Gambol provides basic propositional unification and search capabilities for Common Lisp. The code is a modification of an older library from the CMU AI repository, Frolic.

I have removed from Frolic some code specific to the Cray, as well as references to the "Frobs" system which gives Frolic its name. See AlterationsFromFrolic for all the changes I've made.

A small taste:

GAMBOL> (*- (mortal ?x) (human ?x))
((MORTAL ?X) (HUMAN ?X))
GAMBOL> (*- (human ?x) (man ?x))
((HUMAN ?X) (MAN ?X))
GAMBOL> (*- (human ?x) (woman ?x))
((HUMAN ?X) (WOMAN ?X))
GAMBOL> (*- (woman gertrude.stein))
((WOMAN GERTRUDE.STEIN))
GAMBOL> (*- (man socrates))
((MAN SOCRATES))
GAMBOL> (do-solve-all (?who) '((mortal ?who))
          (format t "~A is mortal~%" ?who))
SOCRATES is mortal
GERTRUDE.STEIN is mortal
NIL
GAMBOL> 

You can drop out to Lisp functions, LOP for operators you care about the results for, LISP for side effects with no truth value to worry about.

GAMBOL> (*- (length nil 0))
((LENGTH NIL 0))
GAMBOL> (*- (length (?? . ?t) ?n)
            (length ?t ?n1)
            (is ?n (lop (1+ ?n1))))
((LENGTH (?? . ?T) ?N) (LENGTH ?T ?N1) (IS ?N (LOP (1+ ?N1))))
GAMBOL> (??- (length (a b c 1 2 3) ?l))

?L = 6
NO
GAMBOL> 

See the API documentation for further details, and Examples for a few common Prolog examples in Gambol.

Second release (0.02) on July 10, 2008. Changes from 0.01:

• some irritations with symbol packages in the operators dealt with (i.e., you don't have to state the package on is)
• added pl-asserta to add rules to the head of the database
• made pl-assert and pl-asserta available to the Gambol language with Prologesque names asserta and assertz.
• added pl-retract to the Lisp API and retract to Gambol
• (fail) is now a hard-coded part of Gambol, so you can't accidentally declare it to be true