Installing Logtalk

This page provides an overview of Logtalk installation requirements and instructions and a description of the files contained on the Logtalk distribution. For detailed, up-to-date installation and configuration instructions, please see the,, and files distributed with Logtalk. The broad compatibility of Logtalk, both with Prolog compilers and operating-systems, together with all the possible user scenarios, means that installation can vary from very simple by running an installer or a couple of scripts to the need of patching both Logtalk and Prolog compilers to workaround the lack of strong Prolog standards or to cope with the requirements of less common operating-systems.

The preferred installation scenario is to have Logtalk installed in a system-wide location, thus available for all users, and a local copy of user-modifiable files on each user home directory (even when you are the single user of your computer). This scenario allows each user to independently customize Logtalk and to freely modify the provided libraries and programming examples. Logtalk installers, installation shell scripts, and Prolog integration scripts favor this installation scenario, although alternative installation scenarios are always possible. The installers set two environment variables, LOGTALKHOME and LOGTALKUSER, pointing, respectively, to the Logtalk installation folder and to the Logtalk user folder.

User applications should preferable be kept outside of the Logtalk user folder created by the installation process, however, as updating Logtalk often results in updating the contents of this folder. If your applications depend on customizations to the distribution files, backup those changes before updating Logtalk.

Hardware and software requirements

Computer and operating system

Logtalk is compatible with almost any computer/operating-system with a modern, standards compliant, Prolog compiler available.

Prolog compiler

Logtalk requires a backend Prolog compiler supporting official and de facto standards. Capabilities needed by Logtalk that are not defined in the official ISO Prolog Core standard include:

  • access to predicate properties
  • operating-system access predicates
  • de facto standard predicates not (yet) specified in the official standard

Logtalk needs access to the predicate property built_in to properly compile objects and categories that contain Prolog built-in predicates calls. In addition, some Logtalk built-ins need to know the dynamic/static status of predicates to ensure correct application. The ISO standard for Prolog modules defines a predicate_property/2 predicate that is already implemented by most Prolog compilers. Note that if these capabilities are not built-in the user cannot easily define them.

For optimal performance, Logtalk requires that the Prolog compiler supports first-argument indexing for both static and dynamic code (most modern compilers support this feature).

Since most Prolog compilers are moving closer to the ISO Prolog standard [ISO95], it is advisable that you try to use the most recent version of your favorite Prolog compiler.

Logtalk installers

Logtalk installers are available for macOS, Linux, and Microsoft Windows. Depending on the chosen installer, some tasks (e.g. setting environment variables or integrating Logtalk with some Prolog compilers) may need to be performed manually.

Source distribution

Logtalk sources are available in a tar archive compressed with bzip2, lgt3xxx.tar.bz2. You may expand the archive by using a decompressing utility or by typing the following commands at the command-line:

% tar -jxvf lgt3xxx.tar.bz2

This will create a sub-directory named lgt3xxx in your current directory. Almost all files in the Logtalk distribution are text files. Different operating-systems use different end-of-line codes for text files. Ensure that your decompressing utility converts the end-of-lines of all text files to match your operating system.

Distribution overview

In the Logtalk installation directory, you will find the following files and directories:

BIBLIOGRAPHY.bib – Logtalk bibliography in BibTeX format – Logtalk end-user customization instructions – Logtalk installation instructions

LICENSE.txt – Logtalk user license

NOTICE.txt – Logtalk copyright notice – Quick start instructions for those that do not like to read manuals – several useful information – release notes for this version – instructions on how to upgrade your programs to the current Logtalk version

VERSION.txt – file containing the current Logtalk version number (used for compatibility checking when upgrading Logtalk)

loader-sample.lgt – sample loader file for user applications

settings-sample.lgt – sample file for user-defined Logtalk settings

tester-sample.lgt – sample file for helping to automate running user application unit tests

adapters – notes on the provided adapter files – template adapter file ... – specific adapter files
coding – notes on syntax highlighter and text editor support files providing syntax coloring for publishing and editing Logtalk source code ... – syntax coloring support files
contributions – notes on the user-contributed code ... – user-contributed code files
core – notes on the current status of the compiler and runtime ... – core source files
docs – notes on the provided documentation for core, library, tools, and contributions entities index.html – root document for all entities documentation ... – other entity documentation files
examples – short description of the provided examples

bricks – example description and other notes SCRIPT.txt – step by step example tutorial loader.lgt – loader utility file for the example objects ... – bricks example source files

