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.. This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2024 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0
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
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.
.. rst-class:: align-right
control construct
.. index:: pair: (::)/2; Control construct .. _control_send_to_object_2:
(::)/2
::
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.
::
+object_identifier::+callable - zero_or_more {+object_identifier}::+callable - zero_or_more
| 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)
::
| ?- 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