Conceptually what's the relationship between false, dif, and \+.
Given this program:
likes(john, mary).
What is being asked exactly when executing the query:
\+ like(john, A).
And why is the answer not:
dif(A, mary).
The more I think about it, the more I'm convinced that I don't understand the meaning of negation in Prolog.
\+ like(john, A)
(This is an malformed query if variable A behind the \+ is unbound at call time: floundering)
\+ is an operator that is defined procedurally:
\+In other words, "if you can't find evidence for it, assume it's false" aka. "default negation". An attitude taken in relational databases for one. It is basically a replacement for the problem that Prolog doesn't have strongly negated statements, but turns out to be a welcome extension that the philosophers of logic hadn't even come up with earlier.
dif(A, mary).
Is much simpler: "Make sure that A and mary do not unify on this branch of the computation". A constraint is set up that will cause unification of mary and A to fail, either immediately if A is already bound to mary or later. Compare with A \= mary which states that A and mary do not unification at the time this statement is encountered, and so is not really a "logic statement" at all.
I collected some notes on negation as failure and dif/2 which need to be reworked. Still useful.