jacksonwillis / ulithp

A micro LISP implementation in ~30 lines of Ruby.

uLithp

A micro LISP implementation in ~30 lines of Ruby, with a reader in 32 lines, and a REPL in 12 lines. Really frickin' small.

• car
• cdr
• quote
• atom
• if
• label
• cons
• eq
• λ

See the lithp.rb file for implementation details and the uLithp blog post.

Embedded usage

The LISP interpreter is just a Ruby class that evals expressions in Ruby data structures, like so:

	 l = Lisp.new

	 l.eval [:label, :a, 42]

	 l.eval :a
	 #=> 42

	 l.eval [:eq, 42, :a]
	 #=> true

	 l.eval [:quote, [1, 2]]
	 #=> [1, 2]

	 l.eval [:car, [:quote, [1, 2]]]
	 #=> 1

	 l.eval [:cdr, [:quote, [1, 2]]]
	 #=> [2]

	 l.eval [:cons, 1, [:quote, [2,3]]]
	 #=> [1, 2, 3]

	 l.eval [:if, [:eq, 1, 2], 42, 43]
	 #=> 43

	 l.eval [:atom, [:quote, [1,2]]]
	 #=> false

	 l.eval [:label, :second, [:quote, [:lambda, [:x], 	 [:car, [:cdr, :x]]]]]

	 l.eval [:second, [:quote, [1, 2, 3]]]
	 #=> 2

Have fun!

REPL usage

In your shell use Ruby 1.9.2 to run the REPL:

    rvm use ruby-1.9.2
	ruby -I . repl.rb

You'll then see a prompt:

>

Start typing uLithp code:

    (car (quote (1 2 3)))
	
	(cdr (quote (1 2 3)))
	
	(label second (quote (λ (x) (car (cdr x))))) 
	(second (quote (1 2 3)))

Enjoy!

thanks

Thanks to Russ Olsen for the reader and REPL.