[[eunit]]
== EUnit

EUnit is the tool of choice for unit testing. Erlang.mk
automates a few things on top of EUnit, including the
discovery and running of unit tests.

=== Writing tests

The http://www.erlang.org/doc/apps/eunit/chapter.html[EUnit user guide]
is the best place to learn how to write tests. Of note is
that all functions ending with `_test` or `_test_` will be
picked up as EUnit test cases.

Erlang.mk will automatically pick up tests found in any of
the Erlang modules of your application. It will also pick up
tests located in the '$(TEST_DIR)' directory, which defaults
to 'test/'.

It is generally a good practice to hide test code from
the code you ship to production. With Erlang.mk, you can
do this thanks to the `TEST` macro. It is only defined
when running tests:

[source,erlang]
----
-ifdef(TEST).

%% Insert tests here.

-endif.
----

Be careful, however, if you include the EUnit header file,
as it also defines the `TEST` macro. Make sure to only include
it inside an `ifdef` block, otherwise tests will always be
compiled.

[source,erlang]
----
-ifdef(TEST).

-include_lib(\"eunit/include/eunit.hrl\").

%% Insert tests here.

-endif.
----

Erlang.mk will automatically recompile your code when you
perform a normal build after running tests, and vice versa.

=== Configuration

The `EUNIT_OPTS` variable allows you to specify additional
EUnit options. Options are documented in the
http://www.erlang.org/doc/man/eunit.html#test-2[EUnit manual].
At the time of writing, the only available option is `verbose`:

[source,make]
EUNIT_OPTS = verbose

The `EUNIT_ERL_OPTS` variable allows you to specify options
to be passed to `erl` when running EUnit tests. For example,
you can load the 'vm.args' and 'sys.config' files:

[source,make]
EUNIT_ERL_OPTS = -args_file config/vm.args -config config/sys.config

=== Usage

To run all tests (including EUnit):

[source,bash]
$ make tests

To run all tests and static checks (including EUnit):

[source,bash]
$ make check

You can also run EUnit separately:

[source,bash]
$ make eunit

EUnit will be quiet by default, only outputting errors.
You can easily make it verbose for a single invocation:

[source,bash]
$ make eunit EUNIT_OPTS=verbose

Erlang.mk allows you to run all tests from a specific
module, or a specific test case from that module, using
the variable `t`.

For example, to run all tests from the `cow_http_hd`
module (instead of all tests from the entire project),
one could write:

[source,bash]
$ make eunit t=cow_http_hd

Similarly, to run a specific test case:

[source,bash]
$ make eunit t=cow_http_hd:parse_accept_test_

To do the same against a multi-application repository,
you can use the `-C` option:

[source,bash]
$ make -C apps/my_app eunit t=my_module:hello_test

Note that this also applies to dependencies. From Cowboy,
you can run the following directly:

[source,bash]
$ make -C deps/cowlib eunit t=cow_http_hd

Finally, xref:coverage[code coverage] is available,
but covered in its own chapter.