functor(?Term, ?Functor, ?Arity)

Term

Prolog term.

Functor

Atomic term or variable.

Arity

Positive integer or variable.

Description

If Term is not instantiated, it is bound to a term with functor Functor and arity Arity.

This predicate regards atomic terms (number, atom or string) as terms with arity 0 and functor equal to the term.

To query only the arity of a term, arity/2 can be used instead of functor/3.

Modes and Determinism

• functor(+, -, -) is det
• functor(-, +, +) is det

Exceptions

(4) instantiation fault

Term and either (or both) of Functor and Arity are uninstantiated (non-coroutine mode only).

(5) type error

Arity is neither a variable nor an integer.

(5) type error

Functor is neither a variable nor an atomic term.

(6) out of range

Arity is a negative integer.

Examples

   Success:
   functor(f(1,2),f,2).
   functor(f(1,2),F,A).  (gives F=f, A=2).
   functor(T,f,3).       (gives T=f(_g50, _g52, _g54)).
   functor(T,.,2).       (gives T=[_g48 | _g50]).
   functor([],F,A).      (gives F=[], A=0).
   functor("s",F,A).     (gives F="s", A=0).
   Fail:
   functor(f(1,2),f,3).
   functor("compound(g)",compound,1).
   functor(f(1,2),"f",2).
   Error:
   functor(T,F,A).                    (Error 4).
   functor("f",[f],X).                (Error 5).
   functor(X,[a],Y).                  (Error 5).
   functor(f(1,2),f,-1).              (Error 6).

See Also