$ make search q=pool
Erlang.mk can fetch and compile the dependencies that your project requires. Erlang.mk improves upon the concepts introduced by Rebar, so they should be familiar to many seasoned Erlang developers.
Erlang.mk is not a package manager, nor is it trying to be, but it does include an index of Erlang packages to make discovering useful projects easier.
This chapter will explain how to use packages, add dependencies to your project or bundle them directly in a single repository.
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 | lessOnce you find the package you need, adding it as a dependency to your project is a one-liner:
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:
Or you could depend on a C project directly, if you are building a NIF:
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.
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:
Erlang.mk comes with additional types of dependencies.
It has TEST_DEPS for dependencies used only for testing:
DOC_DEPS for dependencies used only when building documentation:
REL_DEPS for dependencies required to build the release,
or to include extra applications in the release:
And SHELL_DEPS for dependencies to make available when running
the make shell command:
All these will be documented in more details in their respective chapters.
By default, Erlang.mk will look into its package index to find the project you are looking for, if you only provide its name. This is this case:
If you need a different version, you need to define another variable. There are two ways to do this, each being useful for different reasons.
If you simply want to change the commit number, all you
need to do is to define the dep_$(DEP_NAME)_commit
variable. In the case of Cowboy, this would look like this:
Erlang.mk will use the package index to get all information about Cowboy, except the commit number which will be overriden.
If you need to set the fetch method or repository information
too, for example because you want to use your own fork, or
simply because the project is missing from the index, you
can define the dep_$(DEP_NAME) variable with everything:
This will fetch Cowboy from your fork at the given commit.
Erlang.mk comes with a number of different fetch methods. You can fetch from Git, Mercurial, SVN, to name a few. There are fetch methods that will work everywhere, and fetch methods that will only work in a given environment.
The following table lists all existing methods:
| Name | Format | Description |
|---|---|---|
git |
git repo commit |
Clone the Git repository and checkout the given version |
git-submodule |
git-submodule |
Initialize and update the Git submodule |
hg |
hg repo commit |
Clone the Mercurial repository and update to the given version |
svn |
svn repo |
Checkout the given SVN repository |
cp |
cp path/to/repo |
Recursively copy a local directory |
hex |
hex version |
Download the given project version from hex.pm |
fail |
N/A |
Always fail, reserved for internal use |
legacy |
N/A |
Legacy Erlang.mk fetcher, reserved for internal use |
The git and hg methods both have a repository and commit.
You can use any valid commit, tag or branch in that repository
for the commit value.
For example, to fetch Cowboy with tag 2.0.0-pre.2 from Git:
Or to fetch Ehsa tag 4.0.3 from Mercurial:
Git also comes with a concept of submodules. Erlang.mk can
automatically initializes and updates submodules for dependencies,
as long as they were added beforehand using git submodule add:
The svn method only has a repository value, but that’s
simply because the SVN repository URL can also contain
the path and commit.
This would fetch an example project from the trunk:
And this would fetch a separate example project from a specific commit:
You can copy a directory from your machine using the cp method.
It only takes the path to copy from:
Finally, you can use a package from the Hex repository:
If none of the existing methods fit your use, you can simply
define your own. Erlang.mk will consider all variables that
are named as dep_fetch_$(METHOD) to be available fetch
methods. You can do anything inside this variable, as long
as you create a folder named $(DEPS_DIR)/$(call dep_name,$1).
Or in layman terms, if your dependency is Cowboy, this would
become deps/cowboy.
To give an example, this is what the Git method does:
Note that, like dependency information, this custom fetch method must be written before including erlang.mk.
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:
Fetch all dependencies for the application
Build first dependency
Build Nth dependency
Build last dependency
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:
This will only ignore dependencies that are needed for building. It is therefore safe to write:
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:
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:
They are not fetched
They are not autopatched
They are not deleted on make distclean
They are not automatically added to the application resource file
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:
Fetch the dependency using the configured fetch method
If it contains a configure.ac or configure.in file, run autoreconf -Wall -vif -I m4
If it contains a configure script, run it
Run autopatch on the project
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.
Rebar projects are automatically converted to use Erlang.mk as their build tool. This essentially patches Rebar out, and fixes and converts the project to be compatible with Erlang.mk.
Erlang.mk projects have their erlang.mk file redirect to the top-level project’s Erlang.mk. This is to ensure that functionality works across all dependencies, even if the dependency’s Erlang.mk is outdated.
Other Erlang projects get a small Erlang.mk Makefile generated automatically.
Projects with no source directory and no Makefile get an empty Makefile generated, for compatibility purposes.
Other projects with no Makefile are left untouched.
You can disable the replacing of the erlang.mk file by
defining the NO_AUTOPATCH_ERLANG_MK variable:
You can also disable autopatch entirely for a few select
projects using the NO_AUTOPATCH variable:
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:
Dependencies will not be compiled or downloaded when required
The dependency directory $(DEPS_DIR) will not be removed on make distclean
This variable only applies to remote dependencies.