CODEWITHSUNDEEP
prologprolog-dif
Manuel W.
Manuel W.

Reputation: 65

Why does this prolog program returns false?

woman(lisa).
woman(maggie).

parents(lisa  , homer, marge).
parents(maggie, homer, marge).

sister(X, Y) :-
   X \= Y,
   woman(X),
   parents(X, P, M),
   parents(Y, P, M).

When I ran the following query, I did not expect failure (but got it nonetheless).

?- sister(lisa, X).
false.                         % expected result: X = maggie

Why am I seeing this behavior?

Upvotes: 2

Views: 288

Answers (2)

repeat
repeat

Reputation: 18726

Stay pure! How? Use for expressing term inequality!

sister(X, Y) :-
   dif(X, Y),
   woman(X),
   parents(X, P, M),
   parents(Y, P, M).

Sample query:

?- sister(lisa, X).
X = maggie.

For more on dif/2 see also:

Upvotes: 1

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726579

Assuming that sister(X,Y) says that Y is a sister of X, you want woman(Y), not woman(X).

Re-arranging the rule makes it work (demo):

sister(X, Y):-
  woman(Y),
  parents(X, P, M),
  parents(Y, P, M),
  X \= Y.

The important thing in this re-arrangement is that the X \= Y term is moved to a point after both X and Y have been bound - X is given to the rule as input, and Y is bound through participation in woman(Y).

Upvotes: 1

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