..
   This file is part of Logtalk <https://logtalk.org/>  
   SPDX-FileCopyrightText: 1998-2023 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.


.. _programming_programming:

Writing and running applications
================================

For a successful programming in Logtalk, you need a good working
knowledge of Prolog and an understanding of the principles of
object-oriented programming. Most guidelines for writing good Prolog
code apply as well to Logtalk programming. To those guidelines, you
should add the basics of good object-oriented design.

One of the advantages of a system like Logtalk is that it enable us to
use the currently available object-oriented methodologies, tools, and
metrics [Champaux92]_ in logic programming. That said, writing applications
in Logtalk is similar to writing applications in Prolog: we define new
predicates describing what is true about our domain objects, about our
problem solution. We encapsulate our predicate directives and definitions
inside new objects, categories, and protocols that we create by hand with
a text editor or by using the Logtalk built-in predicates. Some of the
information collected during the analysis and design phases can be
integrated in the objects, categories and protocols that we define by
using the available entity and predicate documenting directives.

.. _programming_starting:

Starting Logtalk
----------------

We run Logtalk inside a normal Prolog session, after loading the
necessary files. Logtalk extends but does not modify your Prolog
compiler. We can freely mix Prolog queries with the sending of messages
and our applications can be made of both normal Prolog clauses and
object definitions.

Depending on your Logtalk installation, you may use a script or a shortcut
to start Logtalk with your chosen Prolog compiler. On POSIX operating-systems,
Bash shell integration scripts should be available from the command-line.
On Windows, PowerShell integration scripts should be available from the
command-line and integration shortcuts should be available from the Start Menu.
Scripts are named upon the used backend Prolog compilers.

For example, assuming a POSIX operating-system and GNU Prolog as the backend:

.. code-block:: bash

   $ gplgt
   ...

Depending on your Logtalk installation, you may need to type instead
``gplgt.sh``. On Windows, using PowerShell 7.2 or later version and
ECLiPSe as the backend:

.. code-block:: powershell

   PS> eclipselgt.ps1
   ...

.. _programming_parallel_processes:

Running parallel Logtalk processes
----------------------------------

Running parallel Logtalk processes is enabled by setting the
:ref:`clean <flag_clean>` flag to ``on``. This is the default flag value
in the backend adapter files. With this setting, the intermediate Prolog
files generated by the Logtalk compiler include the processes identifier
in the names, thus preventing file names clashes when running parallel
processes. When the flag is turned off, the generated intermediate Prolog
file names don't include the process identifier and are kept between runs.
This is usually done to avoid repeated recompilation of stable code when
developing large applications or when running multiple test sets for
performance (by avoiding repeated recompilation of the ``lgtunit`` tool).

To run parallel Logtalk processes with the ``clean`` flag turned off, each
process must use its own :term:`scratch directory`. This is accomplished
by defining the ``scratch_directory`` library alias to a per process
location. For example, assuming we're using GNU Prolog as the backend, a
possible definition could be:

::

   :- multifile(logtalk_library_path/2).
   :- dynamic(logtalk_library_path/2).
   
   logtalk_library_path(scratch_directory, Directory) :-
       temporary_name(lgtXXXXXX, Name),
       decompose_file_name(Name, _, Prefix, _),
       atom_concat('/tmp/', Prefix, Directory),
       (   file_exists(Directory) ->
           true
       ;   make_directory(Directory)
       ).

Assuming the code above is saved in a ``parallel_logtalk_processes_setup.pl``
file, we would then start Logtalk using:

.. code-block:: bash

   $ gplgt --init-goal "consult('parallel_logtalk_processes_setup.pl')"

The details on how to define and load the definition of the ``scratch_directory``
library alias are, however, backend specific (due to the lack of Prolog
standardization) and possibly also operating-system specific (different
locations for the temporary directory). The Logtalk library contains support
for selected backends.

.. _programming_source_files:

Source files
------------

Logtalk source files may define any number of entities (objects,
categories, or protocols). Source files may also contain Prolog code
interleaved with Logtalk entity definitions. Plain Prolog code is usually
copied as-is to the corresponding Prolog output file (except, of course,
if subject to the :ref:`term-expansion mechanism <expansion_expansion>`).
Prolog modules are compiled as objects. The following Prolog directives are
processed when read (thus affecting the compilation of the source code that
follows): ``ensure_loaded/1``, ``use_module/1-2``, ``op/3``, and
``set_prolog_flag/2``. The :ref:`directives_initialization_1` directive may
be used for defining an initialization goal to be executed when loading a
source file.

Logtalk source files can include the text of other files by using the
:ref:`directives_include_1` directive. Although there is also a standard
Prolog ``include/1`` directive, any occurrences of this directive in a
Logtalk source file is handled by the Logtalk compiler,
not by the :term:`backend Prolog compiler`, to improve portability.

When writing a Logtalk source file the following advice applies:

- When practical and when performance is critical, define each entity on
  its own source file.
- Source file loading order can impact performance (e.g. if an object
  imports a category defined in a source file loaded after the object
  source file, no static binding optimizations will be possible).
- Initialization directives that result in the compilation and loading of
  other source files (e.g. libraries) should preferably be written in the
  application loader file to ensure the availability of the entities
  they define when compiling the application source files (thus enabling
  static binding optimizations).

Naming conventions
^^^^^^^^^^^^^^^^^^

When defining each entity in its own source file, it is recommended that
the source file be named after the entity identifier. For parametric objects,
the identifier arity can be appended to the identifier functor. By default,
all Logtalk source files use the extension ``.lgt`` but this is optional
and can be set in the adapter files. For example, we may define an object
named ``vehicle`` and save it in a ``vehicle.lgt`` source file. A ``sort(_)``
parametric object would be saved it on a ``sort_1.lgt`` source file.

Source file text encoding
^^^^^^^^^^^^^^^^^^^^^^^^^

The text encoding used in a source file may be declared using the
:ref:`directives_encoding_1` directive when running Logtalk with
backend Prolog compilers that support multiple encodings (check the
:ref:`encoding_directive <flag_encoding_directive>` flag in the
adapter file of your Prolog compiler).

