..
   This file is part of Logtalk <https://logtalk.org/>  
   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.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.


.. rst-class:: align-right

**directive**

.. index:: pair: if/1; Directive
.. _directives_if_1:

``if/1``
========

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

::

   if(Goal)

Starts an *if-then* branch when performing conditional compilation. The code
following the directive is compiled iff ``Goal`` is true. If ``Goal``
throws an error instead of either succeeding or failing, the error is
reported by the compiler and compilation of the enclosing source file
or entity is aborted. The goal is subjected to
:ref:`goal expansion <expansion_expansion>` when the directive occurs
in a source file. Conditional compilation directives can be nested.

.. warning::

   Conditional compilation goals **cannot** depend on predicate definitions
   contained in the same source file that contains the conditional
   compilation directives (as those predicates only become available after
   the file is successfully compiled and loaded).

Template and modes
------------------

::

   if(@callable)

Examples
--------

A common example is checking if a built-in predicate exists and
providing a definition when the predicate is absent:

::

   :- if(\+ predicate_property(length(_,_), built_in)).

       length(List, Length) :-
           ...

   :- endif.

Another common example is conditionally including code for a specific
backend Prolog compiler:

::

   :- if(current_logtalk_flag(prolog_dialect, swi)).

       % SWI-Prolog specific code
       :- set_prolog_flag(double_quotes, codes).

   :- endif.

If necessary, test goal errors can be converted into failures using the
standard ``catch/3`` control construct. For example:

::

   :- if(catch(\+ log(7,_), _, fail)).

      % define the legacy log/2 predicate
      log(X, Y) :- Y is log(X).

   :- endif.

.. seealso::

   :ref:`directives_elif_1`,
   :ref:`directives_else_0`,
   :ref:`directives_endif_0`
