序言：简单的家庭关系

Question

I am learning Prolog and what i want is to do a simple "calculation"(i dont know how its called in Prolog) of the members of a simle family.

For example i have:

 1)father of steve is petter
 2)brother steve is john
 3)A person is a son to a Father when the brother of the person has  as father the Father

(it seems funny and completely out of logic when its out of my mind :) )

father(steve,petter).
brother(john,steve).
father(X,Y):-brother(X,Z),father(Z,Y)).

and my question is who is the father of john(the correct awnser would be petter)

?-father(john,X).

But it always giving me false.

Answer 1

When you enter father(john, X). , it starts by trying to find a Z such that brother(john, Z) is true. No such Z exists, so it returns false.

Note that brother(steve, john) does not imply brother(john, steve) unless you tell Prolog that it should.

Answer 2

SOLVED:

father(steve,petter).
brother(john,steve).
whoisfather(X,Y):-brother(X,Z),father(Z,Y).
?- whoisfather(john,X).
X = petter.

instead of

father(steve,petter).
brother(john,steve).
father(X,Y):-brother(X,Z),father(Z,Y)).
?- father(john,X).
false.

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