.. _programming_multi_pass_compiler:

Multi-pass compiler
-------------------

Logtalk is implemented using a *multi-pass* compiler. In comparison,
some Prolog systems use a multi-pass compiler while others use a single-pass
compiler. While there are pros and cons with each solution, the most relevant
consequence in this context is for the content of source files. In Logtalk,
entities and predicates only become available (for the runtime system) after
the source file is successfully compiled and loaded. This may prevent some
compiler optimizations, notably :term:`static binding`, if some of the referred
entities are defined in the same source file. On the other hand, the order of
predicate directives and predicate definitions is irrelevant. In contrast,
in a system implemented using a single-pass compiler, the order of the source
file terms can and often is significant for proper and successful compilation.
In these systems, predicates may become available for calling as soon as they
are compiled even if the remaining of the source file is yet to be compiled.

The Logtalk compiler reads source files using the Prolog standard ``read_term/3``
predicate. This ensures compatibility with any syntax extensions that the
used backend may implement. In the first compiler stage, all source file
terms are read and data about all defined entities, directives, predicates,
and grammar rules is collected. Any defined :ref:`term-expansion rules <expansion_expansion>`
are applied to the read terms. Grammar rules are expanded into predicate
clauses unless expanded by user-defined term-expansion rules. The second
stage compiles all initialization goals and clause bodies, taking advantage
of the data collected in the first stage, and applying any defined
goal-expansion rules. Depending on the compilation mode, the generated
code can be instrumented for debugging tools or optimized for performance.
Linter checks are performed during these two first stages. The final step
in the second stage is to write the generated intermediate Prolog code
into a temporary file. In the third and final stage, this intermediate
Prolog file is compiled and loaded by the used backend. These intermediate
files are deleted by default after loading (see the :ref:`clean <flag_clean>`
flag description for details).

.. _programming_compiling:

Compiling and loading your applications
---------------------------------------

Your applications will be made of source files containing your objects,
protocols, and categories. The source files can be compiled to disk by
calling the :ref:`predicates_logtalk_compile_1` built-in predicate:

.. code-block:: text

   | ?- logtalk_compile([source_file1, source_file2, ...]).

This predicate runs the compiler on each file and, if no fatal errors
are found, outputs Prolog source files that can then be consulted or
compiled in the usual way by your Prolog compiler.

To compile to disk and also load into memory the source files we can use
the :ref:`predicates_logtalk_load_1` built-in predicate:

.. code-block:: text

   | ?- logtalk_load([source_file1, source_file2, ...]).

This predicate works in the same way of the predicate
``logtalk_compile/1`` but also loads the compiled files into memory.

Both predicates expect a source file name or a list of source file names
as an argument. The Logtalk source file name extension, as defined in
the adapter file (by default, ``.lgt``), can be omitted.

If you have more than a few source files then you may want to use a
:term:`loader file` helper file containing the calls to the ``logtalk_load/1-2``
predicates. Consulting or compiling the loader file will then compile
and load all your Logtalk entities into memory (see below for details).

With most :term:`backend Prolog compilers <backend Prolog compiler>`, you
can use the shorthands ``{File}`` for ``logtalk_load(File)`` and
``{File1, File2, ...}`` for ``logtalk_load([File1, File2, ...])``. The use
these shorthands should be restricted to the Logtalk/Prolog top-level
interpreter as they are not part of the language specification and may be
commented out in case of conflicts with backend Prolog compiler features.

The built-in predicate :ref:`predicates_logtalk_make_0` can be used to
reload all modified source files. With most backend Prolog compilers,
you can also use the ``{*}`` top-level shortcut. Files are also reloaded
when the compilation mode changes. An extended version of this predicate,
:ref:`predicates_logtalk_make_1`, accepts multiple targets including
``all``, ``clean``, ``check``, ``circular``, ``documentation``, ``caches``,
``debug``, ``normal``, and ``optimal``. For example, assume that you have
loaded your application files and found a bug. You can easily recompile the
files in debug mode by using the ``logtalk_make(debug)`` goal. After
debugging and fixing the bug, you can reload the files in normal mode
using the ``logtalk_make(normal)`` or in optimized mode using the
``logtalk_make(optimal)`` goal. See the predicates documentation for a
complete list of targets and top-level shortcuts. In particular, the
``logtalk_make(clean)`` goal can be specially useful before switching
backend Prolog compilers as the generated intermediate files may not be
compatible. The ``logtalk_make(caches)`` goal is usually used when
benchmarking compiler performance improvements.

.. _programming_errors_warnings_comments:

Compiler errors, warnings, and comments
---------------------------------------

Following a Prolog tradition inherited from Quintus Prolog, the compiler
prefixes (by default) errors with a ``!`` and warnings with a ``*``. For
example:

.. code-block:: text

   !     Existence error: directive object/1 does not exist
   !       in directive end_object/0
   !       in file /home/jdoe/logtalk/examples/errors/unmatched_directive.lgt at or above line 27
   
   *     No matching clause for goal: baz(a)
   *       while compiling object main_include_compiler_warning
   *       in file /home/jdoe/logtalk/examples/errors/include_compiler_warning.lgt between lines 38-39

Compiler comments are prefixed by ``%``. For example:

.. code-block:: text

   ?- {ack(loader)}.
   % [ /home/jdoe/logtalk/examples/ack/ack.lgt loaded ]
   % [ /home/jdoe/logtalk/examples/ack/loader.lgt loaded ]
   % (0 warnings)
   true.

.. _programming_loaders:

Loader files
------------

If you look into the Logtalk distribution, you will notice that most source
code directories (e.g. of tools, libraries, and examples) contain a *driver
file* that can be used to load all included source files and any required
libraries. These loader files are usually named ``loader.lgt`` or contain
the word *loader* in their name. Loader files are ordinary source files and
thus compiled and loaded like any source file. By also defining a loader file
for your project, you can then load it by simply typing:

.. code-block:: text

   | ?- {loader}.

Another driver file, usually named ``tester.lgt`` (or containing the word
*tester* in its name) is commonly used to load and run tests. By also
defining a tester file for your project, you can then run its tests by
simply typing:

.. code-block:: text

   | ?- {tester}.

Usually these driver files contain calls to the built-in predicates
:ref:`predicates_set_logtalk_flag_2` (e.g. for setting global,
*project-specific*, flag values) and :ref:`predicates_logtalk_load_1` or
:ref:`predicates_logtalk_load_2` (for loading project files), wrapped
inside a Prolog ``initialization/1`` directive for portability. For
instance, if your code is split in three source files named
``source1.lgt``, ``source2.lgt``, and ``source3.lgt``, then the contents
of your loader file could be:

::

   :- initialization((
       % set project-specific global flags
       set_logtalk_flag(events, allow),
       % load the project source files
       logtalk_load([source1, source2, source3])
   )).

