What is the logical 'not' in Prolog?
The problem that I face, is a bit trivial.
I want to use logical not in Prolog, but it seems that not/1 is not the thing that I want:
course(ai).
course(pl).
course(os).
have(X,Y) :- course(X),course(Y),not(X = Y).
I query:
have(X,Y), write(X-Y), nl , fail.
And I do not get the result I want :(
Answer
In place of not(X = Y) you need to write \+ X = Y or X \= Y. But consider to use dif(X,Y) instead. dif/2 is present in B, SWI, YAP, SICStus. To see the difference:
?- X = b, dif(a, X).
X = b.
?- X = b, \+ a = X.
X = b.
So up to now everything seems to be fine. But what, if we simply exchange the order of the two goals?
?- \+ a = X, X = b.
false.
?- dif(a, X), X = b.
X = b.
(\+)/1 now gives us a different result, because there is an answer
for a = X, the goal \+ a = X will fail.
(\+)/1 is thus not negation, but means not provable at this point
in time.
A safe approximation of dif/2 is possible in ISO Prolog, too.