lisp
====

![Gem Version][3] ![Gem][1] ![Build Status][2]

Minimal Lisp interpreter using 75LOC and only standard libraries excluding the REPL. Inspired by [Lis.py](http://norvig.com/lispy.html).

```clojure
$ 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
-----

```clojure
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 (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_

[1]: https://img.shields.io/gem/dt/lisp
[2]: https://travis-ci.org/jamesmoriarty/lisp.svg?branch=master
[3]: https://img.shields.io/gem/v/lisp