Another example of directives that are often used in a loader file would
be ``op/3`` directives declaring global operators needed by your
project. Loader files are also often used for setting source
file-specific compiler flags (this is useful even when you only have a
single source file if you always load it with using the same set of
compiler flags). For example:

::

   :- initialization((
       % set project-specific global flags
       set_logtalk_flag(underscore_variables, dont_care),
       set_logtalk_flag(source_data, off),
       % load the project source files
       logtalk_load(
           [source1, source2, source3],
           % source file-specific flags
           [portability(warning)]),
       logtalk_load(
           [source4, source5],
           % source file-specific flags
           [portability(silent)])
   )).

To take the best advantage of loader and tester files, define a clause for
the multifile and dynamic ``logtalk_library_path/2`` predicate for the
directory containing your source files as explained in the next section.

When your project also uses Prolog module resources, the loader file is
also the advised place to load them, preferably without any exports.
For example:

::

   :- use_module(library(clpfd), []).
   ...

   :- initialization((
       ...
   )).

Complex projects often use a main loader file that loads the loader files
of each of the project components. Thus, loader files provide a central
point to understand a project organization and dependencies.

Worth mentioning here a common mistake when first starting working with loader
files. New users sometimes try to set compiler flags using ``logtalk_load/2``
when loading a loader file. For example, by writing:

.. code-block:: text

   | ?- logtalk_load(loader, [optimize(on)]).

This will not work as you might expect as the compiler flags will only
be used in the compilation of the ``loader.lgt`` file itself and will
not affect the compilation of files loaded through the
``initialization/1`` directive contained on the loader file.

.. _programming_libraries:

Libraries of source files
-------------------------

Logtalk defines a *library* simply as a directory containing source
files. Library locations can be specified by defining or asserting
clauses for the dynamic and multifile predicate
:ref:`predicates_logtalk_library_path_2`. For example:

::

   :- multifile(logtalk_library_path/2).
   :- dynamic(logtalk_library_path/2).

   logtalk_library_path(shapes, '$LOGTALKUSER/examples/shapes/').

The first argument of the predicate is used as an alias for the path on
the second argument. Library aliases may also be used on the second
argument. For example:

::

   :- multifile(logtalk_library_path/2).
   :- dynamic(logtalk_library_path/2).

   logtalk_library_path(lgtuser, '$LOGTALKUSER/').
   logtalk_library_path(examples, lgtuser('examples/')).
   logtalk_library_path(viewpoints, examples('viewpoints/')).

This allows us to load a library source file without the need to first
change the current working directory to the library directory and then
back to the original directory. For example, in order to load a
``loader.lgt`` file, contained in a library named ``viewpoints``, we
just need to type:

.. code-block:: text

   | ?- logtalk_load(viewpoints(loader)). 

The best way to take advantage of this feature is to load at startup a source
file containing clauses for the ``logtalk_library_path/2`` predicate needed
for all available libraries (typically, using a :term:`settings file`, as
discussed below). This allows us to load library source files or entire
libraries without worrying about libraries paths, improving code portability.
The directory paths on the second argument should always end with the path
directory separator character. Most backend Prolog compilers allows the use
of environment variables in the second argument of the ``logtalk_library_path/2``
predicate. Use of POSIX relative paths (e.g. ``'../'`` or ``'./'``) for
top-level library directories (e.g. ``lgtuser`` in the example above) is
not advised as different backend Prolog compilers may start with
different initial working directories, which may result in portability
problems of your loader files.

This :term:`library notation` provides functionality inspired by the
``file_search_path/2`` mechanism introduced by Quintus Prolog and later
adopted by some other Prolog compilers but with a key difference: there
is no fragile search mechanism and the Logtalk ``make`` can be used to
check for duplicated library aliases. Multiple definitions for the
same alias are problematic when using external dependencies as any
third-party update to those dependencies can introduce file name clashes.
Note that the potential for these clashes cannot be reliably minimized by
a careful ordering of the ``logtalk_library_path/2`` predicate clauses
due to this predicate being multifile and dynamic.

.. _programming_settings:

Settings files
--------------

Although is always possible to edit the :term:`backend Prolog compiler` adapter
files, the recommended solution to customize compiler flags is to create a
``settings.lgt`` file in the Logtalk user folder or in the user home folder.
Depending on the backend Prolog compiler and on the operating-system,
is also possible to define per-project settings files by creating a
``settings.lgt`` file in the project directory and by starting Logtalk from
this directory. At startup, Logtalk tries to load a ``settings.lgt`` file
from the following directories, searched in sequence:

- Startup directory (``$LOGTALK_STARTUP_DIRECTORY``)
- Logtalk user directory (``$LOGTALKUSER``)
- User home directory (``$HOME``; ``%USERPROFILE%`` on Windows if ``%HOME%`` is not defined)
- Application data directory (``%APPDATA%\Logtalk``; only on Windows)
- Config directory (``$XDG_CONFIG_HOME/logtalk``)
- Default config directory (``$HOME/.config/logtalk/``)

The startup directory is only searched when the read-only
:ref:`settings_file <flag_settings_file>` flag is set to ``allow``.
When no settings files are found, Logtalk will use the default compiler flag
values set on the backend Prolog compiler adapter files. When limitations of
the backend Prolog compiler or on the operating-system prevent Logtalk from
finding the settings files, these can always be loaded manually after Logtalk
startup.

Settings files are normal Logtalk source files (although when automatically
loaded by Logtalk they are compiled and loaded silently with any errors being
reported but otherwise ignored). The usual contents is an
``initialization/1`` Prolog directive containing calls to the
:ref:`predicates_set_logtalk_flag_2`
Logtalk built-in predicate and asserting clauses for the
:ref:`predicates_logtalk_library_path_2`
multifile dynamic predicate. Note that the
:ref:`directives_set_logtalk_flag_2`
directive cannot be used as its scope is local to the source file being
compiled.

One of the troubles of writing portable applications is the different
feature sets of Prolog compilers. Using the Logtalk support for
conditional compilation and the :ref:`prolog_dialect <flag_prolog_dialect>`
flag we can write a single settings file that can be used with several
:term:`backend Prolog compilers <backend Prolog compiler>`:

::

   :- if(current_logtalk_flag(prolog_dialect, yap)).

       % YAP specific settings
       ...

   :- elif(current_logtalk_flag(prolog_dialect, gnu)).

