Search code examples
prologprolog-toplevel

Question mark in SWI-Prolog - nondeterministic functor

I've just tried to implement absolute function in Prolog and I've got some strange behavior. My code was:

absval(X, RESULT) :- X >= 0, RESULT is X.
absval(X, RESULT) :- X < 0, RESULT is -X.

And when I try in SWI-Prolog absval(-2,X). I get

X = 2

yes

as expected. But otherwise when I invoke absval(2,X), I get X = 2 ? and I should insert another input. After pressing enter I get also yes.

What does mean the second one result? What's wrong with my solution?

Solution

  • what you report doesn't match the behaviour I get here:

    ?- absval(2,X).
X = 2 ;
false.

    that's actually what is expected.

    If you need to make it deterministic, use a cut, or better, the 'if/then/else' construct:

    absval(X, RESULT) :- X >= 0 -> RESULT is X ; RESULT is -X.