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Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

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.. rst-class:: align-right

control construct

.. index:: pair: (::)/2; Control construct .. _control_send_to_object_2:

(::)/2

Description

::

Object::Message {Proxy}::Message

Sends a message to an object. The message argument must match a :ref:`public <directives_public_1>` predicate of the receiver object. When the message corresponds to a :ref:`protected <directives_protected_1>` or :ref:`private <directives_private_1>` predicate, the call is only valid if the :term:sender matches the :term:`predicate scope container`. When the predicate is declared but not defined, the message simply fails (as per the :term:`closed-world assumption`).

When the predicate used to answer the message is a :term:meta-predicate, the :term:`calling context <predicate calling context>` for the predicate meta-arguments is the object sending the message.

The {Proxy}::Message syntax allows simplified access to :term:`parametric object proxies <parametric object proxy>`. Its operational semantics is equivalent to the conjunction (call(Proxy), Proxy::Message). I.e. Proxy is proved within the context of the pseudo-object :ref:`user <objects_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 :ref:`optimize <flag_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 :ref:`performance <performance_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

.. seealso::

:ref:control_send_to_self_1, :ref:control_call_super_1, :ref:control_delegate_message_1