       % GNU Prolog specific settings
       ...

   :- else.

       % generic Prolog settings

   :- endif.

The Logtalk distribution includes a ``settings-sample.lgt`` sample file with
commented out code snippets for common settings.

.. _programming_linter:

Compiler linter
---------------

The compiler includes a :doc:`../devtools/linter` that checks for a wide range
of possible problems in source files. Notably, the compiler checks for unknown
entities, unknown predicates, undefined predicates (i.e. predicates that
are declared but not defined), missing directives (including missing
``dynamic/1`` and ``meta_predicate/1`` directives), redefined built-in
predicates, calls to non-portable predicates, singleton variables, goals that
are always true or always false (i.e. goals that are can be replaced by
``true`` or ``fail``), and trivial fails (i.e. calls to predicates with no
match clauses). Most of the linter warnings are controlled by
:ref:`compiler flags <programming_flags_lint>`. See the next section
for details.

.. _programming_flags:

Compiler flags
--------------

The :ref:`predicates_logtalk_load_1` and :ref:`predicates_logtalk_compile_1`
always use the current set of default compiler flags as specified in
your settings file and the Logtalk adapter files or changed for the
current session using the built-in predicate
:ref:`predicates_set_logtalk_flag_2`.
Although the default flag values cover the usual cases, you may want to
use a different set of flag values while compiling or loading some of
your Logtalk source files. This can be accomplished by using the
:ref:`predicates_logtalk_load_2` or the :ref:`predicates_logtalk_compile_2`
built-in predicates. These two predicates accept a list of options
affecting how a Logtalk source file is compiled and loaded:

.. code-block:: text

   | ?- logtalk_compile(Files, Options).

or:

.. code-block:: text

   | ?- logtalk_load(Files, Options).

In fact, the ``logtalk_load/1`` and ``logtalk_compile/1`` predicates are
just shortcuts to the extended versions called with the default compiler
flag values. The options are represented by a compound term where the
functor is the flag name and the sole argument is the flag value.

We may also change the default flag values from the ones loaded from the
adapter file by using the :ref:`predicates_set_logtalk_flag_2`
built-in predicate. For example:

.. code-block:: text

   | ?- set_logtalk_flag(unknown_entities, silent).

The current default flags values can be enumerated using the
:ref:`predicates_current_logtalk_flag_2` built-in predicate:

.. code-block:: text

   | ?- current_logtalk_flag(unknown_entities, Value).

   Value = silent
   yes

Logtalk also implements a :ref:`directives_set_logtalk_flag_2`
directive, which can be used to set flags within a source file or within
an entity. For example:

::

   % compile objects in this source file with event support
   :- set_logtalk_flag(events, allow).

   :- object(foo).

       % compile this object with support
       % for dynamic predicate declarations
       :- set_logtalk_flag(dynamic_declarations, allow).
       ...

   :- end_object.

   ...

Note that the scope of the ``set_logtalk_flag/2`` directive is local to
the entity or to the source file containing it.

.. note::

   Applications should never rely on default flag values for working
   properly.  Whenever the compilation of a source file or an entity
   requires a specific flag value, the flag should be set explicitly
   in the entity, in the source file, or in the loader file.

Read-only flags
^^^^^^^^^^^^^^^

Some flags have read-only values and thus cannot be changed at runtime. Their
values are defined in the Prolog backend :term:`adapter files <adapter file>`
These are:

.. _flag_settings_file:
.. index:: pair: settings_file; Flag

``settings_file``
   Allows or disables loading of a :term:`settings file` at startup.
   Possible values are ``allow``, ``restrict``, and ``deny``. The usual
   default value is ``allow`` but it can be changed by editing the adapter
   file when e.g. embedding Logtalk in a compiled application. With a value
   of ``allow``, settings files are searched in the startup directory,
   in the Logtalk user directory, in the user home directory, in the
   ``APPDATA`` if running on Windows, and in the XDG configuration directory.
   With a value of ``restrict``, the search for the settings files skips the
   startup directory.

.. _flag_prolog_dialect:
.. index:: pair: prolog_dialect; Flag

``prolog_dialect``
   Identifier of the :term:`backend Prolog compiler` (an atom). This flag can be used
   for :ref:`conditional compilation <conditional_compilation_directives>`
   of Prolog compiler specific code.

.. _flag_prolog_version:
.. index:: pair: prolog_version; Flag

``prolog_version``
   Version of the :term:`backend Prolog compiler` (a compound term,
   ``v(Major, Minor, Patch)``, whose arguments are integers). This flag
   availability depends on the Prolog compiler. Checking the value of
   this flag fails for any Prolog compiler that does not provide access
   to version data.

.. _flag_prolog_compatible_version:
.. index:: pair: prolog_compatible_version; Flag

``prolog_compatible_version``
   Compatible version of the :term:`backend Prolog compiler` (a compound term,
   usually with the format ``@>=(v(Major, Minor, Patch))``, whose
   arguments are integers). This flag availability depends on the Prolog
   compiler. Checking the value of this flag fails for any Prolog
   compiler that does not provide access to version data.

.. _flag_unicode:
.. index:: pair: unicode; Flag

``unicode``
   Informs Logtalk if the :term:`backend Prolog compiler` supports the Unicode
   standard. Possible flag values are ``unsupported``, ``full`` (all
   Unicode planes supported), and ``bmp`` (supports only the Basic
   Multilingual Plane).

.. _flag_encoding_directive:
.. index:: pair: encoding_directive; Flag

``encoding_directive``
   Informs Logtalk if the :term:`backend Prolog compiler` supports the
   :ref:`directives_encoding_1` directive.
   This directive is used for declaring the text encoding of source
   files. Possible flag values are ``unsupported``, ``full`` (can be
   used in both Logtalk source files and compiler generated Prolog
   files), and ``source`` (can be used only in Logtalk source files).

.. _flag_tabling:
.. index:: pair: tabling; Flag

``tabling``
   Informs Logtalk if the :term:`backend Prolog compiler` provides tabling
   programming support. Possible flag values are ``unsupported`` and
   ``supported``.

.. _flag_engines:
.. index:: pair: engines; Flag

``engines``
   Informs if the :term:`backend Prolog compiler` provides the required low
   level multi-threading programming support for Logtalk
   :term:`threaded engines <threaded engine>`. Possible flag values
   are ``unsupported`` and ``supported``.

