.. This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2024 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0

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.. _messages_messages:

Messages

Messages allows us to ask an object to prove a goal and must always match a declared predicate within the scope of the sender object. Note that sending a message is fundamentally different from calling a predicate. When calling a predicate, the caller decides implicitly which predicate definition will be executed. When sending a message, it is the receiving object, not the sender, that decides which predicate definition (if any) will be called to answer the message. The predicate definition that is actually used to answer a message depends on the relations between the object and its imported categories and ancestor objects (if any). See the :ref:inheritance_inheritance section for details on the predicate declaration and predicate definition lookup procedures.

When a message corresponds to a :term:meta-predicate, the meta-arguments are always called in the context of the object (or category) sending the message.

Logtalk uses nomenclature similar to in other object-oriented programming languages such as Smalltalk. Therefore, the terms query and message are used interchangeably when referring to a declared predicate that is part of an object interface. Likewise, the terms predicate and method are used interchangeably when referring to the predicate definition (inside an object or category) that is called to answer a message.

.. _messages_operators:

Operators used in message sending

Logtalk declares the following operators for the message sending control constructs:

::

:- op(600, xfy, ::). :- op(600, fy, ::). :- op(600, fy, ^^).

It is assumed that these operators remain active (once the Logtalk compiler and runtime files are loaded) until the end of the Prolog session (this is the usual behavior of most Prolog compilers). Note that these operator definitions are compatible with the predefined operators in the Prolog ISO standard.

.. _messages_sending:

Sending a message to an object

Sending a message to an object is accomplished by using the :ref:control_send_to_object_2 control construct:

::

..., Object::Message, ...

The message must match a public predicate declared for the receiving object. The message may also correspond to a protected or private predicate if the sender matches the predicate scope container. If the predicate is declared but not defined, the message simply fails (as per the :term:`closed-world assumption`).

.. _messages_delegating:

Delegating a message to an object

It is also possible to send a message to an object while preserving the original sender and meta-call context by using the :ref:control_delegate_message_1 delegation control construct:

::

..., [Object::Message], ....

This control construct can only be used within objects and categories (at the top-level interpreter, the sender is always the pseudo-object user so using this control construct would be equivalent to use the (::)/2 message sending control construct).

Sending a message to self

While defining a predicate, we sometimes need to send a message to self, i.e., to the same object that has received the original message. This is done in Logtalk through the :ref:control_send_to_self_1 control construct:

::

..., ::Message, ....

The message must match either a public or protected predicate declared for the receiving object or a private predicate within the scope of the sender otherwise an error will be thrown. If the message is sent from inside a category or if we are using private inheritance, then the message may also match a private predicate. Again, if the predicate is declared but not defined, the message simply fails (as per the :term:`closed-world assumption`).

.. _messages_broadcasting:

Broadcasting

In the Logtalk context, broadcasting is interpreted as the sending of several messages to the same object. This can be achieved by using the message sending control construct described above. However, for convenience, Logtalk implements an extended syntax for message sending that may improve program readability in some cases. This extended syntax uses the (,)/2, (;)/2, and (->)/2 control constructs (plus the (*->)/2 soft-cut control construct when provided by the backend Prolog compiler). For example, if we wish to send several messages to the same object, we can write:

.. code-block:: text

| ?- Object::(Message1, Message2, ...).

This is semantically equivalent to:

.. code-block:: text

| ?- Object::Message1, Object::Message2, ... .

This extended syntax may also be used with the (::)/1 message sending control construct.

.. _messages_super:

Calling imported and inherited predicates

When redefining a predicate, sometimes we need to call the inherited definition in the new code. This functionality, introduced by the Smalltalk language through the super primitive, is available in Logtalk using the :ref:control_call_super_1 control construct:

::

..., ^^Predicate, ....

Most of the time we will use this control construct by instantiating the pattern:

::

Predicate :- ..., % do something ^^Predicate, % call inherited definition ... . % do something more

This control construct is generalized in Logtalk where it may be used to call any imported or inherited predicate definition. This control construct may be used within objects and categories. When combined with :term:`static binding`, this control construct allows imported and inherited predicates to be called with the same performance of local predicates. As with the message sending control constructs, the (^^)/1 call simply fails when the predicate is declared but not defined (as per the :term:`closed-world assumption`).

.. _messages_events:

Message sending and event generation

Assuming the :ref:`events <flag_events>` flag is set to allow for the object (or category) sending a message using the :ref:control_send_to_object_2 control construct, two events are generated, one before and one after the message execution. Messages that are sent using the :ref:control_send_to_self_1 (message to self) control construct or the :ref:control_call_super_1 super mechanism described above do not generate any events. The rationale behind this distinction is that messages to self and super calls are only used internally in the definition of methods or to execute additional messages with the same target object (represented by self). In other words, events are only generated when using an object's public interface; they cannot be used to break object encapsulation.

If we need to generate events for a public message sent to self, then we just need to write something like:

::

Predicate :- ..., % get self reference self(Self), % send a message to self using (::)/2 Self::Message, ... .

If we also need the sender of the message to be other than the object containing the predicate definition, we can write:

::

Predicate :- ..., % send a message to self using (::)/2 % sender will be the pseudo-object user self(Self), {Self::Message}, ... .

When events are not used, is possible to turn off event generation globally or on a per entity basis by using the events compiler flag to optimize message sending performance (see the :ref:events_events section for more details).

.. _messages_from_module:

Sending a message from a module

Messages can be sent to objects from within Prolog modules. Depending on the backend support for goal-expansion and on the :ref:`optimize <flag_optimize>` flag being turned on, the messages will use static binding when possible. This optimization requires the object to be compiled and loaded before the module. Note that the module can be user. This is usually the case when sending the message from the top-level interpreter. Thus, the same conditions apply in this case. Note that loading Prolog modules using Prolog directives or built-in predicates necessarily limits the range of possible optimizations for message sent from the modules.

