SWI-Prolog -- functor/4

Availability:built-in

functor(?Term, ?Name, ?Arity, ?Type)

As functor/3, but designed to work with zero-arity terms (e.g., a(), see section 5). Type is one of atom, compound, callable or atomic. Type must be instantiated if Name is an atom and Arity is 0 (zero). In other cases Type may be a variable. This predicate is true if Term (either initially or after haveing been created from Name and Type) and Type are related as below

If Term is compound (including zero-arity compounds), Type must be compound or callable. If Type is unbound is is unified with compound.
If Term is an atom, Type must be atom or callable. If Type is unbound is is unified with atom.
Else Type is unified with atomic.

This predicate provides a safe round trip for zero-arity compounds and atoms. It can also be used as a variant of functor/3 that only processes compound or callable terms. See also compound/1, callable/1 and compound_name_arity/3.