control construct

# `(::)/2`

## Description

    Object::Message
    {Proxy}::Message

Sends a message to an object. The message argument must match a public predicate of the receiver object. When the message corresponds to a protected or private predicate, the call is only valid if the sender matches the predicate scope container. When the predicate is declared but not defined, the message simply fails (as per the closed-world assumption).

When the predicate used to answer the message is a meta-predicate, the calling context for the predicate meta-arguments is the object sending the message.

The `{Proxy}::Message` syntax allows simplified access to parametric object proxies. Its operational semantics is equivalent to the conjunction `(call(Proxy), Proxy::Message)`. I.e. `Proxy` is proved within the context of the pseudo-object user and, if successful, the `Proxy` term is used as an object identifier. Exceptions thrown when proving `Proxy` are handled by the `(::)/2` control construct. This construct construct supports backtracking over the `{Proxy}` goal.

The lookups for the message declaration and the corresponding method are performed using a depth-first strategy. Depending on the value of the optimize flag, these lookups are performed at compile time whenever sufficient information is available. When the lookups are performed at runtime, 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

    +object_identifier::+callable - zero_or_more
    {+object_identifier}::+callable - zero_or_more

## Errors

Either `Object` or `Message` is a variable:
`instantiation_error`
`Object` is neither a variable nor a valid object identifier:
`type_error(object_identifier, Object)`
`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 declared protected:
`permission_error(access, protected_predicate, Name/Arity)`
`Message`, with predicate indicator `Name/Arity`, is not declared:
`existence_error(predicate_declaration, Name/Arity)`
`Object` does not exist:
`existence_error(object, Object)`
`Proxy` is a variable:
`instantiation_error`
`Proxy` is neither a variable nor a callable term:
`type_error(callable, Proxy)`
`Proxy`, with predicate indicator `Name/Arity`, does not exist in the `user` pseudo-object:
`existence_error(procedure, Name/Arity)`

## Examples

    | ?- list::member(X, [1, 2, 3]).

    X = 1 ;
    X = 2 ;
    X = 3
    yes

See also

(::)/1, (^^)/1, []/1