# 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.

## 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, the scripts should be available from the command-line; scripts are named upon the used backend Prolog compilers. On Windows, the shortcuts should be available from the Start Menu.

If no scripts or shortcuts are available for your installation, operating-system, or Prolog compiler, you can always start a Logtalk session by performing the following steps:

1. Start your Prolog compiler.

2. Load the appropriate adapter file for your compiler. Adapter files for most common Prolog compilers can be found in the `adapters` subdirectory.

3. Load the library paths file corresponding to your Logtalk installation contained in the `paths` subdirectory.

4. Load the Logtalk compiler/runtime files contained in the `core` subdirectory.

Note that the adapter files, compiler/runtime files, and library paths file are Prolog source files. The predicate called to load (and compile) them depends on your Prolog compiler. In case of doubt, consult your Prolog compiler reference manual or take a look at the definition of the predicate `'$lgt_load_prolog_code'/3` in the corresponding adapter file.

Most Prolog compilers support automatic loading of an initialization file, which can include the necessary directives to load both the Prolog adapter file and the Logtalk compiler. This feature, when available, allows automatic loading of Logtalk when you start your Prolog compiler.

## 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 term-expansion mechanism). 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 `initialization/1` Prolog directive may be used for defining an initialization goal to be executed when loading a source file. Most calls to Logtalk built-in predicates from file `initialization/1` directives are compiled for better performance.

Logtalk source files can include the text of other files by using the 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 backend Prolog compiler.

### Multi-pass compiler

Logtalk is currently 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 fully compiled and loaded. This may prevent some compiler optimizations, notably 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. 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).

- File 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.

### Naming conventions

If you prefer to define each entity in its own source file, then 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. Intermediate Prolog source files (generated by the Logtalk compiler) have, by default, a `_lgt` suffix and a `.pl` extension. Again, this can be set to match the needs of a particular Prolog compiler in the corresponding adapter file. For example, we may define an object named `vehicle` and save it in a `vehicle.lgt` source file that will be compiled to a `vehicle_lgt.pl` Prolog file. If we have a `sort(_)` parametric object we can save it on a `sort_1.lgt` source file that will be compiled to a `sort_1_lgt.pl` Prolog file. This name scheme helps avoid file name conflicts (remember that all Logtalk entities share the same namespace). To further prevent file name conflicts, specially when embedding applications, and depending on the backend compiler, the names of the intermediate Prolog files may include a directory hash.

### Source file text encoding

The text encoding used in a source file may be declared using the encoding/1 directive when running Logtalk with backend Prolog compilers that support multiple encodings (check the encoding_directive flag in the adapter file of your Prolog compiler).

## 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 logtalk_compile/1 built-in predicate:

    | ?- 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 logtalk_load/1 built-in predicate:

    | ?- 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 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 backend Prolog compilers, 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 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, 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 Reference Manual 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.

## Loader files

Most examples directories contain an utility 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. For an example loader file named `loader.lgt` we would type:

    | ?- logtalk_load(loader).

Usually these files contain a call to the built-in predicates set_logtalk_flag/2 (e.g. for setting global, project-specific, flag values) and logtalk_load/1 or logtalk_load/2 (for loading project files), wrapped inside a Prolog `initialization/1` directive for poertability. 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 application. 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 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 application 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((
        ...
    )).

A common mistake is to try to set compiler flags using `logtalk_load/2` when loading a loader file. For example, by writing:

    | ?- 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.

## 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 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:

    | ?- 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 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 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.

## Settings files

Although is always possible to edit the 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 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 set_logtalk_flag/2 Logtalk built-in predicate and asserting clauses for the logtalk_library_path/2 multifile dynamic predicate. Note that the 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 prolog_dialect flag we can write a single settings file that can be used with several backend Prolog compilers:

    :- 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.

## Compiler linter

The compiler includes a 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 compiler flags. See the next section for details.

