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microKanren in Ruby

microKanren is a minimalist relational (logic) programming language. This project is a port of microKanren to Ruby. It is an almost exact translation of the original implementation, which was written for Petite Chez Scheme.


Add this line to your application's Gemfile:

gem 'micro_kanren'

And then execute:

$ bundle

Or install it yourself as:

$ gem install micro_kanren


The following example demonstrates how MicroKanren can be used from the console:

> require 'micro_kanren'
> include MicroKanren::Core
> include MicroKanren::MiniKanrenWrappers

> res = call_fresh(-> (q) { eq(q, 5) }).call(empty_state)
> res.to_s
(((([0] . 5)) . 1))

See the spec file for more examples. The spec file is almost an exact port of the microKanren tests written in Scheme.


The code in this gem is closely based on the following sources:

  • The microKanren paper by Jason Hemann and Daniel P. Friedman. I read this paper a couple of times, and will probably have to read it a few more before I have a better understanding about what this code is doing and how to write more effective logic programs.
  • This code is also in parts copied from Scott Vokes' port of microKanren to Lua. It was great to have the Lua code as a second example of the implementation in the paper, and it made my job especially easy since Lua is so similar to Ruby.


This project requires Ruby 2.0 or higher. You can see which Rubies work with this project in the Travis CI Build Status.

Note that there does seem to be a particular early patch level of of a Ruby interpreter that breaks Ruby ruby_ukanren! If the unit tests don't pass for you locally, or the example in this file doesn't work, try upgrading your Ruby to a recent patch level of one of the supported Ruby versions that is currently passing on Travis CI. See this issue for more.


  1. Fork it ( )
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request


See LICENSE.txt.


See credits for source of original code. This port was written by [Justin Leitgeb] (

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A port of microKanren to Ruby




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