Lisp Interpreter in the Ruby Programming Language - 70 LOC. Inspired by Lis.py.
$ lisp-rb
ctrl-c to exit
> (begin
(> (define incf
((> (lambda (x)
(((> (set! x (+ x 1))))
(> (define one 1)
(> (incf one))
2
>
gem install lisp
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constant literal number - A number evaluates to itself. Example: 12 or -3.45e+6
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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
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variable reference - var A symbol is interpreted as a variable name; its value is the variable's value. Example: x
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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))).
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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)))
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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
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quotation - (quote exp) Return the exp literally; do not evaluate it. Example: (quote (a b c)) ⇒ (a b c)
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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))
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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