functor/3
Module: builtins
functor/3 — builds structures and retrieves information about them
FORMS
functor(Structure, Functor, Arity)
DESCRIPTION
The principal functor of term Structure has name Functor and arity Arity, where Functor is an atom. Either Structure must be instantiated to a term or an atom, or Functor and Arity must be instantiated to an atom and a non-negative integer, respectively.
In the case where Structure is initially unbound, functor/3 will unify Structure with a structured term of Arity arguments, where the principal functor of the term is Functor. Each argument of the new structure will be a new uninstantiated variable.
When Structure is instantiated to a structured term, Functor will be unified with the principal functor of Structure and Arity will be unified with the arity. functor/3 treats atoms as structured terms with arity 0. The principal functor of a list is ‘ . ‘ with arity 2.
EXAMPLES
?- functor(Structure,fish,2).
Structure=fish(_123,_124)
yes.
?- functor(city('SantaMonica','CA','USA'), Functor, Arity).
Functor=city
Arity=3
yes.