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.

Installation:

Installing Mutaml

$ git clone https://github.com/jmid/mutaml.git
$ cd mutaml
$ opam install .

Instructions:

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):

   (library
     (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/mytests.ml test driver. This creates/overwrites an individual lib.muts file for each instrumented lib.ml 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.

Environment Variables and Instrumentation Options

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:

 (executable
  (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.

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)

Status

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.

Acknowledgements

Mutaml was developed with support from the OCaml Software Foundation.