Writing and running tests — dune documentation

# Writing and running tests

Dune tries to streamline the testing story as much as possible, so that you can focus on the tests themselves and not bother with setting up with various test frameworks.

In this section, we will explain the workflow to deal with tests in dune. In particular we will see how to run the testsuite of a project, how to describe your tests to dune and how to promote tests result as expectation.

We distinguish two kinds of tests: inline tests and custom tests. Inline tests are usually written directly inside the ml files of a library. They are the easiest to work with and usually requires nothing more than writing `(inline_tests)` inside your library stanza. Custom tests consist on executing an executable and sometimes do something afterwards, such as diffing its output.

## Running tests

Whatever the tests of a project are, the usual way to run tests with dune is to call `dune runtest` from the shell (or the command alias `dune test`). This will run all the tests defined in the current directory and any sub-directory recursively.

Note that in any case, `dune runtest` is simply a short-hand for building the `runtest` alias, so you can always ask dune to run the tests in conjunction with other targets by passing `@runtest` to `dune build`. For instance:

    $ dune build @install @runtest
    $ dune build @install @test/runtest

### Running a single test

If you would only like to run a single test for your project, you may use `dune exec` to run the test executable (for the sake of this example, `project/tests/myTest.ml`):

    dune exec project/tests/myTest.exe

### Running tests in a directory

You can also pass a directory argument to run the tests from a sub-tree. For instance `dune runtest test` will only run the tests from the `test` directory and any sub-directory of `test` recursively.

## Inline tests

There are several inline tests framework available for OCaml, such as ppx_inline_test and qtest. We will use ppx_inline_test as an example as at the time of writing this document it has the necessary setup to be used with dune out of the box.

ppx_inline_test allows one to write tests directly inside ml files as follows:

    let rec fact n = if n = 1 then 1 else n * fact (n - 1)

    let%test _ = fact 5 = 120

The file has to be preprocessed with the ppx_inline_test ppx rewriter, so for instance the `dune` file might look like this:

    (library
     (name foo)
     (preprocess (pps ppx_inline_test)))

In order to instruct dune that our library contains inline tests, all we have to do is add an `inline_tests` field:

    (library
     (name foo)
     (inline_tests)
     (preprocess (pps ppx_inline_test)))

We can now build and execute this test by running `dune runtest`. For instance, if we make the test fail by replacing `120` by `0` we get:

    $ dune runtest
    [...]
    File "src/fact.ml", line 3, characters 0-25: <<(fact 5) = 0>> is false.

    FAILED 1 / 1 tests

Note that in this case Dune knew how to build and run the tests without any special configuration. This is because ppx_inline_test defines an inline tests backend and it is used by the library. Some other frameworks, such as qtest don’t have any special library or ppx rewriter. To use such a framework, you must tell dune about it since it cannot guess it. You can do that by adding a `backend` field:

    (library
     (name foo)
     (inline_tests (backend qtest.lib)))

In the example above, the name qtest.lib comes from the public_name field in qtest’s own dune file.

### Inline expectation tests

Inline expectation tests are a special case of inline tests where you write a bit of OCaml code that prints something followed by what you expect this code to print. For instance, using ppx_expect:

    let%expect_test _ =
      print_endline "Hello, world!";
      [%expect{|
        Hello, world!
      |}]

The test procedure consist of executing the OCaml code and replacing the contents of the `[%expect]` extension point by the real output. You then get a new file that you can compare to the original source file. Expectation tests are a neat way to write tests as the following test elements are clearly identified:

- the code of the test
- the test expectation
- the test outcome

You can have a look at this blog post to find out more about expectation tests. To dune, the workflow for expectation tests is always as follows:

- write the test with some empty expect nodes in it
- run the tests
- check the suggested correction and promote it as the original source file if you are happy with it

Dune makes this workflow very easy, simply add `ppx_expect` to your list of ppx rewriters as follows:

    (library
     (name foo)
     (inline_tests)
     (preprocess (pps ppx_expect)))

Then calling `dune runtest` will run these tests and in case of mismatch dune will print a diff of the original source file and the suggested correction. For instance:

    $ dune runtest
    [...]
    -src/fact.ml
    +src/fact.ml.corrected
    File "src/fact.ml", line 5, characters 0-1:
    let rec fact n = if n = 1 then 1 else n * fact (n - 1)

    let%expect_test _ =
      print_int (fact 5);
    -  [%expect]
    +  [%expect{| 120 |}]

In order to accept the correction, simply run:

    $ dune promote

You can also make dune automatically accept the correction after running the tests by typing:

    $ dune runtest --auto-promote

Finally, some editor integration is possible to make the editor do the promotion and make the workflow even smoother.

### Running a subset of the test suite

You may also run a group of tests located under a directory with:

    dune runtest mylib/tests

The above command will run all tests defined in tests and its sub-directories.

### Running tests in bytecode or JavaScript

By default Dune run inline tests in native mode, except if native compilation is not available in which case it runs them in bytecode.

