluate function bodyform?
intrinsic function application?
quote
return unevaluated argument
condandor
begin evaluating arguments according to operator semantics
list
expand to repeated applications of consother?
push function variable, arguments
lambda function application?
push lambda function, bodyother?
eval car, cdr
When the evaluation stack is emptied, the result is popped from the argument stack and printed. A
stack is again used to manage recursion.
Conclusion.
The goal of writing a small LISP interpreter and extending it in LISP has been realized. Though it
was not my original intention, it would be easy to incorporate the LISP functions as intrinsics, and manyother extensions (such as numeric functions) could be made, in which case the interpreter might fulfill
more than a pedagogic function. Even so, it can be used as is for an introduction to LISP programmingand implementation concepts. I hope it also inspires more of us to learn how to program in awk!
References.
[1]
McCarthy, J. Recursive Functions of Symbolic Expressions and their Computation by Machine, PartI. Comm. ACM, 3, 4, pp. 185-195 April 1960
[2]Aho, A., Weinberger, P., & Kernighan, B.W. The Awk Programming Language. Addison-Wesley,Reading, MA 1988