What are the uses of the fail predicate in Prolog?

Question

I can't come up with a situation where I would need it.

Answer 1

One case (taken from Constraint Logic Programming using Eclipse) is an implementation of not/1:

:- op(900, fy, not).
not Q :- Q, !, fail.
not _ .

If Q succeeds, the cut (!) causes the second not clause to be discarded, and the fail ensures a negative result. If Q fails, then the second not clause fires first.

Answer 2

Elegant systems provide false/0 as a declarative synonym for the imperative "fail/0". An example where it is useful is when you manually want to force backtracking for side-effects, like:

?- between(1,3,N), format("line ~w\n", [N]), false.
line 1
line 2
line 3

Instead of false/0, you can also use any goal that fails, for example a bit shorter:

?- between(1,3,N), format("line ~w\n", [N]), 0=1.
line 1
line 2
line 3

Thus, false/0 is not strictly needed but quite nice.

Answer 3

Another use for fail is to force backtracking through alternatives when using predicates with side effects:

writeall(X) :- member(A,X), write(A), fail.
writeall(_).

Some people might not consider this particularly good programming style though. :)