... – other examples

integration – notes on scripts for Logtalk integration with Prolog compilers ... – Prolog integration scripts
library – short description of the library contents all_loader.lgt – loader utility file for all library entities ... – library source files
... – POSIX man pages for the shell scripts
manuals – notes on the provided documentation bibliography.html – bibliography glossary.html – glossary index.html – root document for all documentation ... – other documentation files
paths – description on how to setup library and examples paths – default library and example paths
scratch – notes on the scratch directory
scripts – notes on scripts for Logtalk user setup, packaging, and installation ... – packaging, installation, and setup scripts
tests – notes on the current status of the unit tests ... – unit tests for built-in features
tools – notes on the provided programming tools ... – programming tools

Adapter files

Adapter files provide the glue code between the Logtalk compiler/runtime and a Prolog compiler. Each adapter file contains two sets of predicates: ISO Prolog standard predicates and directives not built-in in the target Prolog compiler and Logtalk specific predicates.

Logtalk already includes ready to use adapter files for most academic and commercial Prolog compilers. If an adapter file is not available for the compiler that you intend to use, then you need to build a new one, starting from the included file. Start by making a copy of the template file. Carefully check (or complete if needed) each listed definition. If your Prolog compiler conforms to the ISO standard, this task should only take you a few minutes. In most cases, you can borrow code from the predefined adapter files. If you are unsure that your Prolog compiler provides all the ISO predicates needed by Logtalk, try to run the system by setting the unknown predicate error handler to report as an error any call to a missing predicate. Better yet, switch to a modern, ISO compliant, Prolog compiler. If you send me your adapter file, with a reference to the target Prolog compiler, maybe I can include it in the next release of Logtalk.

The adapter files specify default values for most of the Logtalk compiler flags. Most of these compiler flags are described in the next section. A few of these flags have read-only values and cannot be changed at runtime. These are:

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, and in the user home directory. With a value of restrict, settings files are only searched in the Logtalk user directory and in the user home directory.
Name of the backend Prolog compiler (an atom). This flag can be used for conditional compilation of Prolog specific code.
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.
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.
Level of conformance of the backend Prolog compiler with the ISO Prolog Core standard. The possible values are strict for compilers claiming strict conformance and lax for compilers claiming only broad conformance.
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).
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).
Informs Logtalk if the backend Prolog compiler provides tabling programming support. Possible flag values are unsupported and supported.
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.
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.
Informs Logtalk if the backend Prolog compiler provides suitable module support. Possible flag values are unsupported and supported (Logtalk provides limited support for compiling Prolog modules as objects).
Informs Logtalk if the backend Prolog compiler provides the minimal support for cyclic terms necessary for working with coinductive predicates. Possible flag values are unsupported and supported.

Settings files

Although is always possible to edit the backend Prolog compiler adapter files, the recommended solution to customize compiler flags is to edit the 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 startup directory (assuming that the read-only settings_file flag is set to allow). If not found, Logtalk tries to load a settings.lgt file from the Logtalk user folder. If still not found, Logtalk tries to load a settings.lgt file from the user home folder. 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 silently with any errors being simply 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. For example, one of the troubles of writing portable applications is the different feature sets of Prolog compilers. A typical issue is the lack of support for tabling. Using the Logtalk support for conditional compilation you could write:

:- if(current_logtalk_flag(tabling, supported)).

    % add tabling directives to the source code
    :- table(foo/1).
    :- table(bar/2).

:- endif.

The prolog_dialect flag may also be used with the conditional compilation directives in order to define a single settings file that can be used with several backend Prolog compilers. For example:

:- 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 and runtime

The core sub-directory contains the Prolog and Logtalk source files that implement the Logtalk compiler and the Logtalk runtime. The compiler and the runtime may be split in two (or more) separate files or combined in a single file, depending on the Logtalk release that you are installing.


Starting from version 2.7.0, Logtalk contains a standard library of useful objects, categories, and protocols. Read the corresponding file for details about the library contents.


Logtalk 2.x and 3.x contain new implementations of some of the examples provided with previous 1.x versions. The sources of each one of these examples can be found included in a subdirectory with the same name, inside the directory examples. The majority of these examples include a file named SCRIPT.txt that contains cases of simple utilization. Some examples may depend on other examples and library objects to work properly. Read the corresponding file for details before running an example.

Logtalk source files

Logtalk source files are text files containing one or more entity definitions (objects, categories, or protocols). The Logtalk source files may also contain plain Prolog code. The extension .lgt is normally used. Logtalk compiles these files to plain Prolog by appending to the file name a suffix derived from the extension and by replacing the .lgt extension with .pl (.pl is the default Prolog extension; if your Prolog compiler expects the Prolog source filenames to end with a specific, different extension, you can set it in the corresponding adapter file).