You can change this setting to choose which modes tests should run in. To do that, add a `modes` field to the `inline_tests` field. Available modes are:

- `byte` for running tests in byte code
- `native` for running tests in native mode
- `best` for running tests in native mode with fallback to byte code if native compilation is not available
- `js` for running tests in JavaScript using Node.js

For instance:

    (library
     (name foo)
     (inline_tests (modes byte best js))
     (preprocess (pps ppx_expect)))

### Specifying inline test dependencies

If your tests are reading files, you must say it to dune by adding a `deps` field the the `inline_tests` field. The argument of this `deps` field follows the usual Dependency specification. For instance:

    (library
     (name foo)
     (inline_tests (deps data.txt))
     (preprocess (pps ppx_expect)))

### Passing special arguments to the test runner

Under the hood, a test executable is built by dune. Depending on the backend used this runner might take useful command line arguments. You can specify such flags by using a `flags` field, such as:

    (library
     (name foo)
     (inline_tests (flags (-foo bar)))
     (preprocess (pps ppx_expect)))

The argument of the `flags` field follows the Ordered set language.

### Using additional libraries in the test runner

When tests are not part of the library code, it is possible that tests require additional libraries than the library being tested. This is the case with qtest as tests are written in comments. You can specify such libraries using a `libraries` field, such as:

    (library
     (name foo)
     (inline_tests (backend qtest)
                   (libraries bar)))

### Defining your own inline test backend

If you are writing a test framework, or for specific cases, you might want to define your own inline tests backend. If your framework is naturally implemented by a library or ppx rewriter that the user must use when they want to write tests, then you should define this library has a backend. Otherwise simply create an empty library with the name you want to give for your backend.

In order to define a library as an inline tests backend, simply add an `inline_tests.backend` field to the library stanza. An inline tests backend is specified by thee parameters:

1. How to create the test runner
2. How to build the test runner
3. How to run the test runner

These three parameters can be specified inside the `inline_tests.backend` field, which accepts the following fields:

    (generate_runner   <action>)
    (runner_libraries (<ocaml-libraries>))
    (flags             <flags>)
    (extends          (<backends>))

For instance:

`<action>` follows the User actions specification. It describe an action that should be executed in the directory of libraries using this backend for their tests. It is expected that the action produces some OCaml code on its standard output. This code will constitute the test runner. The action can use the following additional variables:

- `%{library-name}` which is the name of the library being tested
- `%{impl-files}` which is the list of implementation files in the library, i.e. all the `.ml` and `.re` files
- `%{intf-files}` which is the list of interface files in the library, i.e. all the `.mli` and `.rei` files

The `runner_libraries` field specifies what OCaml libraries the test runner uses. For instance, if the `generate_runner` actions generates something like `My_test_framework.runtests ()`, the you should probably put `my_test_framework` in the `runner_libraries` field.

If you test runner needs specific flags, you should pass them in the `flags` field. You can use the `%{library-name}` variable in this field.

Finally, a backend can be an extension of another backend. In this case you must specify by in the `extends` field. For instance, ppx_expect is an extension of ppx_inline_test. It is possible to use a backend with several extensions in a library, however there must be exactly one root backend, i.e. exactly one backend that is not an extension of another one.

When using a backend with extensions, the various fields are simply concatenated. The order in which they are concatenated is unspecified, however if a backend `b` extends of a backend `a`, then `a` will always come before `b`.

#### Example of backend

In this example, we put tests in comments of the form:

    (*TEST: assert (fact 5 = 120) *)

The backend for such a framework looks like this:

    (library
     (name simple_tests)
     (inline_tests.backend
      (generate_runner (run sed "s/(\\*TEST:\\(.*\\)\\*)/let () = \\1;;/" %{impl-files}))
      ))

Now all you have to do is write `(inline_tests ((backend simple_tests)))` wherever you want to write such tests. Note that this is only an example, we do not recommend using `sed` in your build as this would cause portability problems.

## Custom tests

We said in Running tests that to run tests dune simply builds the `runtest` alias. As a result, to define custom tests, you simply need to add an action to this alias in any directory. For instance if you have a binary `tests.exe` that you want to run as part of running your testsuite, simply add this to a dune file:

    (rule
     (alias  runtest)
     (action (run ./tests.exe)))

Hence to define an a test a pair of alias and executable stanzas are required. To simplify this common pattern, dune provides a tests stanza to define multiple tests and their aliases at once:

    (tests (names test1 test2))

### Diffing the result

It is often the case that we want to compare the output of a test to some expected one. For that, dune offers the `diff` command, which in essence is the same as running the `diff` tool, except that it is more integrated in dune and especially with the `promote` command. For instance let’s consider this test:

    (rule
     (with-stdout-to tests.output (run ./tests.exe)))

    (rule
     (alias runtest)
     (action (diff tests.expected test.output)))

After having run `tests.exe` and dumping its output to `tests.output`, dune will compare the latter to `tests.expected`. In case of mismatch, dune will print a diff and then the `dune promote` command can be used to copy over the generated `test.output` file to `tests.expected` in the source tree.

Alternatively, the tests also supports this style of tests.

    (tests (names tests))

Where dune expects a `tests.expected` file to exist to infer that this is an expect tests.

This provides a nice way of dealing with the usual write code, run, promote cycle of testing. For instance:

    $ dune runtest
    [...]
    -tests.expected
    +tests.output
    File "tests.expected", line 1, characters 0-1:
    -Hello, world!
    +Good bye!
    $ dune promote
    Promoting _build/default/tests.output to tests.expected.

Note that if available, the diffing is done using the patdiff tool, which displays nicer looking diffs that the standard `diff` tool. You can change that by passing `--diff-command CMD` to dune.