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   Copyright 1998-2021 Paulo Moura <pmoura@logtalk.org>
   SPDX-License-Identifier: Apache-2.0

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.. 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 (``'/'``).

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 settings 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 also 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).

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),
      assertz(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`
