lisp-rb

Lisp Interpreter in Ruby. Inspired by Lis.py.

$ lisp-rb
ctrl-c to exit
> (define pi 3.14)
3.14
> (* pi 2)
6.28
> (define area (lambda (r) (* 3.141592653 (* r r))))
#<Proc:0x007f8af9c59ff0@./lisp.rb:44>
> (area 3)
28.274333877
> (define fact (lambda (n) (if (<= n 1) 1 (* n (fact (- n 1))))))
#<Proc:0x007f9a4cc4acf0@./lisp.rb:63 (lambda)>
> (fact 10)
3628800.0

Install

gem install lisp

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 (r) (* 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