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<h1 class="lined-header"><span>External plugins</span></h1>
<div class="paragraph"><p>It is often convenient to be able to keep the build files
used by all your projects in one place. Those files could
be Makefiles, configuration files, templates and more.</p></div>
<div class="paragraph"><p>Erlang.mk allows you to automatically load plugins from
dependencies. Plugins can do anything, including defining
new variables, defining file templates, hooking themselves
inside the normal Erlang.mk processing or even adding new
rules.</p></div>
<div class="paragraph"><p>You can load plugins using one of two methods. You can
either load all plugins from a dependency, or just one.
We will also cover conventions about writing external
plugins.</p></div>
<div class="sect1">
<h2 id="_loading_all_plugins_from_a_dependency">Loading all plugins from a dependency</h2>
<div class="sectionbody">
<div class="paragraph"><p>To load plugins from a dependency, all you need to do is add
the dependency name to <code>DEP_PLUGINS</code> in addition to the list
of dependencies.</p></div>
<div class="paragraph"><p>For example, if you have <code>cowboy</code> in <code>DEPS</code>, add <code>cowboy</code> in
<code>DEP_PLUGINS</code> also:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">DEPS =</span> cowboy
<span style="color: #009900">DEP_PLUGINS =</span> cowboy</tt></pre></div></div>
<div class="paragraph"><p>This will load the file <em>plugins.mk</em> in the root folder of
the Cowboy repository.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_loading_one_plugin_from_a_dependency">Loading one plugin from a dependency</h2>
<div class="sectionbody">
<div class="paragraph"><p>Now that we know how to load all plugins, let’s take a look
at how to load one specific plugin from a dependency.</p></div>
<div class="paragraph"><p>To do this, instead of writing only the name of the dependency,
we will write its name and the path to the plugin file. This
means that writing <code>DEP_PLUGINS = cowboy</code> is equivalent to
writing <code>DEP_PLUGINS = cowboy/plugins.mk</code>.</p></div>
<div class="paragraph"><p>Knowing this, if we were to load the plugin <em>mk/dist.mk</em>
from Cowboy and no other, we would write the following in
our Makefile:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">DEPS =</span> cowboy
<span style="color: #009900">DEP_PLUGINS =</span> cowboy/mk/dist.mk</tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_writing_external_plugins">Writing external plugins</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <em>plugins.mk</em> file is a convention. It is meant to load
all the plugins from the dependency. The code for the plugin
can be written directly in <em>plugins.mk</em> or be separate.</p></div>
<div class="paragraph"><p>If you are providing more than one plugin with your repository,
the recommended way is to create one file per plugin in the
<em>mk/</em> folder in your repository, and then include those
individual plugins in <em>plugins.mk</em>.</p></div>
<div class="paragraph"><p>For example, if you have two plugins <em>mk/dist.mk</em> and
<em>mk/templates.mk</em>, you could write the following <em>plugins.mk</em>
file:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #990000">THIS :=</span> <span style="color: #009900">$(</span>dir <span style="color: #009900">$(</span>realpath <span style="color: #009900">$(</span>lastword <span style="color: #009900">$(MAKEFILE_LIST))))</span>
include <span style="color: #009900">$(THIS)</span>/mk/dist.mk
include <span style="color: #009900">$(THIS)</span>/mk/templates.mk</tt></pre></div></div>
<div class="paragraph"><p>The <code>THIS</code> variable is required to relatively include files.</p></div>
<div class="paragraph"><p>This allows users to not only be able to select individual
plugins, but also select all plugins from the dependency
in one go if they wish to do so.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_early_stage_plugins">Early-stage plugins</h2>
<div class="sectionbody">
<div class="paragraph"><p>Plugins declared in <code>DEP_PLUGINS</code> are loaded near the end of Erlang.mk.
That’s why you have access to all previously initialized variables.
However, if you want your plugin to add common dependencies to
your applications, a regular is loaded too late in the process.
You need to use "Early-stage plugins". They are declared using the
<code>DEP_EARLY_PLUGINS</code> variable instead. Plugins listed in this variable
are loaded near the beginning of Erlang.mk Otherwise, they work exactly
the same.</p></div>
<div class="paragraph"><p>If you only give the name of a dependency, the default file loaded is
<em>early-plugins.mk</em>. You can specify a filename exactly like you would
have done it with regular plugins.</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-style: italic"><span style="color: #9A1900"># In your application's Makefile</span></span>
<span style="color: #009900">BUILD_DEPS =</span> common_deps
<span style="color: #009900">DEP_EARLY_PLUGINS =</span> common_deps</tt></pre></div></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-style: italic"><span style="color: #9A1900"># In the plugin's early-plugins.mk</span></span>
DEPS <span style="color: #990000">+=</span> cowboy
<span style="color: #009900">TEST_DEPS =</span> ct_helper
<span style="color: #009900">dep_ct_helper =</span> git https<span style="color: #990000">:</span>//github.com/ninenines/ct_helper master</tt></pre></div></div>
</div>
</div>
<div class="sect1">
<h2 id="_loading_plugins_local_to_the_application">Loading plugins local to the application</h2>
<div class="sectionbody">
<div class="paragraph"><p>If the Erlang.mk plugin lives in the same directory or repository as your
application or library, then you can load it exactly like an external
plugin: the dependency name is simply the name of your application or
library.</p></div>
<div class="paragraph"><p>For example, the following Makefile loads a plugin in the <em>mk</em>
subdirectory:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">DEP_PLUGINS =</span> <span style="color: #009900">$(PROJECT)</span>/mk/dist.mk</tt></pre></div></div>
<div class="paragraph"><p>This also works with early-stage plugins:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="color: #009900">DEP_EARLY_PLUGINS =</span> <span style="color: #009900">$(PROJECT)</span>/mk/variables.mk</tt></pre></div></div>
<div class="paragraph"><p>Like external plugins, if you do not specify the path to the plugin, it
defaults to <em>plugins.mk</em> or <em>early-plugins.mk</em>, located at the root of
your application:</p></div>
<div class="listingblock">
<div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><span style="font-style: italic"><span style="color: #9A1900"># Loads ./early-plugins.mk</span></span>
<span style="color: #009900">DEP_EARLY_PLUGINS =</span> <span style="color: #009900">$(PROJECT)</span>
<span style="font-style: italic"><span style="color: #9A1900"># Loads ./plugins.mk</span></span>
<span style="color: #009900">DEP_PLUGINS =</span> <span style="color: #009900">$(PROJECT)</span></tt></pre></div></div>
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