Well i’m just wondering why does it work, where prolog loose variable.
?- \+ \+ member(X,[a]),X=b.
X=b.
Where X=a. automagicly vanished.
Well i’m just wondering why does it work, where prolog loose variable. ?- \+
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Well i’m just wondering why does it work, where prolog loose variable.
?- \+ \+ member(X,[a]),X=b.
X=b.
Where X=a. automagicly vanished.
\+/1is not “not” in the logical sense, but is implemented through “negation as failure”. That means,\+ Goalsucceeds iffGoalfails.Think of
\+/1as being implemented as:As you can see, in both cases can no variable in
Goalbecome bound. In the “if” branch, any binding will be undone by backtracking, and in the “else” branch, no variable will have been become bound anyway.In your example,
member(X,[a]succeeds by bindingXtoa. This will have\+ member(X,[a])fail, and so\+\+ member(X,[a])succeeds because of this failure. Due to the intermediate failure,Xwill not be bound toa.