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Unifycle: A Unification Library for Clojure

Sep 27, 2010

As a first step toward a much grander Clojure project I needed a unification library to supplement a simple logic engine (the second step in the larger plan). There are a few options floating around, but nothing that was readily available nor complete. Therefore, I decided to package a simple unification library in hopes that others might find it useful and hopefully eliminate the need to search around fruitlessly like I did. A basic use case for Unifycle is as follows:

[sourcecode lang=”clojure” gist=”599021″](use ‘[me.fogus.unifycle :only [unifier]])

;; unify two expressions with an occurs check

(unifier ‘((?a * ?x ** 2) + (?b * ?x) + ?c) ‘(?z + (4 * 5) + 3))

;=> ((?a * 5 ** 2) + (4 * 5) + 3)[/sourcecode]

Unifycle exposes a number of canned functions, starting with unifier, try-subst, and garner-unifiers. These functions use an occurs check internally, so use them with that fact in mind. I have also exposed versions of these functions without internal occurs check named unifier-, try-subst-, and garner-unifiers-. If you know what unification is then you know what an occurs check is — if not, then this whole post is probably moot.

I have also exposed factory functions named make-occurs-unify-fn, make-occurs-subst-fn, make-occurs-unifier-fn, make-unify-fn, make-subst-fn, and make-unifier-fn. The first three create versions using and occurs check and the last three create versions without. Each of these factory functions take a single predicate function that is used to determine if a symbol in a (potentially) unified expression refers to a variable. From the example above, you’ll notice that the default variable function is preceded with a question mark (e.g. ?snigglet).

I have created a documentation page for Unifycle that I plan to expand. Any and all help is appreciated.


3 Comments, Comment or Ping

  1. Paul Hobbs

    This is very useful, thank you! I’m re-implementing a Prolog system in Clojure because it requires Java interop. This will help immensely! You rock :-)

  2. Thanks so much for this. I will be using this and you just saved me months of work (yeah, I am that slow)

  3. Nathan Sorenson

    This looks cool. I’m doing a bit of logic programming in Clojure myself. I’m assuming you’ve seen the mini-kanren port to Clojure? I don’t think it unifies arbitrary forms like yours seems to; it just handles sequences.

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