I'm new to Prolog. Trying to write a predicate named leaftree, that is True if the given tree is a leaf tree. A tree is a leaf tree if all its leaves are atoms, or if it's empty. There's my predicate:
leaftree(nil).
leaftree(t(nil, X, nil)):-
atom(X).
leaftree(t(L, X, R)):-
leaftree(L), leaftree(R).
There's the input I'm using to test this predicate:
leaftree(t(t(nil, 5, nil), X, nil)).
Well, the output is True but it should be False because 5 isn't an atom. I played around with the leaftree(t(nil, X, nil)):- atom(X) part; it seems like the predicate is called, but it always returns True. Even if I explicitly write fail or other false statements. If matters, I'm using Swish. I'd love your thoughts!
Thanks so much in advance. Orel
The result of your input is true because it matches the third definition of the predicate. So it exists a predicate that is true for your query.
This happens because in the third definition X is free to be anything. When consulting your file with swi-prolog i receive, in fact, a warning saying: Singleton variable [X].
You could replace the second and the third definition with the following (which include both)
leaftree(t(L, X, R)):-
leaftree(L), atom(X), leaftree(R).