11.3.86 functor/3 [ISO]

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.
   • otherwise, 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 atomic, and it is unified with Term.

Exceptions

instantiation_error

Term and either Name or Arity are uninstantiated.

type_error

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

domain_error

Arity is an integer < 0.

representation_error

Term is uninstantiated and Arity > 255.

Examples

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

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

See Also

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