allsea9 / clisp

Lisp interpreter in C++

Clisp

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A lightweight (~550 lines of code) Lisp interpreter with Scheme dialect. I was inspired to make this after watching all the SICP lectures.

Features:

• file inclusion e.g. (include test.scm), can be nested inside files
• optimized tail recursion
• first class procedures and by extension higher order procedures
• lexical scoping so you don't have to worry about local variables clashing in called procedures
• free typed arguments, e.g. (define (add x) ...) x is expected by the semantics to be a list, but not enforced
• local binding (essentially giving code blocks) with keyword let
• ; comments

I originally intended to include the boost parts required, but as you can see from include_list, you're better off getting the proper boost distribution. To compensate, this repository contains a binary built on Ubuntu 14.04 which should work on most Unix based machines.

Example:

(include funcs.scm)     ; don't do this! recursive inclusion is bad D:

(define (if test a b) (cond ((test) a) (else b)))

(define compose (lambda (f g)   ; fundamental higher order procedure
        (lambda (x)
                (f (g x)))))

(define expt (lambda (x n)      ; exponential 
              (cond ((= n 1) x)
                    (else (* x 
                             (expt x (- n 1)))))))

(define nth-power (lambda (n)
        (lambda (x)
                (expt x n))))

(define square (nth-power 2))

(define cube (nth-power 3))

(define square_and_cube (compose square cube))

(define (add x)
        (cond ((empty? x) 0)
              (else (+ (car x) (add (cdr x))))))

(begin (do something) (do somethingelse) result)    ; basic sequencing

(define (add4 n)
        (let ((x 4)) (+ x n)))  ; basic local binding

(cat 'something 'somethingelse)

(square 5)

(add (1 2 3 4))

Tips:

• build by typing "make" in the same directory
• build benchmark version by replacing main.cpp with timing.cpp in makefile
• list parameters (such as x for add above) treated same as 'normal' parameters
• interpret files with ./clisp [filename] [-p], add -p or -print option to force printing of file evaluation, silent by default (assumes a lot of definitions)
  • include files with (include filename), which can be nested
• build debug information (with step by step info) by changing build target and source in makefile to "testing" and "testing.cpp" respectively
• requires a compiler supporting C++11
• uses boost::variant (link above)
• keywords (so far): define, lambda, cond, cons, cdr, list, else, and, or, not, empty?, include, begin
• use 'quote to signify string
  • string will raise an error if it's not defined, but 'string will return string
• use cat primitive instead of + to concatenate strings
• expressions can extend over different lines, terminated by appropriate )!