CODEWITHSUNDEEP
prolog
921kiyo
921kiyo

Reputation: 572

Why is [] = _. true in Prolog?

I am learning Prolog, and do not understand why ?- [] = _. returns true in SWI-Prolog. [] means an empty list, and _ means it is not empty, right?
Could someone explain the above logic?

Upvotes: 3

Views: 989

Answers (1)

Isabelle Newbie
Isabelle Newbie

Reputation: 9378

_ is a logical variable, just like X or anything else that starts with an underscore or a capital letter. Free variables (i.e., variables that are not already bound to some term) can be unified with anything. A goal like [] = X says "unify X with []", with the effect that, if this succeeds, every use of X will refer to the term [].

With _ as the variable this is the same as for X, with the exception that _ is the anonymous variable: It cannot be reused, its name does not matter, and different occurrences of _ refer to different variables. So _ can never be bound before encountering the goal [] = _. Thus this unification succeeds, which is why you get the answer true.

_ does not by itself mean "not empty". But you may be confused by its use as a placeholder: L = [_,_,_] means that L is a list of three elements (that we know nothing about). In this sense _ means "there is something here". But it has to be inside the list for this meaning.

Upvotes: 11

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