artificial intelligence - is it possible to prove nested rule in prolog?

Question

I'm trying to call a rule inside another rule my code is to fined the grandpa just like:

male(jack).
male(john).
male(mark).
parent(jack,john).
parent(john,mark).
father(X,Y):-male(X),parent(X,Y).
grandpa(X,Y):-father(X,father(F,Y)).

and the query in GNU Prolog is

grandpa(X,mark).

it just returns no and when I tried to trace the call I noticed that it doesn't even call the nested rule. So is there any way to do this in prolog?

Answer 1

Rules aren't functions (which is why they aren't called functions): you don't call them, you prove them. To do what you seem to be asking, you would write:

grandpa(X,Y) :- father(X,Z), father(Z,Y).

That is: X is Y's grandpa if there exists a Z such that X is Z's father and Z is Y's father.