Erlang.mk gives you access to nearly 500 packages, with more being added regularly.

To find a package, search for it:

$ make search q=pool

This will return all packages matching this word, like worker pool and acceptor pool projects.

You can also list everything and use regular command line tools to find what you need, for example:

$ make search | less

Once you find the package you need, adding it as a dependency to your project is a one-liner:

DEPS = cowboy

And that’s it! The next time you run make, Erlang.mk will fetch and compile Cowboy. Erlang.mk will also ensure Cowboy is available whenever you use the shell, run tests and any other operations.

Erlang.mk will fill in the application resource file with all applications found in DEPS. But not all dependencies are Erlang applications, and not all dependencies need to be a runtime dependency. That’s where the BUILD_DEPS variable comes in: it works just like DEPS, except the dependencies listed there will not be added as runtime dependencies.

For example, you could add a parse transform project like this to make it available only at build time:

BUILD_DEPS = erlando

Or you could depend on a C project directly, if you are building a NIF:

BUILD_DEPS = leveldb
dep_leveldb = git https://github.com/basho/leveldb 2.1.3

This dependency will be built before your application, so you could easily copy the resulting shared file into your priv/ directory as part of the build process. More information about that in the NIFs and port drivers Chapter 8, NIFs and port drivers chapter.

Another variable, LOCAL_DEPS, allows specifying runtime dependencies which are part of Erlang/OTP itself, but also dependencies that are included in the repository. Since they are already on your system, there is no need to fetch them. Do note that there is no way to choose the version, the application used will be the one already on your system.

You could depend on the Crypto application, for example:

LOCAL_DEPS = crypto

Erlang.mk comes with additional types of dependencies. It has TEST_DEPS for dependencies used only for testing:

TEST_DEPS = ct_helper
dep_ct_helper = git https://github.com/ninenines/ct_helper master

DOC_DEPS for dependencies used only when building documentation:

DOC_DEPS = edown

REL_DEPS for dependencies required to build the release, or to include extra applications in the release:

REL_DEPS = recon

And SHELL_DEPS for dependencies to make available when running the make shell command:

SHELL_DEPS = tddreloader

All these will be documented in more details in their respective chapters.

DEPS = cowboy

DEPS = cowboy
dep_cowboy_commit = 2.0.0-pre.2

DEPS = cowboy
dep_cowboy = git https://github.com/essen/cowboy 2.0.0-pre.2

dep_cowboy = git https://github.com/ninenines/cowboy 2.0.0-pre.2

dep_ehsa = hg https://bitbucket.org/a12n/ehsa 4.0.3

dep_cowboy = git-submodule

dep_ex1 = svn https://example.com/svn/trunk/project/ex1

dep_ex2 = svn svn://example.com/svn/branches/erlang-proj/ex2@264

dep_cowboy = cp $(HOME)/ninenines/cowboy

dep_cowboy = hex 1.0.3

define dep_fetch_git
    git clone -q -n -- $(call dep_repo,$1) $(DEPS_DIR)/$(call dep_name,$1); \
    cd $(DEPS_DIR)/$(call dep_name,$1) && git checkout -q $(call dep_commit,$1);
endef

The order in which dependencies are fetched and built is well defined. This means that Erlang.mk will get the same applications regardless of the command or options being used.

In tree traversal terms, where the list of dependencies is a tree, Erlang.mk fetches everything using the pre-order traversal method. The steps can be summarized like this, starting from the root application:

Every time a dependency is built, these same steps are followed, recursively.

Do note that the first step, fetching all dependencies of an application, is not guaranteed to be ordered. The reason for this is that it is not possible to have the same dependency listed twice in a single application, and therefore there can be no conflicts. Remember, this step only fetches, at no point are different applications built in parallel.

What about conflicts between the dependencies of different applications? Simple. Since builds are ordered, this means that the first version of an application that is fetched will be the one that wins.

This means that if project A depends on projects B and C, in this order, and that both B and C depend on a different version of D, it will always be B’s version of D that wins, because we fetch the dependencies of B before fetching those from C.

