functor/3 meta-logical

Synopsis

functor(+Term, -Name, -Arity)

functor(-Term, +Name, +Arity)

Succeeds if the principal functor of term Term has name Name and arity Arity.

Arguments

Term term

Name atom

Arity arity

Description

There are two ways of using this predicate:

1. If Term is initially instantiated, then
   • if Term is a compound term, Name and Arity are unified with the name and arity of its principal functor.
   • if Term is an atom or number, Name is unified with Term, and Arity is unified with 0.
2. If Term is initially uninstantiated, Name and Arity must both be instantiated, and
   • if Arity is an integer in the range 1..255, then Name must be an atom, and Term becomes instantiated to the most general term having the specified Name and Arity; that is, a term with distinct variables for all of its arguments.
   • if Arity is 0, then Name must be an atom or a number, and it is unified with Term.

Examples

     
     | ?- functor(foo(a,b), N, A).
     
     N = foo,
     A = 2
     
     | ?- functor(X, foo, 2).
     
     X = foo(_1,_2)
     

Please note: _1 and _2 are anonymous variables. The term foo(_1,_2) is the "most general" term that has name foo and arity 2.

     | ?- functor(X, 2, 0).
     
     X = 2
     

Exceptions

instantiation_error

Term and either Name or Arity are uninstantiated.

type_error

Name is not atomic when Arity is 0, or Arity is not an integer.

representation_error

Term is uninstantiated and Arity is an integer > 255.

See Also

arg/3, name/2, =../2 ref-lte-act