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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 control 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 for 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`
