SWI-Prolog -- functor/3

Availability:built-in

[ISO]functor(?Term, ?Name, ?Arity)

True when Term is a term with functor Name/Arity. If Term is a variable it is unified with a new term whose arguments are all different variables (such a term is called a skeleton). If Term is atomic, Arity will be unified with the integer 0, and Name will be unified with Term. Raises instantiation_error() if Term is unbound and Name/Arity is insufficiently instantiated.

SWI-Prolog also supports terms with arity 0, as in a() (see section 5). Such terms must be processed using functor/4 or compound_name_arity/3. The predicate functor/3 and =../2 raise a domain_error when faced with these terms. Without this precaution a round trip of a term with arity 0 over functor/3 would create an atom.