prologprolog-setofprolog-tabling

Prolog Query Duplicates

I've seen a few questions on this topic, however none of them really answers my question properly. I'll write a small example, here are some facts:

football(john).
football(sam).

tennis(john).
tennis(pete).

netball(sandy).

I want to create a rule that states pete likes anyone that plays football or tennis.

likes(pete, X) :- (football(X) ; tennis(X)), X \= pete.

But obviously when I query it in Prolog, john will come up twice as john plays both football and tennis. I want it to come up with john only once. How can I amend my code to do that?

Thanks in advance - Dan

Solution

  • One clean solution is to use your Prolog system's tabling mechanism.

    For example, in SWI-Prolog, you can do this by adding the following directive at the top :

    :- table likes/2.

    With this directive, you get:

    ?- likes(pete, X).
X = john ;
X = sam.

    Whereas without it, you get:

    ?- likes(pete, X).
X = john ;
X = sam ;
X = john.

    Tabling is also known as SLG resolution.