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# functor(?Term, ?Functor, ?Arity)

Succeeds if the compound term Term has functor Functor and arity Arity or
if Term and Functor are atomic and equal, and Arity is 0.
*Term*
- Prolog term.
*Functor*
- Atomic term or variable.
*Arity*
- Positive integer or variable.

## Description

If Term is instantiated, its functor is unified with Functor and its
arity with Arity.
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.

### 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

=.. / 2, arg / 3