.. warning::

If you want to benchmark the performance of a message sending goal at the top-level interpreter, be careful to check first if the goal is pre-compiled to use static binding, otherwise you will also be benchmarking the Logtalk compiler itself.

.. _messages_performance:

Message sending performance

For a detailed discussion on message sending performance, see the :ref:performance_performance section.

.. .. _messages_performance:

Message sending performance ---------------------------

Logtalk supports both :term:`static binding` and :term:dynamic binding. Static binding is used whenever messages are sent (using the (::)/2 control construct) to static objects already loaded and with the :ref:`optimize <flag_optimize>` compiler flag turned on. When that is not the case (or when using the (::)/1 control construct), Logtalk uses dynamic binding coupled with a caching mechanism that avoids repeated lookups of predicate declarations and predicate definitions. This is a solution common to other programming languages supporting dynamic binding. :term:`Message lookups <message lookup>` are automatically cached the first time a message is sent. Cache entries are automatically removed when loading entities or using Logtalk dynamic features that invalidate the cached lookups.

Whenever static binding is used, message sending performance is roughly the same as a predicate call in plain Prolog. When discussing Logtalk dynamic binding performance, two distinct cases should be considered: messages sent by the user from the top-level interpreter and messages sent from compiled objects. In addition, the message declaration and definition lookups may, or may not be already cached by the runtime engine. In what follows, we will assume that the message lookups are already cached.

.. _messages_inferences:

Translating message processing to predicate calls ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In order to better understand the performance trade-offs of using Logtalk dynamic binding when compared to plain Prolog or to Prolog module systems, is useful to translate message processing in terms of predicate calls. However, in doing this, we should keep in mind that the number of predicate calls is not necessarily proportional to the time taken to execute them.

With event-support turned on, a message sent from a compiled object (or category) to another object translates to a minimum of five predicate calls:

checking for before events one call to the built-in predicate (\+)/1 and a call to its argument, assuming that no events are defined method call using the cached lookup one call to a dynamic predicate (the cache entry) checking for after events one call to the built-in predicate (\+)/1 and a call to its argument, assuming that no events are defined

Given that events can be dynamically defined at runtime, there is no room for reducing the number of predicate calls without turning off support for event-driven programming. When events are defined, the number of predicate calls grows proportional to the number of events and event handlers (monitors). Event-driven programming support can be switched off for specific object using the :ref:`events <flag_events>` compiler flag. Doing so, reduces the number of predicate calls from three to just one.

Messages to self are transparent regarding events and, as such, imply only one predicate call (to the cache entry, a dynamic predicate).

When a message is sent by the user from the top-level interpreter, Logtalk needs to perform a runtime translation of the message term in order to prove the corresponding goal. Thus, while sending a message from a compiled object corresponds to either three predicate calls (event-support on) or one predicate call (event-support off), the same message sent by the user from the top-level interpreter necessarily implies an overhead. Considering the time taken for the user to type the goal and read the reply, this overhead is of no practical consequence.

When a message is not cached, the number of predicate calls depends on the number of steps needed for the Logtalk runtime engine to lookup the corresponding predicate scope declaration (to check if the message is valid) and then to lookup a predicate definition for answering the message.

.. _messages_cputime:

Processing time ~~~~~~~~~~~~~~~

Not all predicate calls take the same time. Moreover, the time taken to process a specific predicate call depends on the Prolog compiler implementation details. As such, the only valid performance measure is the time taken for processing a message.

The usual way of measuring the time taken by a predicate call is to repeat the call a number of times and than to calculate the average time. A sufficient large number of repetitions would hopefully lead to an accurate measure. Care should be taken to subtract the time taken by the repetition code itself. In addition, we should be aware of any limitations of the predicates used to measure execution times. One way to make sense of numbers we get is to repeat the test with the same predicate using plain Prolog and with the predicate encapsulated in a module.

A simple predicate for helping benchmarking predicate calls could be:

::

benchmark(N, Goal) :- repeat(N), call(Goal), fail.

benchmark(_, _).

The rational of using a failure-driven loop is to try to avoid any interference on our timing measurements from garbage-collection or memory expansion mechanisms. Based on the predicate benchmark/2, we may define a more convenient predicate for performing our benchmarks. For example:

::

benchmark(Goal) :- % some sufficiently large number of repetitions N = 10000000, write('Number of repetitions: '), write(N), nl, % replace by your Prolog-specific predicate get_cpu_time(Seconds1), benchmark(N, Goal), get_cpu_time(Seconds2), Average is (Seconds2 - Seconds1)/N, write('Average time per call: '), write(Average), write(' seconds'), nl, Speed is 1.0/Average, write('Number of calls per second: '), write(Speed), nl.

We can get a baseline for our timings by doing:

.. code-block:: text

| ?- benchmark(true).

For comparing message sending performance across several Prolog compilers, we would call the benchmark/1 predicate with a suitable argument. For example:

.. code-block:: text

| ?- benchmark(list::length([1,2,3,4,5,6,7,8,9,0], _)).

For comparing message sending performance with predicate calls in plain Prolog and with calls to predicates encapsulated in modules, we should use exactly the same predicate definition in the three cases.

It should be stressed that message sending is only one of the factors affecting the performance of a Logtalk application (and often not the most important one). The strengths and limitations of the chosen Prolog compiler play a crucial role on all aspects of the development, reliability, usability, and performance of a Logtalk application. It is advisable to take advantage of the Logtalk wide compatibility with most Prolog compilers to test for the best match for developing your Logtalk applications.