fogus/ulithp

---

fogus/ulithp.json

{
  "createdAt": "2012-01-25T19:23:59Z",
  "defaultBranch": "master",
  "description": "A micro LISP implementation in 24 lines of Ruby.",
  "fullName": "fogus/ulithp",
  "homepage": "http://fogus.github.com/ulithp/",
  "language": "Ruby",
  "name": "ulithp",
  "pushedAt": "2020-02-13T16:24:16Z",
  "stargazersCount": 271,
  "topics": [
    "langdev"
  ],
  "updatedAt": "2025-10-24T10:16:37Z",
  "url": "https://github.com/fogus/ulithp"
}

μLithp

Official website and deep analysis

A micro LISP implementation in 24 lines of Ruby, with a reader in 33 lines, REPL in 35 lines, a compiler in 15 lines and Ruby interop in 13 lines. Really frickin’ small.

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

See the src/lithp.rb file for source and doc/index.org for implementation details and the original announcement in the μLithp blog post.

this is my entry into the December 2012 PLT Games

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 third (quote (lambda (x) (car (cdr (cdr x))))))
  (third (quote (1 2 3 4 5)))

Enjoy!

thanks

Thanks to Russ Olsen for the reader and REPL.

License

This software is provided as-is under the MIT license.