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A tribute to:

Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I

(as found in paper/recursive.pdf)

A micro-subset of scheme / the original LISP in a single C file: komplott.c


  • Single file implementation.
  • Scheme-compliant enough for the test programs to be executable by GNU Guile.
  • Copying garbage collector based on Cheney's Algorithm.
  • Limited tail call optimization (not true TCO; see tests/true-tco.scm).
  • Near-zero error handling.
  • Zero thread safety or security.

Also includes:


An implementation of the core of LISP 1.5 from 1962


  • To build the komplott executable, run make. The only dependency aside from make is gcc.

  • To run the LISP 1.5 interpreter and a couple of test cases, run make lisp15.

LISP 1.5

The version presented in the README is slightly tweaked from the one that can be found in tests/lisp15.scm in order to more closely resemble early LISP rather than scheme: #t and #f are written as t and nil.

(define pairlis (lambda (x y a)
                  (cond ((null? x) a)
                        (t (cons (cons (car x) (car y))
                                 (pairlis (cdr x) (cdr y) a))))))

(define assoc (lambda (x a)
                (cond ((equal? (caar a) x) (car a))
                      (t (assoc x (cdr a))))))

(define atom? (lambda (x)
                 ((null? x) t)
                 ((atom? x) t)
                 (t nil))))

(define evcon (lambda (c a)
                 ((eval (caar c) a) (eval (cadar c) a))
                 (t (evcon (cdr c) a)))))

(define evlis (lambda (m a)
                 ((null? m) nil)
                 (t (cons (eval (car m) a)
                             (evlis (cdr m) a))))))

(define apply (lambda (fun x a)
                 ((atom? fun)
                   ((equal? fun (quote CAR)) (caar x))
                   ((equal? fun (quote CDR)) (cdar x))
                   ((equal? fun (quote CONS)) (cons (car x) (cadr x)))
                   ((equal? fun (quote ATOM)) (atom? (car x)))
                   ((equal? fun (quote EQ)) (equal? (car x) (cadr x)))
                   (t (apply (eval fun a) x a))))

                 ((equal? (car fun) (quote LAMBDA))
                  (eval (caddr fun) (pairlis (cadr fun) x a)))

                 ((equal? (car fun) (quote LABEL))
                   (caddr fun)
                    (cons (cadr fun) (caddr fun))

(define eval (lambda (e a)
                ((atom? e) (cdr (assoc e a)))
                ((atom? (car e))
                  ((equal? (car e) (quote QUOTE)) (cadr e))
                  ((equal? (car e) (quote COND)) (evcon (cdr e) a))
                  (t (apply (car e) (evlis (cdr e) a) a))))
                (t (apply (car e) (evlis (cdr e) a) a)))))

(define evalquote (lambda (fn x) (apply fn x (quote ()))))

Here is an example of actual LISP 1.5 code:

        (LAMBDA (FN SEQ)
                  ((EQ NIL SEQ) NIL)
                  (T (CONS (FN (CAR SEQ))
                           (MAPCAR FN (CDR SEQ)))))))

; where
; DUP -> (LAMBDA (X) (CONS X X))
; LST -> (A B C)

To prevent reading from continuing indefinitely, each packet should end with STOP followed by a large number of right parentheses. An unpaired right parenthesis will cause a read error and terminate reading.

STOP )))))))))))))))))