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instance(?Instance, ?Term)

Succeeds if Instance is an instance of Term.
?Instance
An arbitrary term.
?Term
An arbitrary term.

Description

Succeeds if it is possible to find an instantiation of free variables in Term such that Term and Instance are equal. The result is undefined if Term and Instance share variables. Note that no unification actually occurs.

Fail Conditions

Fails if Instance is not an instance of Term.

Resatisfiable

No

Examples

   Success:
   instance(atom,X).
   instance(f(a,b),X).
   instance(f(a,b),f(X,Y)).
   instance(f(a,X),f(Y,X)).
   instance(f(a,X),f(X,Y)).
   instance(f(X,Y),f(Y,X)).
   instance([a,b,c],[A,B,C]).
   instance([a,f(1,b,X),Y|Z],T).
   instance(f(X),X).         % with occur_check off
   Fail:
   instance(f(a,b),f(X,X)).
   instance(X,a).
   instance(f(X),X).         % with occur_check on

See Also

compare_instances / 3, prune_instances / 2, variant / 2