[SOLVED] One common hobby between two persons in Prolog

One common hobby between two persons in Prolog

I found this old question ( Prolog Predicate to return true when two people have same hobby ) for writing some code to find the same hobby between two persons using dif. I have a similar situation, to display the names of two persons that have the same hobbies,but the hobbies element is defined as a list. Whatever I try it doesn't seem to work, any ideas would be appreciated.

This is what I have so far:

one_common_hobby(P1,P2):-
   dif(P1,P2),
   person(P1,_,_,hobbies([H|Tail])),
   person(P2,_,_,hobbies([H|Tail])),
   member(P1,Tail),
   member(P2,Tail).

Solution

For simplicity, suppose the predicate person is defined as follows:

person(ann, hobbies([fishing, pottery, chess])).
person(bob, hobbies([parkour, painting, origami])).
person(coy, hobbies([hunting, fishing, parkour, chess])).

To find two different persons (P1 and P2) who have a common hobby you must:

Access the list of hobbies of the first person (H1).
Access the list of hobbies of the second person (H2).
Check if there is at least one hobby X that belongs simultaneously to lists H1 and H2.

have_common_hobby(P1, P2) :-
    dif(P1, P2),
    person(P1, hobbies(H1)),
    person(P2, hobbies(H2)),
    once( ( member(X, H1),
            member(X, H2) ) ).

Example:

?- have_common_hobby(A, B).
A = ann,
B = coy ;
A = bob,
B = coy ;
A = coy,
B = ann ;
A = coy,
B = bob ;
false.