.. _flag_threads:
.. index:: pair: threads; Flag

``threads``
   Informs if the :term:`backend Prolog compiler` provides the required low
   level multi-threading programming support for all high-level Logtalk
   :ref:`multi-threading features <threads_threads>`. Possible flag
   values are ``unsupported`` and ``supported``.

.. _flag_modules:
.. index:: pair: modules; Flag

``modules``
   Informs Logtalk if the :term:`backend Prolog compiler` provides suitable
   module support. Possible flag values are ``unsupported`` and
   ``supported`` (independently of this flag, Logtalk provides limited support
   for compiling Prolog modules as objects).

.. _flag_coinduction:
.. index:: pair: coinduction; Flag

``coinduction``
   Informs Logtalk if the :term:`backend Prolog compiler` provides the
   required minimal support for cyclic terms necessary for working with
   :term:`coinductive predicates <coinductive predicate>`. Possible flag
   values are ``unsupported`` and ``supported``.

Version flags
^^^^^^^^^^^^^

.. _flag_version_data:
.. index:: pair: version_data; Flag

``version_data(Value)``
   Read-only flag whose value is the compound term
   ``logtalk(Major,Minor,Patch,Status)``. The first three arguments are
   integers and the last argument is an atom, possibly empty,
   representing version status: ``aN`` for alpha versions, ``bN`` for
   beta versions, ``rcN`` for release candidates (with ``N`` being a
   natural number), and ``stable`` for stable versions. The
   ``version_data`` flag is also a de facto standard for Prolog
   compilers.

.. _programming_flags_lint:

Lint flags
^^^^^^^^^^

.. _flag_unknown_entities:
.. index:: pair: unknown_entities; Flag

``unknown_entities(Option)``
   Controls the unknown entity warnings, resulting from loading an
   entity that references some other entity that is not currently
   loaded. Possible option values are ``warning`` (the usual default)
   and ``silent``. Note that these warnings are not always avoidable,
   specially when using reflective designs of class-based hierarchies.

.. _flag_unknown_predicates:
.. index:: pair: unknown_predicates; Flag

``unknown_predicates(Option)``
   Defines the compiler behavior when unknown messages or calls to unknown
   predicates (or non-terminals) are found. An unknown message is a message
   sent to an object that is not part of the object protocol. An unknown
   predicate is a called predicate that is neither locally declared or
   defined. Possible option values are ``error``, ``warning`` (the usual
   default), and ``silent`` (not recommended).

.. _flag_undefined_predicates:
.. index:: pair: undefined_predicates; Flag

``undefined_predicates(Option)``
   Defines the compiler behavior when calls to declared but undefined
   predicates (or non-terminals) are found. Note that these calls will
   fail at runtime as per closed-world assumption. Possible option values
   are ``error``, ``warning`` (the usual default), and ``silent`` (not
   recommended).

.. _flag_steadfastness:
.. index:: pair: steadfastness; Flag

``steadfastness(Option)``
   Controls warnings about *possible* non :term:`steadfast <steadfastness>`
   predicate definitions due to variable aliasing at a clause head and a cut
   in the clause body. Possible option values are ``warning`` and ``silent``
   (the usual default due to the possibility of false positives).

.. _flag_portability:
.. index:: pair: portability; Flag

``portability(Option)``
   Controls the non-ISO specified Prolog built-in predicate and non-ISO
   specified Prolog built-in arithmetic function calls warnings plus use
   of non-standard Prolog flags and/or flag values. Possible option
   values are ``warning`` and ``silent`` (the usual default).

.. _flag_deprecated:
.. index:: pair: deprecated; Flag

``deprecated(Option)``
   Controls the deprecated predicate warnings. Possible option
   values are ``warning`` (the usual default) and ``silent``.

.. _flag_missing_directives:
.. index:: pair: missing_directives; Flag

``missing_directives(Option)``
   Controls the missing predicate directive warnings. Possible option
   values are ``warning`` (the usual default) and ``silent`` (not
   recommended).

.. _flag_duplicated_directives:
.. index:: pair: duplicated_directives; Flag

``duplicated_directives(Option)``
   Controls the duplicated predicate directive warnings. Possible option
   values are ``warning`` (the usual default) and ``silent`` (not
   recommended). Note that conflicting directives for the same predicate
   are handled as errors, not as duplicated directive warnings.

.. _flag_trivial_goal_fails:
.. index:: pair: trivial_goal_fails; Flag

``trivial_goal_fails(Option)``
   Controls the printing of warnings for calls to local static predicates
   with no matching clauses. Possible option values are ``warning`` (the
   usual default) and ``silent`` (not recommended).

.. _flag_always_true_or_false_goals:
.. index:: pair: always_true_or_false_goals; Flag

``always_true_or_false_goals(Option)``
   Controls the printing of warnings for goals that are always true or
   false. Possible option values are ``warning`` (the usual default) and
   ``silent`` (not recommended).

.. _flag_grammar_rules:
.. index:: pair: grammar_rules; Flag

``grammar_rules(Option)``
   Controls the printing of grammar rules related warnings. Possible
   option values are ``warning`` (the usual default) and ``silent`` (not
   recommended).

.. _flag_arithmetic_expressions:
.. index:: pair: arithmetic_expressions; Flag

``arithmetic_expressions(Option)``
   Controls the printing of arithmetic expressions related warnings. Possible
   option values are ``warning`` (the usual default) and ``silent`` (not
   recommended).

.. _flag_lambda_variables:
.. index:: pair: lambda_variables; Flag

``lambda_variables(Option)``
   Controls the printing of lambda variable related warnings. Possible
   option values are ``warning`` (the usual default) and ``silent`` (not
   recommended).

.. _flag_suspicious_calls:
.. index:: pair: suspicious_calls; Flag

``suspicious_calls(Option)``
   Controls the printing of suspicious call warnings. Possible option
   values are ``warning`` (the usual default) and ``silent`` (not
   recommended).

.. _flag_redefined_built_ins:
.. index:: pair: redefined_built_ins; Flag

``redefined_built_ins(Option)``
   Controls the Logtalk and Prolog built-in predicate redefinition warnings.
   Possible option values are ``warning`` and ``silent`` (the usual default).
   Warnings about redefined Prolog built-in predicates are often the result
   of running a Logtalk application on several Prolog compilers as each
   Prolog compiler defines its set of built-in predicates.

