package mutaml

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A mutation tester for OCaml







Mutaml is a mutation testing tool for OCaml. It uses a ppxlib-based preprocessor to make a series of small breaking changes to a program's source code and then runs the program's testsuite for each of them to catch uncaught misbehaviour.


test mutation testing

Published: 17 Jul 2023


Mutaml: A Mutation Tester for OCaml

Mutaml is a mutation testing tool for OCaml.
Briefly, that means Mutaml tries to change your code randomly to see if the changes are caught by your tests.

In more detail: Mutation testing is a form of fault injection used to assess the quality of a program's testsuite. Mutation testing works by repeatedly making small, breaking changes to a program's text, such as turning a + into -, negating the condition of an if-then-else, ..., and subsequently rerunning the testsuite to see if each such 'mutant program' is 'killed' (caught) by one or more tests in the testsuite. By finding examples of uncaught wrong behaviour, mutation testing can thereby reveal limitations of an existing testsuite and indirectly suggest improvements.

Since OCaml already prevents many potential programming errors at compile time due to its strong type system, pattern-match compiler warnings, etc. Mutaml favors mutations that

  • preserve typing and

  • would not be caught statically, e.g., changes in the values computed.

Mutaml consists of:

  • a ppxlib-preprocessor that first transforms the program under test.

  • mutaml-runner that loops through a range of possible program mutations, and saves the output from running the test suite on each of the mutants

  • mutaml-report that prints a test report to the console.


Installing Mutaml

$ git clone
$ cd mutaml
$ opam install .


How you can use mutaml depends on your project's build setup. For now it has only been tested with dune, but it should work with other build systems supporting an explicit two-staged build process.

Using Mutaml with dune

  1. Mark the target code for instrumentation in your dune file(s):

      (public_name your_library)
      (instrumentation (backend mutaml)))

    Using dune's instrumentation stanza, your project's code is only instrumented when you pass the --instrument-with mutaml option.

  2. Compile your test code with mutaml instrumentation enabled:

    $ dune build test --instrument-with mutaml

    assuming you have a test/ test driver. This creates/overwrites an individual lib.muts file for each instrumented file and an overview file mutaml-mut-files.txt listing them. These files are written to dune's current build context.

  3. Start mutaml-runner, passing the name of the test executable to run:

    $ mutaml-runner _build/default/test/mytests.exe

    This reads from the files written in step 2. Running the command also creates/overwrites the file mutaml-report.json. You can also pass a command that runs the executable through dune if you prefer:

    $ mutaml-runner "dune exec --no-build test/mytests.exe"
  4. Generate a report, optionally passing the json-file (mutaml-report.json) created above:

    $ mutaml-report

    By default this prints diffs for each mutation that flew under the radar of your test suite. The diff output can be suppressed by passing --no-diff.

Steps 3 and 4 output a number of additional files. These are all written to a dedicated directory named _mutations.

Instrumentation Options and Environment Variables

The preprocessor's behaviour can be configured through either environment variables or instrumentation options in the dune file:

  • MUTAML_SEED - an integer value to seed mutaml-ppx's randomized mutations (overridden by instrumentation option -seed)

  • MUTAML_MUT_RATE - a integer between 0 and 100 to specify the mutation frequency (0 means never and 100 means always - overridden by instrumentation option -mut-rate)

  • MUTAML_GADT - allow only pattern mutations compatible with GADTs (true or false, overridden by instrumentation option -gadt)

For example, the following dune file sets all three instrumentation options:

  (name test)
  (instrumentation (backend mutaml -seed 42 -mut-rate 75 -gadt false))

We could achieve the same behaviour by setting three environment variables:

  $ export MUTAML_SEED=42
  $ export MUTAML_MUT_RATE=75
  $ export MUTAML_GADT=false

If you do both, the values passed as instrumentation options in the dune file takes precedence.

Runner Options and Environment Variables

By default, mutaml-runner expects to find the preprocessor's output files in the default build context _build/default. This can be configured via an environment variable or a command-line option, e.g., if instrumentation is enabled via another dune-workspace build context:

  • MUTAML_BUILD_CONTEXT - a path prefix string (overridden by command-line option --build-context)

mutaml-runner also repeats test suite runs for all instrumented files by default. An option --muts muts-file is available to enable more targeted mutation testing. Running, e.g.,

mutaml-runner --muts lib/lib2.muts _build/default/test/mytests.exe

will only consider mutations of the corresponding library lib/, which the runner searches for in the build context.

Report Options and Environment Variables

Currently mutaml-report uses diff --color -u as its default command to print diffs. It falls back to diff -u when the environment variable CI is true. The used command can also be configured an environment variable:

  • MUTAML_DIFF_COMMAND - the command and options to use instead, e.g. MUTAML_DIFF_COMMAND="diff -U 5" will disable colored outputs and add 5 lines of unified context. Mutaml expects the specified command to support --label options.

Passing the option --no-diff to mutaml-report prevents any mutation diffs from being printed.


This is an alpha release. There are therefore rough edges:

  • Mutaml is designed to avoid repeated recompilation for each mutation. It does so by writing files during preprocessing which are later read during the mutaml-runner testing loop. As a consequence, if you attempt to merge steps 2 and 3 above into one step this will not work:

    $ mutaml-runner "dune test --force --instrument-with mutaml"

    The preprocessor in this case only writes the relevant files when mutaml-runner first calls the command, and thus after it needs the information contained in the files...

  • There are issues to force dune to rebuild. This can affect Mutaml, e.g., in case just an environment variable changed. dune clean is a crude but effective work-around to this issue.

  • The output files to _build/default are not registered with dune. This means rerunning steps 2,3,4 above will fail, as the additional output files in _build/default are not cached by dune and hence deleted. Again dune clean is a crude but effective work-around.

  • ...

Mutations should not introduce compiler errors, be it type errors or from the pattern-match compiler. If you encounter a situation where this happens please report it in an issue.


Mutaml was developed with support from the OCaml Software Foundation.

Dependencies (8)

  1. conf-diffutils
  2. conf-which
  3. conf-timeout
  4. stdlib-random
  5. ppx_yojson_conv >= "v0.14.0"
  6. ppxlib >= "0.26.0" & < "0.28.0"
  7. dune >= "3.0.0"
  8. ocaml >= "4.12.0"

Dev Dependencies (1)

  1. ounit2 with-test

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