Three simple interface functions call logic. They are any, findall, and findallset. Each takes two arguments: a result expression to instantiate and a goal expression. any returns the first solution found. findall returns all solutions. findallset returns all solutions deleting duplicates.
Assuming the definitions for fact and color from the previous examples.
|(any '(?x is the factorial of 5) '(fact 5 ?x))
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|(120 IS THE FACTORIAL OF 5)
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|(findall '(?x is a color) '(color ?x))
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|((RED IS A COLOR) (BLUE IS A COLOR)
(GREEN IS A COLOR))
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|(findall '?y '(or (= ?y 5) (= ?y 5)))
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|(5 5)
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|(findallset '?y '(or (= ?y 5) (= ?y 5)))
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|(5)
findall and findallset will hang if a goal expression generates an infinite solution set.
More powerful all solution predicates (bagof and setof) are available from within Common Prolog.
KnowledgeWorks and Prolog User Guide (Windows version) - 24 Mar 2017