Erlang.mk

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Chapter 7. Packages and dependencies

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.

7.1. Searching packages

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

7.2. Adding dependencies to your project

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.

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.

7.2.1. Modifying the dependency source or version

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:

DEPS = cowboy

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:

DEPS = cowboy
+dep_cowboy_commit = 2.0.0-pre.2

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:

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

This will fetch Cowboy from your fork at the given commit.

7.2.2. Fetch methods

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
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:

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

Or to fetch Ehsa tag 4.0.3 from Mercurial:

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

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:

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

And this would fetch a separate example project from a +specific commit:

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

You can copy a directory from your machine using the cp method. +It only takes the path to copy from:

dep_cowboy = cp $(HOME)/ninenines/cowboy

Finally, you can use a package from the +Hex repository:

dep_cowboy = hex 1.0.3

7.2.3. Custom fetch methods

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:

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

Note that, like dependency information, this custom fetch method +must be written before including erlang.mk.

7.3. How deps are fetched and built

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.

7.4. Ignoring unwanted dependencies

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.

7.5. Dependencies directory

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.

7.6. Dependencies local to the repository

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

7.7. Repositories with no application at the root level

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.

7.8. Autopatch

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:

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

7.9. Skipping deps

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.

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