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`functor/3`

`functor(+Term, -Name, -Arity)`

`functor(-Term, +Name, +Arity)`

Succeeds if the principal functor of term `Term` has name
`Name` and arity `Arity`.

`Term`*term*`Name`*atom*`Arity`*arity*

There are two ways of using this predicate:

- If
`Term`is initially instantiated, then- if
`Term`is a compound term, then`Name`and`Arity`are unified with the name and arity of its principal functor. - otherwise,
`Name`is unified with`Term`, and`Arity`is unified with 0.

- if
- If
`Term`is initially uninstantiated, then`Name`and`Arity`must both be instantiated, and- if
`Arity`is an integer in the range 1..255, then`Name`must be an atom, and`Term`becomes instantiated to the most general term having the specified`Name`and`Arity`; that is, a term with distinct variables for all of its arguments. - if
`Arity`is 0, then`Name`must be atomic, and it is unified with`Term`.

- if

`instantiation_error`

`Term`and either`Name`or`Arity`are uninstantiated.`type_error`

`Name`is not atomic, or`Arity`is not an integer, or`Name`is not an atom when`Arity`> 0.`domain_error`

`Arity`is an integer < 0.`representation_error`

`Term`is uninstantiated and`Arity`> 255.

| ?-functor(foo(a,b), N, A).N = foo, A = 2 | ?-functor(X, foo, 2).X = foo(_A,_B)

| ?-functor(X, 2, 0).X = 2

`arg/3`

, `name/2`

, `=../2`

, ref-lte-act.

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