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   Copyright 1998-2019 Paulo Moura <pmoura@logtalk.org>

   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

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.. index:: logtalk_library_path/2
.. _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, 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
--------

::

   | ?- 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

.. seealso::

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