An implementation of miniKanren in Haskell
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README.md

MiniKanrenT

An implementation of miniKanren in Haskell

To get started

clone the repository and open examples.hs like so:

ghci -isrc examples.hs

If you want to make it work with your own type you can basically just replicate what's in Control.Monad.MiniKanrenT.Term replacing Term with your own type.

Differences in semantics from miniKanren

As far as I know the semantics are essantially the same except:

  • Logic variables are typed (and you can define your own custom types)
  • conde is actually condi and it doesn't have an else clause (as it doesn't do anything special anyway)
  • My implementation will not go into an infinite loop if a unification is circular. There is no "occurs check" and it doesn't need one.

Origins

This year at strangeloop I was totally blown away by the miniKanren talk by Daniel Friedman and William Byrd

I highly recommend watching the presentation. It is simultaneously entertaining and mind blowing.

They used an extended version of miniKanren, a logic programming language (a la prolog) embedded in scheme, to do several remarkable things:

  • Generate programs that evaluate to a given value
  • Generate programs that satisfy a given type
  • And most awesomely, generate Quines! (and Twines!!)

This was something that I clearly did not know enough about yet. Usually when I want to really learn and understand something in CS I try to implement it.

So I grabbed a copy of The Reasoned Schemer and did just that!

My weapon of choice is Haskell, so here you go, a MiniKanren monad transformer in Haskell.

It turns out there is a very nice monad transformer by Dan Doel based on a paper (co-authored by Friedman) that does the backtracking and interleaving for me. I'll still likely have to take a shot at making my own version of logict (again, just to learn how it works) but this will work for now.

However, I couldn't find a suitable Haskell logic variable implementation. So I made one that can be safely used in a monad transformer. That means it will work correctly even with the backtracking and branching that the logic monad does. And it also means that I can basically plug the two monads together to make a miniKanren monad!

This is not quite the MiniKanren in the talk. They have 'does not unify' operator =/= that I don't have implemented... yet... But! I think it does correctly implement the core features of miniKanren as described in The Reasoned Schemer.

I'm not quite satisfied with implementation yet. I still want to implement the remaining features necessary to be able to generate my own quines :) I also want to implement some of the "libraries" for working with numbers and lists and such.

So there should be more to come :)