.. _flag_redefined_operators:
.. index:: pair: redefined_operators; Flag

``redefined_operators(Option)``
   Controls the Logtalk and Prolog built-in operator redefinition warnings.
   Possible option values are ``warning`` (the usual default) and ``silent``.
   Redefining Logtalk operators or standard Prolog operators can break term
   parsing causing syntax errors or change how terms are parsed introducing
   bugs.

.. _flag_singleton_variables:
.. index:: pair: singleton_variables; Flag

``singleton_variables(Option)``
   Controls the singleton variable warnings. Possible option values are
   ``warning`` (the usual default) and ``silent`` (not recommended).

.. _flag_underscore_variables:
.. index:: pair: underscore_variables; Flag

``underscore_variables(Option)``
   Controls the interpretation of variables that start with an
   underscore (excluding the anonymous variable) that occur once in a
   term as either don't care variables or singleton variables. Possible
   option values are ``dont_care`` and ``singletons`` (the usual
   default). Note that, depending on your Prolog compiler, the
   ``read_term/3`` built-in predicate may report variables that start
   with an underscore as singleton variables. There is no standard
   behavior, hence this option.

.. _flag_naming:
.. index:: pair: naming; Flag

``naming(Option)``
   Controls warnings about entity, predicate, and variable names per
   official coding guidelines (which advise using underscores for entity
   and predicate names and camel case for variable names). Additionally,
   variable names should not differ only on case. Possible option values
   are ``warning`` and ``silent`` (the usual default due to the current
   limitation to ASCII names and the computational cost of the checks).

.. _flag_duplicated_clauses:
.. index:: pair: duplicated_clauses; Flag

``duplicated_clauses(Option)``
   Controls warnings of duplicated entity clauses (and duplicated entity
   grammar rules). Possible option values are ``warning`` and ``silent``
   (the usual default due to the required heavy computations). When the
   term-expansion mechanism is used and results in duplicated clauses,
   the reported line numbers are for lines of the original clauses that
   were expanded.

.. _flag_disjunctions:
.. index:: pair: disjunctions; Flag

``disjunctions(Option)``
   Controls warnings on clauses where the body is a disjunction. Possible
   option values are ``warning`` (the usual default) and ``silent``. As
   per coding guidelines, in most cases, these clauses can be rewritten
   using a clause per disjunction branch for improved code readability.

.. _flag_conditionals:
.. index:: pair: conditionals; Flag

``conditionals(Option)``
   Controls warnings on if-then-else and soft-cut control constructs. Possible
   option values are ``warning`` (the usual default) and ``silent``. Warnings
   include misuse of cuts, potential bugs in the test part, and missing else
   part (lack of compliance with coding guidelines).

.. _flag_catchall_catch:
.. index:: pair: catchall_catch; Flag

``catchall_catch(Option)``
   Controls warnings on ``catch/3`` goals that catch all exceptions. Possible
   option values are ``warning`` and ``silent`` (the usual default). Lack of
   standardization often makes it tricky or cumbersome to avoid too generic
   ``catch/3`` goals when writing portable code.

.. _flag_tail_recursive:
.. index:: pair: tail_recursive; Flag

``tail_recursive(Option)``
   Controls warnings of non-tail recursive predicate (and non-terminal)
   definitions. The lint check does not detect all cases of non-tail
   recursive predicate definitions, however. Also, definitions that
   make two or more recursive calls are not reported as usually they
   cannot be changed to be tail recursive. Possible option values are
   ``warning`` and ``silent`` (the usual default).

Optional features compilation flags
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. _flag_complements:
.. index:: pair: complements; Flag

``complements(Option)``
   Allows objects to be compiled with support for complementing
   categories turned off in order to improve performance and security.
   Possible option values are ``allow`` (allow complementing categories
   to override local object predicate declarations and definitions),
   ``restrict`` (allow complementing categories to add predicate
   declarations and definitions to an object but not to override them),
   and ``deny`` (ignore complementing categories; the usual default).
   This option can be used on a per-object basis. Note that changing
   this option is of no consequence for objects already compiled and
   loaded.

.. _flag_dynamic_declarations:
.. index:: pair: dynamic_declarations; Flag

``dynamic_declarations(Option)``
   Allows objects to be compiled with support for dynamic declaration of
   new predicates turned off in order to improve performance and
   security. Possible option values are ``allow`` and ``deny`` (the
   usual default). This option can be used on a per-object basis. Note
   that changing this option is of no consequence for objects already
   compiled and loaded. This option is only checked when sending an
   :ref:`methods_asserta_1` or :ref:`methods_assertz_1` message to an
   object. Local asserting of new predicates is always allowed.

.. _flag_events:
.. index:: pair: events; Flag

``events(Option)``
   Allows message sending calls to be compiled with or without
   :ref:`event-driven programming <events_events>` support. Possible
   option values are ``allow`` and ``deny`` (the usual default). Objects
   (and categories) compiled with this option set to ``deny`` use
   optimized code for message-sending calls that does not trigger
   events. As such, this option can be used on a per-object (or
   per-category) basis. Note that changing this option is of no
   consequence for objects already compiled and loaded.

.. _flag_context_switching_calls:
.. index:: pair: context_switching_calls; Flag

``context_switching_calls(Option)``
   Allows context switching calls (``(<<)/2``) to be either allowed or
   denied. Possible option values are ``allow`` and ``deny``. The
   default flag vale is ``allow``. Note that changing this option is of
   no consequence for objects already compiled and loaded.

Backend Prolog compiler and loader flags
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. _flag_prolog_compiler:
.. index:: pair: prolog_compiler; Flag

``prolog_compiler(Flags)``
   List of compiler flags for the generated Prolog files. The valid
   flags are specific to the used Prolog backend compiler. The usual
   default is the empty list. These flags are passed to the backend
   Prolog compiler built-in predicate that is responsible for compiling
   to disk a Prolog file. For Prolog compilers that don't provide
   separate predicates for compiling and loading a file, use instead
   the :ref:`prolog_loader <flag_prolog_loader>` flag.

.. _flag_prolog_loader:
.. index:: pair: prolog_loader; Flag

``prolog_loader(Flags)``
   List of loader flags for the generated Prolog files. The valid flags
   are specific to the used Prolog backend compiler. The usual default
   is the empty list. These flags are passed to the backend Prolog
   compiler built-in predicate that is responsible for loading a
   (compiled) Prolog file.

