SWI-Prolog -- functor/3

Documentation
- Reference manual
  - Built-in Predicates
    - Analysing and Constructing Terms
- Packages

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 compound_name_arity/3. The predicate functor/3 and =../2 raise a domain_error when faced with these terms. Without this precaution, the inconsistency demonstrated below could happen silently.^{95Raising
a domain error was suggested by Jeff Schultz.}

?- functor(a(), N, A).
N = a, A = 0.
?- functor(T, a, 0).
T = a.