How to force a result to be false in Prolog

Question

I want to know if it is possible to force a result e.g.

test(0, 0, 0).

to be false in Prolog.

Well usually you do this by writing the logic of `test/3` that way. — Willem Van Onsem, Oct 21 '17 at 19:39
You could write `test(0,0,0) :- fail.`. But, the most appropriate way to ensure a query fails is to not present facts and rules that enable it to succeed. Forcing failure could be a bad code smell. — lurker, Oct 21 '17 at 19:41

score 3 · Answer 1 · answered Oct 22 '17 at 15:57

Yes, you can do it with a little helper predicate like so:

:- use_module(library(clpfd)).

test_(X,Y,Z) :-                        % base relation
   Z #= X+Y.                           % here: integer sum

test(X,Y,Z) :-                         % exclude (0,0,0) as a solution
   abs(X) + abs(X-Y) + abs(Y-Z) #\= 0,
   test_(X,Y,Z).

Sample queries:

?- test(1,1,2).
true
?- test(1,-1,0).
true

Note how the unwanted solution is excluded:

?- test_(0,0,0).            % 0+0 = 0
true
?- test(0,0,0).
false

How to force a result to be false in Prolog

1 Answers1