Other flags
^^^^^^^^^^^

.. _flag_scratch_directory:
.. index:: pair: scratch_directory; Flag

``scratch_directory(Directory)``
   Sets the directory to be used to store the temporary files generated
   when compiling Logtalk source files. This directory can be specified
   using an atom or using :term:`library notation`. The directory must
   always end with a slash. The default value is a sub-directory of the
   source files directory, either ``'./lgt_tmp/'`` or ``'./.lgt_tmp/'``
   (depending on the backend Prolog compiler and operating-system).
   Relative directories must always start with ``'./'`` due to the lack
   of a portable solution to check if a path is relative or absolute.
   The default value set on the :term:`backend Prolog compiler` adapter
   file can be overriden by defining the ``scratch_directory`` library
   alias (see the :ref:`predicates_logtalk_library_path_2`
   predicate documentation for details).

.. _flag_report:
.. index:: pair: report; Flag

``report(Option)``
   Controls the default printing of messages. Possible option values are
   ``on`` (by usual default, print all messages that are not intercepted
   by the user), ``warnings`` (only print warning and error messages
   that are not intercepted by the user), and ``off`` (do not print any
   messages that are not intercepted by the user).

.. _flag_code_prefix:
.. index:: pair: code_prefix; Flag

``code_prefix(Character)``
   Enables the definition of prefix for all functors of Prolog code
   generated by the Logtalk compiler. The option value must be a single
   character atom. Its default value is ``'$'``. Specifying a code
   prefix provides a way to solve possible conflicts between Logtalk
   compiled code and other Prolog code. In addition, some Prolog
   compilers automatically hide predicates whose functor start with a
   specific prefix such as the character ``$``. Although this is not a
   read-only flag, it should only be changed at startup time and **before**
   loading any source files. When changing this flag (e.g. from a
   :term:`settings file`), restart with the :ref:`clean <flag_clean>`
   flag turned on to ensure that any compiled files using the old
   ``code_prefix`` value will be recompiled.

.. _flag_optimize:
.. index:: pair: optimize; Flag

``optimize(Option)``
   Controls the compiler optimizations. Possible option values are
   ``on`` (used by default for deployment) and ``off`` (used by default
   for development). Compiler optimizations include the use of static
   binding whenever possible, the removal of redundant calls to
   ``true/0`` from predicate clauses, the removal of redundant
   unifications when compiling grammar rules, and inlining of predicate
   definitions with a single clause that links to a local predicate, to
   a plain Prolog built-in (or foreign) predicate, or to a Prolog module
   predicate with the same arguments. Care should be taken when
   developing applications with this flag turned on as changing and
   reloading a file may render :term:`static binding` optimizations
   invalid for code defining in other loaded files. Turning on this
   flag automatically turns off the :ref:`debug <flag_debug>` flag.

.. _flag_source_data:
.. index:: pair: source_data; Flag

``source_data(Option)``
   Defines how much information is retained when compiling a source
   file. Possible option values are ``on`` (the usual default for
   development) and ``off``. With this flag set to ``on``, Logtalk will
   keep the information represented using documenting directives plus
   source location data (including source file names and line numbers).
   This information can be retrieved using the
   :ref:`reflection API <reflection_reflection>` and is useful for
   documenting, debugging, and integration with third-party development
   tools. This flag can be turned off in order to generate more compact
   code.

.. _flag_debug:
.. index:: pair: debug; Flag

``debug(Option)``
   Controls the compilation of source files in debug mode (the Logtalk
   default debugger can only be used with files compiled in this mode).
   Also controls, by default, printing of ``debug>`` and
   ``debug(Topic)`` messages. Possible option values are ``on`` and
   ``off`` (the usual default). Turning on this flag automatically turns
   off the :ref:`optimize <flag_optimize>` flag.

.. _flag_reload:
.. index:: pair: reload; Flag

``reload(Option)``
   Defines the reloading behavior for source files. Possible option
   values are ``skip`` (skip reloading of already loaded files; this value
   can be used to get similar functionality to the Prolog directive
   ``ensure_loaded/1`` but should be used only with fully debugged
   code), ``changed`` (the usual default; reload files only when they
   are changed since last loaded provided that any explicit flags
   and the compilation mode are the same as before), and ``always``
   (always reload files).

.. _flag_relative_to:
.. index:: pair: relative_to; Flag

``relative_to(Directory)``
   Defines a base directory for resolving relative source file paths.
   The default value is the directory of the source file being compiled.

.. _flag_hook:
.. index:: pair: hook; Flag

``hook(Object)``
   Allows the definition of an object (which can be the pseudo-object
   :ref:`user <apis:user/0>`) implementing the
   :ref:`expanding <apis:expanding/0>` built-in
   protocol. The hook object must be compiled and loaded when this option
   is used. It's also possible to specify a Prolog module instead of a
   Logtalk object but the module must be pre-loaded and its identifier
   must be different from any object identifier.

.. _flag_clean:
.. index:: pair: clean; Flag

``clean(Option)``
   Controls cleaning of the intermediate Prolog files generated when
   compiling Logtalk source files. Possible option values are ``off``
   and ``on`` (the usual default). When turned on, intermediate files
   are deleted after loading and all source files are recompiled
   disregarding any existing intermediate files. When turned off, the
   intermediate files are kept. This is useful when embedding applications,
   which requires collecting the intermediate code, and when working on
   large applications to avoid repeated recompilation of stable code.
   The flag must be turned on when changing compilation modes, changing
   flags such as :ref:`code_prefix <flag_code_prefix>`, or when turning
   on linter flags that are off by default without at the same time making
   changes to the application source files themselves as any existing
   intermediate files would not be recompiled as necessary due to file
   timestamps not changing.

User-defined flags
^^^^^^^^^^^^^^^^^^

Logtalk provides a :ref:`predicates_create_logtalk_flag_3`
predicate that can be used for defining new flags.

.. _programming_reloading:

Reloading source files
----------------------

As a general rule, reloading source files should never occur in
production code and should be handled with care in development code.
Reloading a Logtalk source file usually requires reloading the
intermediate Prolog file that is generated by the Logtalk compiler. The
problem is that there is no standard behavior for reloading Prolog
files. For static predicates, almost all Prolog compilers replace the
old definitions with the new ones. However, for dynamic predicates, the
behavior depends on the Prolog compiler. Most compilers replace the old
definitions but some of them simply append the new ones, which usually
leads to trouble. See the compatibility notes for the backend Prolog
compiler you intend to use for more information. There is an additional
potential problem when using multi-threading programming. Reloading a
threaded object does not recreate from scratch its old message queue,
which may still be in use (e.g. threads may be waiting on it).

