unique results from prolog

Question

Is there a easy way to make a query in prolog only return each result once?

for instance I'm trying something like:

deadly(Xn) :- scary(X), Xn is X - 1, Xp is X + 1, not(safe(Xn)), safe(Xp).
deadly(Xp) :- scary(X), Xn is X - 1, Xp is X + 1, not(safe(Xp)), safe(Xn).

deadly(X).

and getting

X = 5

X = 5

X = 5

X = 5

....

Not to usefull for me.

Answer 1

One thing that you can do is to apply setof/3 to the predicate that generates the solutions. But note that setof/3 is implemented by applying sort/2 to the result delivered by bagof/3 (at least this is the case in SWI-Prolog). So, if your solution generator goes on forever, then setof/3 will never be applied...

So I would say that try to program so that duplicates are not generated, i.e. by using the cut (!) where it makes sense.

Answer 2

If I remember correctly there is a predicate solutions (or similar, it's been a while since I programmed Prolog) which collects unique solutions in a list.

Edit: setof/3 is the one I was thinking of. Thanks, Kaarel.