..
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   SPDX-FileCopyrightText: 1998-2024 Paulo Moura <pmoura@logtalk.org>
   SPDX-License-Identifier: Apache-2.0

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
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       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
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   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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.. rst-class:: align-right

**built-in predicate**

.. index:: pair: logtalk_library_path/2; Built-in predicate
.. _predicates_logtalk_library_path_2:

``logtalk_library_path/2``
==========================

Description
-----------

::

   logtalk_library_path(Library, Path)

Dynamic and multifile user-defined predicate, allowing the declaration
of aliases to :term:`library` paths. Library aliases may also be used on
the second argument (using the notation *alias(path)*). Paths must always
end with the path directory separator character (``'/'``).

.. warning::

   Library aliases should be unique. The :ref:`predicates_logtalk_make_1`
   built-in predicate can be used to detect and report duplicated library 
   aliases using the ``check`` target.

Clauses for this predicate should preferably be facts. Defining rules to
dynamically compute at runtime both library alias names and their paths,
although sometimes handy, can inadvertently result in endless loops when
those rules also attempt to expand library paths. When asserting facts
for this predicate, use preferably ``asserta/1`` instead of ``assertz/1``
to help ensure the facts will be used before any rules.

Relative paths (e.g. ``'../'`` or ``'./'``) should only be used within
the *alias(path)*) notation so that library paths can always be expanded
to absolute paths independently of the (usually unpredictable) current
directory at the time the ``logtalk_library_path/2`` predicate is
called.

When working with a relocatable application, the actual application
installation directory can be retrieved by calling the
:ref:`predicates_logtalk_load_context_2` predicate with the ``directory``
key and using the returned value to define the ``logtalk_library_path/2``
predicate. On a :term:`settings file` file or a :term:`loader file` file,
simply use an :ref:`directives_initialization_1` directive to wrap the
call to the ``logtalk_load_context/2`` predicate and the assert of the
``logtalk_library_path/2`` fact.

This predicate may be used to override the default :term:`scratch directory`
by defining the library alias ``scratch_directory`` in a backend Prolog
initialization file (assumed to be loaded prior to Logtalk loading). This
allows e.g. Logtalk to be installed in a read-only directory by setting
this alias to the operating-system directory for temporary files. It also
allows several Logtalk instances to run concurrently without conflict by
using a unique scratch directory per instance (e.g. using a process ID or
a UUID generator).

This predicate may be used to override the default location used by the
:doc:`../../devtools/packs` tool to store registries and packs by defining
the ``logtalk_packs`` library alias in settings file or in a backend
Prolog initialization file (assumed to be loaded prior to Logtalk loading).

The :ref:`logtalk <objects_logtalk>` built-in object provides an
:ref:`expand_library_path/2 <apis:logtalk/0::expand_library_path/2>`
predicate that can be used to expand library aliases and files expressed
using library notation.

Modes and number of proofs
--------------------------

::

   logtalk_library_path(?atom, -atom) - zero_or_more
   logtalk_library_path(?atom, -compound) - zero_or_more

Errors
------

(none)

Examples
--------

::

   :- initialization((
      logtalk_load_context(directory, Directory),
      asserta(logtalk_library_path(my_application_root, Directory))
   )).

::

   | ?- logtalk_library_path(viewpoints, Path).

   Path = examples('viewpoints/')
   yes

   | ?- logtalk_library_path(Library, Path).

   Library = home,
   Path = '$HOME/' ;

   Library = logtalk_home,
   Path = '$LOGTALKHOME/' ;

   Library = logtalk_user
   Path = '$LOGTALKUSER/' ;

   Library = examples
   Path = logtalk_user('examples/') ;

   Library = library
   Path = logtalk_user('library/') ;

   Library = viewpoints
   Path = examples('viewpoints/')
   yes

::

   | ?- logtalk::expand_library_path(viewpoints, Path).

   Path = '/Users/pmoura/logtalk/examples/viewpoints/'.
   yes

   | ?- logtalk::expand_library_path(viewpoints('loader.lgt'), Path).

   Path = '/Users/pmoura/logtalk/examples/viewpoints/loader.lgt'.
   yes
   

.. seealso::

   :ref:`predicates_logtalk_compile_1`,
   :ref:`predicates_logtalk_compile_2`,
   :ref:`predicates_logtalk_load_1`,
   :ref:`predicates_logtalk_load_2`