Similarly, if project A depends on projects B, C and D, regardless of the order, and A, B and C depend on a different version of D, it will always be A’s version that wins, because we fetch all dependencies of A before fetching those from B or C.

Sometimes, you may want to ignore dependencies entirely. Not even fetch them. You may want to do this because a project you depend on depends on an application you do not need (like a dependency for building documentation or testing). Or maybe the dependency is already installed on your system.

To ignore a dependency, simply add it to the IGNORE_DEPS variable:

IGNORE_DEPS += edown proper

This will only ignore dependencies that are needed for building. It is therefore safe to write:

IGNORE_DEPS += edown proper
TEST_DEPS = proper

The PropEr application will be fetched as intended when running make tests or make check. It will however not be fetched when running make or make deps.

Dependencies are fetched in $(DEPS_DIR). By default this is the deps directory. You can change this default, but you should only do so if it was not defined previously. Erlang.mk uses this variable to tell dependencies where to fetch their own dependencies.

You will therefore need to use ?= instead of =. Of course, if you know you will never use this project as a dependency, = will work. But to avoid it biting you later on, do this:

DEPS_DIR ?= $(CURDIR)/libs

The $(CURDIR) part is important, otherwise dependencies of dependencies will be fetched in the wrong directory.

Erlang.mk will also export the REBAR_DEPS_DIR variable for compatibility with Rebar build tools, as long as they are recent enough.

In addition to the dependencies that are fetched, Erlang.mk also allows you to have dependencies local to your repository. This kind of layout is sometimes called multi-application repositories, or repositories with multiple applications.

They work exactly the same as remote dependencies, except:

To properly fill the application resource file, you will need to define the LOCAL_DEPS variable for each relevant application, the same as for OTP applications.

If there is a conflict between a local dependency and a remote dependency, then the local dependency always wins; an error will be triggered when trying to fetch the conflicting remote dependency.

To start using dependencies local to the repository, simply create a folder named $(APPS_DIR). By default, this folder is the apps/ directory.

You can use Erlang.mk to bootstrap local dependencies by using the command make new-app or make new-lib. This command will create the necessary directories and bootstrap the application.

For example, to create a full fledged OTP application as a local dependency:

$ make new-app in=webchat

Or, the same as an OTP library:

$ make new-lib in=webchat

Templates also work with local dependencies, from the root directory of the project. You do need however to tell Erlang.mk to create the files in the correct application:

$ make new t=gen_server n=my_server in=webchat

It’s possible to use Erlang.mk with only applications in $(APPS_DIR), and nothing at the root of the repository. Just create a folder, put the erlang.mk file in it, write a Makefile that includes it, and start creating your applications.

Similarly, it’s possible to have a repository with only dependencies found in $(DEPS_DIR). You just need to create a Makefile and specify the dependencies you want. This allows you to create a repository for handling the building of releases, for example.

Erlang.mk will automatically patch all the dependencies it fetches. It needs to do this to ensure that the dependencies become compatible with not only Erlang.mk, but also with the version of Erlang.mk that is currently used.

When fetching a dependency, the following operations are performed:

Autopatch first checks if there is any project-specific patch enabled. There are currently two: RABBITMQ_CLIENT_PATCH for the amqp_client dependency, and RABBITMQ_SERVER_PATCH for the rabbit dependency. These are needed only for RabbitMQ versions before 3.6.0 (assuming you are using upstream RabbitMQ, and not a fork).

Otherwise, autopatch performs different operations depending on the kind of project it finds the dependency to be.

You can disable the replacing of the erlang.mk file by defining the NO_AUTOPATCH_ERLANG_MK variable:

NO_AUTOPATCH_ERLANG_MK = 1

You can also disable autopatch entirely for a few select projects using the NO_AUTOPATCH variable:

NO_AUTOPATCH = cowboy ranch cowlib

It is possible to temporarily skip all dependency operations. This is done by defining the SKIP_DEPS variable. Use cases include being somewhere with no connection to download them, or perhaps a peculiar setup.

A typical usage would be:

$ make SKIP_DEPS=1

When the variable is defined:

This variable only applies to remote dependencies.