lisp

Minimal Lisp interpreter using 75LOC and only standard libraries excluding the REPL. Inspired by Lis.py.

$ lisp-repl
ctrl-c to exit
> (begin                                                                        
(>   (define incf                                                               
((>     (lambda (x)                                                             
(((>       (set! x (+ x 1))))                                                   
(>   (define one 1)                                                             
(>   (incf one))                                                                
2
>

Install

gem install lisp

Usage

require "lisp"

Lisp.eval(<<-eos)
  (begin
    (define fact
      (lambda (n)
        (if (<= n 1)
          1
          (* n (fact (- n 1))))))
    (fact 10))
eos # => 3628800

Commandline

lisp-repl

Features

• [x] constant literal number - A number evaluates to itself. Example: 12 or -3.45e+6

• [x] procedure call - (proc exp…) If proc is anything other than one of the symbols if, set!, define, lambda, begin, or quote then it is treated as a procedure. It is evaluated using the same rules defined here. All the expressions are evaluated as well, and then the procedure is called with the list of expressions as arguments. Example: (square 12) ⇒ 144

• [x] variable reference - var A symbol is interpreted as a variable name; its value is the variable's value. Example: x

• [x] definition - (define var exp) Define a new variable and give it the value of evaluating the expression exp. Examples: (define r 3) or (define square (lambda (x) (* x x))).

• [x] procedure - (lambda (var…) exp) Create a procedure with parameter(s) named var… and the expression as the body. Example: (lambda ® (* 3.141592653 (* r r)))

• [x] conditional - (if test conseq alt) Evaluate test; if true, evaluate and return conseq; otherwise evaluate and return alt. Example: (if (< 10 20) (+ 1 1) (+ 3 3)) ⇒ 2

• [ ] quotation - (quote exp) Return the exp literally; do not evaluate it. Example: (quote (a b c)) ⇒ (a b c)

• [x] assignment - (set! var exp) Evaluate exp and assign that value to var, which must have been previously defined (with a define or as a parameter to an enclosing procedure). Example: (set! x2 (* x x))

• [x] sequencing - (begin exp…) Evaluate each of the expressions in left-to-right order, and return the final value. Example: (begin (set! x 1) (set! x (+ x 1)) (* x 2)) ⇒ 4