I have some problems with prolog, specifically I can't compare a value of a predicate with a constant.
predicate(9).
compare(X,Y) :- X<Y.
Running the program:
?-compare(predicate(X),10).
Why doesn't it work? Thank you for your answers.
Like @Boris said before "Predicates don't return values in the way that a function does." Here you must try to instantiate the variables in the head of your rule.
If you are trying with you predicate compare/2 to find a number X greater than Y, and at the same time this number X should be a fact predicate/1, then add both conditions to the body of your rule or predicate compare/2
predicate(9).
compare(X,Y) :- predicate(X), X<Y.
Now if you consult:
?- compare(X,10).
The answer will be
X = 9
As you can see, 9 is smaller than 10, and at the same time 9 is a fact predicate/1. And that is the return value you are looking for.
Note that the operator >/2, requires that both sides are instantiated, so in this case you won't be able ask for the value Y in your predicate
?- compare(9, Y)
</2: Arguments are not sufficiently instantiated
Maybe and if it make sense, you can try to instantiate this variable to a fact predicate/1 too.
predicate(9).
predicate(10).
compare(X,Y) :- predicate(X), predicate(Y), X<Y.
?- compare(9,Y).
Y = 10