## Compiler flags

The logtalk_load/1 and 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 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 logtalk_load/2 or the logtalk_compile/2 built-in predicates. These two predicates accept a list of options affecting how a Logtalk source file is compiled and loaded:

    | ?- logtalk_compile(Files, Options).

or:

    | ?- 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 set_logtalk_flag/2 built-in predicate. For example:

    | ?- set_logtalk_flag(unknown_entities, silent).

The current default flags values can be enumerated using the current_logtalk_flag/2 built-in predicate:

    | ?- current_logtalk_flag(unknown_entities, Value).

    Value = silent
    yes

Logtalk also implements a 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 adapter files These are:

`settings_file`

  Allows or disables loading of a 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.

`prolog_dialect`

  Identifier of the backend Prolog compiler (an atom). This flag can be used for conditional compilation of Prolog compiler specific code.

`prolog_version`

  Version of the 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.

`prolog_compatible_version`

  Compatible version of the 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.

`unicode`

  Informs Logtalk if the 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).

`encoding_directive`

  Informs Logtalk if the backend Prolog compiler supports the 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).

`tabling`

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

`engines`

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

`threads`

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

`modules`

  Informs Logtalk if the 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).

`coinduction`

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

### Version flags

`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.

### Lint flags

`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.

`unknown_predicates(Option)`

  Defines the compiler behavior when calls to unknown predicates (or non-terminals) are found. 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).

`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).

`steadfastness(Option)`

  Controls warnings about possible non steadfast 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).

`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).

`deprecated(Option)`

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

`missing_directives(Option)`

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

`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.

`trivial_goal_fails(Option)`

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

`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).

`lambda_variables(Option)`

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

`suspicious_calls(Option)`

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

`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.

`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.

`singleton_variables(Option)`

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

`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.

`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).

`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.

`disjunctions(Option)`

  Controls warnings on clauses where the body is a disjunction. Possible option values are `warning` (the usual default) and `silent`.

`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

`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.

`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 asserta/1 or assertz/1 message to an object. Local asserting of new predicates is always allowed.

`events(Option)`

  Allows message sending calls to be compiled with or without event-driven programming 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.

`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

`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 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

`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 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 backend Prolog compiler adapter file can be overriden by defining the `scratch_directory` library alias (see the logtalk_library_path/2 predicate documentation for details).

`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).

`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 settings file), restart with the clean flag turned on to ensure that any compiled files using the old `code_prefix` value will be recompiled.

`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 static binding optimizations invalid for code defining in other loaded files. Turning on this flag automatically turns off the debug 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 reflection API 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.

`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 optimize flag.

`reload(Option)`

  Defines the reloading behavior for source files. Possible option values are `skip` (skip loading 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 the any explicit flags and the compilation mode are the same as before), and `always` (always reload files).

`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.

`hook(Object)`

  Allows the definition of an object (which can be the pseudo-object user) implementing the expanding 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.

`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, this flag also forces recompilation of all source files, disregarding any existing intermediate files. Thus, it is strongly advised to turn on this flag when switching backend Prolog compilers or changing flags such as code_prefix as the intermediate files generated by the compilation of source files may not be portable (due to differences in the implementation of the standard `write_canonical/2` predicate) or valid (due to changes to the intermediate code format).

### User-defined flags

Logtalk provides a create_logtalk_flag/3 predicate that can be used for defining new flags.

## 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 reload compiler flag to `skip`. For code under development, you can turn off the 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:

    | ?- 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).

## Batch processing

When doing batch processing, you probably want to turn off the 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.

## 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 source_data flag off. To optimize runtime performance, turn on the 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 static binding optimizations. For a clear picture of file dependencies, use the 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. assertz/1 and retract/1) that modify the predicate.

Sending a message to self implies dynamic binding but there are often cases where ::/1 is misused to call an imported or inherited predicate that is never going to be redefined in a descendant. In these cases, a super call, ^^/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 section on performance for a detailed discussion on Logtalk performance.

## 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 portability compiler flag.

## Conditional compilation

Logtalk supports conditional compilation within source files using the if/1, elif/1, else/0, and 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.

## 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 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.

## 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.