# ::/1

## Description

    ::Message

Sends a message to self. Can only used in the body of a predicate definition. The argument should match a public or protected predicate of self. It may also match a private predicate if the predicate is within the scope of the object where the method making the call is defined, if imported from a category, if used from within a category, or when using private inheritance. When the predicate is declared but not defined, the message simply fails (as per the closed-world assumption).

The lookups for the message declaration and the corresponding method are performed using a depth-first strategy. A message to self necessarily implies the use of dynamic binding but a caching mechanism is used to improve performance in subsequent messages. See the User Manual section on performance for details.

## Modes and number of proofs

    ::+callable - zero_or_more

## Errors

`Message` is a variable:
`instantiation_error`
`Message` is neither a variable nor a callable term:
`type_error(callable, Message)`
`Message`, with predicate indicator `Name/Arity`, is declared private:
`permission_error(access, private_predicate, Name/Arity)`
`Message`, with predicate indicator `Name/Arity`, is not declared:
`existence_error(predicate_declaration, Name/Arity)`

## Examples

    area(Area) :-
        ::width(Width),
        ::height(Height),
        Area is Width * Height.

See also

::/2, ^^/1, []/1