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.
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.
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.
To load plugins from a dependency, all you need to do is add
the dependency name to DEP_PLUGINS in addition to the list
of dependencies.
For example, if you have cowboy in DEPS, add cowboy in
DEP_PLUGINS also:
DEPS = cowboy DEP_PLUGINS = cowboy
This will load the file plugins.mk in the root folder of the Cowboy repository.
Now that we know how to load all plugins, let’s take a look at how to load one specific plugin from a dependency.
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 DEP_PLUGINS = cowboy is equivalent to
writing DEP_PLUGINS = cowboy/plugins.mk.
Knowing this, if we were to load the plugin mk/dist.mk from Cowboy and no other, we would write the following in our Makefile:
DEPS = cowboy DEP_PLUGINS = cowboy/mk/dist.mk
The plugins.mk 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 plugins.mk or be separate.
If you are providing more than one plugin with your repository, the recommended way is to create one file per plugin in the mk/ folder in your repository, and then include those individual plugins in plugins.mk.
For example, if you have two plugins mk/dist.mk and mk/templates.mk, you could write the following plugins.mk file:
THIS := $(dir $(realpath $(lastword $(MAKEFILE_LIST)))) include $(THIS)/mk/dist.mk include $(THIS)/mk/templates.mk
The THIS variable is required to relatively include files.
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.