Prolog repeating the same answer infinitely

Question

These are my facts:

married(man, widow).
married(father,daughter).
married(widow, man).
married(daughter,father).

parent(father, man).
parent(widow, daughter).
parent(man, bboy).

% here is the rule giving me problems,

parent(X,Y):-
   married(Z,X),
   parent(Z,Y).

when I run parent(X,Y) it returns the same answers infinitely as such

  ?- parent(X,Y).

X = father
Y = man ? ;

X = widow
Y = daughter ? ;

X = man
Y = bboy ? ;

X = widow
Y = bboy ? ;

X = widow
Y = daughter ? ;

X = widow
Y = bboy ? ;

X = widow
Y = daughter ? ;

How do I stop it from repeating, because when another rule calls the parent rule it crashes prolog

Answer 1

Your problem is here:

parent(X,Y):-
    married(Z,X),
    parent(Z,Y).

Actually, you can see it a little more clearly if you remove married/2:

parent(X,Y):-
    parent(Z,Y).

This is going to lead to unbounded recursion. The simple thing to do is rename either the rule or the facts:

parent(father, man).
parent(widow, daughter).
parent(man, bboy).

parent_of(X, Y) :-
    married(Z, X),
    parent(Z, Y).

No more recursion.