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<h1 class="lined-header"><span>External plugins</span></h1>
<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>
<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>
<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>
<h2 id="_loading_all_plugins_from_a_dependency">Loading all plugins from a dependency</h2>
<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>
<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 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><font color="#009900">DEPS =</font> cowboy
<font color="#009900">DEP_PLUGINS =</font> cowboy</tt></pre>
</div></div>
<p>This will load the file <em>plugins.mk</em> in the root folder of the Cowboy repository.</p>
<h2 id="_loading_one_plugin_from_a_dependency">Loading one plugin from a dependency</h2>
<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>
<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>
<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 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><font color="#009900">DEPS =</font> cowboy
<font color="#009900">DEP_PLUGINS =</font> cowboy/mk/dist.mk</tt></pre>
</div></div>
<h2 id="_writing_external_plugins">Writing external plugins</h2>
<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>
<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>
<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 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><font color="#990000">THIS :=</font> <font color="#009900">$(</font>dir <font color="#009900">$(</font>realpath <font color="#009900">$(</font>lastword <font color="#009900">$(MAKEFILE_LIST))))</font>
include <font color="#009900">$(THIS)</font>/mk/dist.mk
include <font color="#009900">$(THIS)</font>/mk/templates.mk</tt></pre>
</div></div>
<p>The <code>THIS</code> variable is required to relatively include files.</p>
<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>
<p>Plugins can include some help text by extending the target <code>help-plugins</code>:</p>
<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><font color="#990000">help-plugins::</font>
<font color="#009900">$(verbose)</font> <b><font color="#0000FF">printf</font></b> <font color="#FF0000">"%s</font><font color="#CC33CC">\n</font><font color="#FF0000">"</font> <font color="#FF0000">""</font> <font color="#FF0000">"Run benchmark: $(MAKE) perfs"</font></tt></pre>
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<h2 id="_early_stage_plugins">Early-stage plugins</h2>
<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>
<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 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><i><font color="#9A1900"># In your application's Makefile</font></i>
<font color="#009900">BUILD_DEPS =</font> common_deps
<font color="#009900">DEP_EARLY_PLUGINS =</font> 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><i><font color="#9A1900"># In the plugin's early-plugins.mk</font></i>
DEPS <font color="#990000">+=</font> cowboy
<font color="#009900">TEST_DEPS =</font> ct_helper
<font color="#009900">dep_ct_helper =</font> git https<font color="#990000">:</font>//github.com/ninenines/ct_helper master</tt></pre>
</div></div>
<h2 id="_loading__plugins_local_to_the_application">Loading plugins local to the application</h2>
<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>
<p>For example, the following Makefile loads a plugin in the <em>mk</em> subdirectory:</p>
<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><font color="#009900">DEP_PLUGINS =</font> <font color="#009900">$(PROJECT)</font>/mk/dist.mk</tt></pre>
</div></div>
<p>This also works with early-stage plugins:</p>
<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><font color="#009900">DEP_EARLY_PLUGINS =</font> <font color="#009900">$(PROJECT)</font>/mk/variables.mk</tt></pre>
</div></div>
<p>Like external plugins, if you do not specify the path to the plugin, it defaults to <em>plugins.mk</em> or <em>early-plugins.mk</em>, located at the root of your application:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><i><font color="#9A1900"># Loads ./early-plugins.mk</font></i>
<font color="#009900">DEP_EARLY_PLUGINS =</font> <font color="#009900">$(PROJECT)</font>
<i><font color="#9A1900"># Loads ./plugins.mk</font></i>
<font color="#009900">DEP_PLUGINS =</font> <font color="#009900">$(PROJECT)</font></tt></pre>
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