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-?-> ?Body

The matching operator. The head of the clause which contains it will not be unified with the caller, one-way matching will be used instead.

Body: Callable term or a variable.

Description

This operator is used to produce matching clauses, i.e. clauses whose head is unified with the caller only in one direction, namely without binding any variables in the caller. Therefore, only those clause will be selected, which are more general than the call, i.e. the call must be an instance of the head. If the clause head is unifiable with the call, but this unification would bind any of the variables in the call, the unification fails.

-?-> must occur at the beginning of the clause body, directly behind the :- symbol, and it must be followed by a non-empty body. Matching clauses with no body must use true/0 after the matching operator.

The matching operator can be also used to decompose attributed variables. When an attributed variable occurs in the head of a matching clause, it is not unified with the call argument (which would trigger the unification handlers) but instead, the call argument is decomposed into the variable and its attribute(s):

    get_attr(X{A}, Attr) :-
	-?->
	A = Attr.

This predicate can be used to return the attribute of its argument if it is an attributed variable and to fail if it is not.

Clause matching is not supported by dynamic predicates. A run-time exception will be raised when executing a matching clause head that is dynamic.