When using library entities and stable code, you can avoid reloading the
corresponding source files (and, therefore, recompiling them) by setting
the :ref:`reload <flag_reload>` compiler flag to ``skip``. For code under
development, you can turn off the :ref:`clean <flag_clean>` flag to avoid
recompiling files that have not been modified since last compilation
(assuming that backend Prolog compiler that you are using supports
retrieving of file modification dates). You can disable deleting the
intermediate files generated when compiling source files by changing the
default flag value in your settings file, by using the corresponding
compiler flag with the compiling and loading built-in predicates, or,
for the remaining of a working session, by using the call:

.. code-block:: text

   | ?- set_logtalk_flag(clean, off).

Some caveats that you should be aware. First, some warnings that might
be produced when compiling a source file will not show up if the
corresponding object file is up-to-date because the source file is not
being (re)compiled. Second, if you are using several Prolog compilers
with Logtalk, be sure to perform the first compilation of your source
files with the ``clean`` flag turned off: the intermediate Prolog files
generated by the Logtalk compiler may be not compatible across Prolog
compilers or even for the same Prolog compiler across operating systems
(e.g. due to the use of different character encodings or end-of-line
characters).

.. _programming_batch:

Batch processing
----------------

When doing batch processing, you probably want to turn off the
:ref:`report <flag_report>` flag to suppress all messages of type
``banner``, ``comment``, ``comment(_)``, ``warning``, and ``warning(_)``
that are normally printed. Note that error messages and messages providing
information requested by the user will still be printed.

.. _programming_performance:

Optimizing performance
----------------------

The default compiler flag settings are appropriated for the
**development** but not necessarily for the **deployment** of
applications. To minimize the generated code size, turn the
:ref:`source_data <flag_source_data>` flag off. To optimize runtime
performance, turn on the :ref:`optimize <flag_optimize>` flag.
Your chosen backend Prolog compiler may also provide performance
related flags; check its documentation.

Pay special attention to file compilation/loading order. Whenever
possible, compile and load your files taking into account file dependencies.
By default, the compiler will print a warning whenever a file references
an entity that is not yet loaded. Solving these warnings is key for optimal
performance by enabling :term:`static binding` optimizations. For a clear
picture of file dependencies, use the :doc:`../devtools/diagrams` tool to
generate a file dependency diagram for your application.

Minimize the use of dynamic predicates. Parametric objects can often be
used in alternative. When dynamic predicates cannot be avoided, try to
make them private. Declaring a dynamic predicate also as a private
predicate allows the compiler to optimize local calls to the database
methods (e.g. :ref:`methods_assertz_1` and :ref:`methods_retract_1`) that
modify the predicate.

Sending a :term:`message to self` implies :term:`dynamic binding` but
there are often cases where :ref:`control_send_to_self_1` is misused
to call an imported or inherited predicate that is never going to be
redefined in a descendant. In these cases, a :term:`super call`,
:ref:`control_call_super_1`, can be used instead with
the benefit of often enabling static binding. Most of the guidelines for
writing efficient Prolog code also apply to Logtalk code. In particular,
define your predicates to take advantage of first-argument indexing. In
the case of recursive predicates, define them as tail-recursive predicates
whenever possible.

See the :ref:`section on performance <performance_performance>`
for a detailed discussion on Logtalk performance.

.. _programming_portability:

Portable applications
---------------------

Logtalk is compatible with most modern standards compliant Prolog compilers.
However, this does not necessarily imply that your Logtalk applications will
have the same level of portability. If possible, you should only use in your
applications Logtalk built-in predicates and ISO Prolog specified
built-in predicates and arithmetic functions. If you need to use
built-in predicates (or built-in arithmetic functions) that may not be
available in other Prolog compilers, you should try to encapsulate the
non-portable code in a small number of objects and provide a portable
**interface** for that code through the use of Logtalk protocols. An
example will be code that access operating-system specific features. The
Logtalk compiler can warn you of the use of non-ISO specified built-in
predicates and arithmetic functions by using the
:ref:`portability <flag_portability>` compiler flag.

.. _programming_cc:

Conditional compilation
-----------------------

Logtalk supports conditional compilation within source files using the
:ref:`directives_if_1`, :ref:`directives_elif_1`,
:ref:`directives_else_0`, and :ref:`directives_endif_0` directives. This
support is similar to the support found in several Prolog systems such
as ECLiPSe, GNU Prolog, SICStus Prolog, SWI-Prolog, XSB, and YAP.

.. _programming_errors:

Avoiding common errors
----------------------

Try to write objects and protocol documentation **before** writing any
other code; if you are having trouble documenting a predicate perhaps we
need to go back to the design stage.

Try to avoid lengthy hierarchies. Composition is often a better choice
over inheritance for defining new objects (Logtalk supports
component-based programming through the use of
:ref:`categories <categories_categories>`). In addition, prototype-based
hierarchies are semantically simpler than class-based hierarchies.

Dynamic predicates or dynamic entities are sometimes needed, but we
should always try to minimize the use of non-logical features such as
asserts and retracts.

Since each Logtalk entity is independently compiled, if an object
inherits a dynamic or a meta-predicate predicate, then the respective
directives must be repeated to ensure a correct compilation.

In general, Logtalk does not verify if a user predicate call/return
arguments comply with the declared modes. On the other hand, Logtalk
built-in predicates, built-in methods, and message sending control
structures are fully checked for calling mode errors.

Logtalk error handling strongly depends on the ISO compliance of the
chosen Prolog compiler. For instance, the error terms that are generated
by some Logtalk built-in predicates assume that the Prolog built-in
predicates behave as defined in the ISO standard regarding error
conditions. In particular, if your Prolog compiler does not support a
``read_term/3`` built-in predicate compliant with the ISO Prolog
Standard definition, then the current version of the Logtalk compiler
may not be able to detect misspell variables in your source code.

.. _programming_style:

Coding style guidelines
-----------------------

It is suggested that all code between an entity opening and closing
directives be indented by one tab stop. When defining entity code, both
directives and predicates, Prolog coding style guidelines may be
applied. All Logtalk source files, examples, and standard library
entities use tabs (the recommended setting is a tab width equivalent to
4 spaces) for laying out code. Closed related entities can be defined in
the same source file. However, for best performance, is often necessary
to have an entity per source file. Entities that might be useful in
different contexts (such as library entities) are best defined in their
own source files.

A detailed coding style guide is available at the Logtalk official website.
