Code coverage for OCaml

Bisect_ppx is a code coverage tool for OCaml and Reason. It helps
you test thoroughly by showing what's not tested.

You can browse the report seen above online here.

Table of contents



Refer to aantron/bisect-starter-dune, which produces
this report.

  1. Depend on Bisect_ppx
    in your opam file:

    depends: [
      "bisect_ppx" {dev & >= "2.5.0"}
      "dune" {>= "2.7.0"}
  2. Mark the code under test for instrumentation by
    in your dune file:

     (public_name my_lib)
     (instrumentation (backend bisect_ppx)))
  3. Build and run your test binary. In addition to testing your code, when
    exiting, it will write one or more files with names like

    find . -name '*.coverage' | xargs rm -f
    dune runtest --instrument-with bisect_ppx --force

    The --force flag forces all your tests to run, which is needed for an
    accurate coverage report.

    To run tests without coverage, do

    dune runtest
  4. Generate the coverage report in _coverage/index.html:

    bisect-ppx-report html

    You can also generate a short summary in the terminal:

    bisect-ppx-report summary


Refer to aantron/bisect-starter-esy, which produces this

The instructions are the same as for regular Dune usage, but...

  1. Depend on Bisect_ppx in package.json,
    instead of in an opam file:

    "devDependencies": {
      "@opam/bisect_ppx": "^2.5.0"
    "dependencies": {
      "@opam/dune": "^2.7.0"
  2. Use the esy command for the build and for running binaries:

    esy install
    esy dune runtest --instrument-with bisect_ppx --force
    esy bisect-ppx-report html


Refer to aantron/bisect-starter-rescript, which produces
this report.

  1. Depend on Bisect_ppx in package.json,
    and install it:

    "devDependencies": {
      "bisect_ppx": "^2.0.0"
    "dependencies": {
      "rescript": "*"
    npm install

    If pre-built binaries aren't available for your system, the build will
    automatically fall back to building Bisect_ppx from source using
    esy, which will take a few minutes the first time. If this
    happens, you may need to install esy, if it is not already installed:

    npm install -g esy
    npm install
  2. Add Bisect_ppx to your bsconfig.json:

    "bs-dependencies": [
    "ppx-flags": [
  3. If you are using Jest, add this to your package.json:

    "jest": {
      "setupFilesAfterEnv": [

    Or, if you have enabled the flag in
    bsconfig.json, replace the path by


    You can exclude your test cases from the coverage report by adding this to

    "ppx-flags": [
      ["bisect_ppx/ppx", "--exclude-files", ".*_test\\.res$$"]

    Usage with Jest requires Bisect_ppx version 2.4.0 or higher. See the
    aantron/bisect-starter-jest for a complete minimal example
    project. That repo produces this report.

    If the tests will be running in the browser, at the end of testing, call


    This returns binary coverage data in a string option, which you should
    upload or otherwise get out of the browser, and write into a .coverage

  4. Build in development with BISECT_ENABLE=yes, run tests, and generate the
    coverage report in _coverage/index.html:

    BISECT_ENABLE=yes npm run build
    npm run test
    npx bisect-ppx-report html

    To exclude your test files from the report, change your PPX flags like so:

    "ppx-flags": [
      ["bisect_ppx/ppx", "--exclude-files", ".*test\\.re"]

    The last argument is a regular expression in the syntax of OCaml's Str
    . Note
    that backslashes need to be escaped both inside the regular expression, and
    again because they are inside a JSON string.

    Multiple --exclude-files option can be specified if you want to provide
    multiple patterns.

  5. If your project uses both ReScript and native Dune, native Dune will start
    picking up OCaml files that are part of the ReScript bisect_ppx package.
    To prevent this, add a dune file with the following contents to the root of
    your project:

    (data_only_dirs node_modules)


Refer to aantron/bisect-starter-jsoo, which produces
this report.

  1. Follow the Dune instructions above, except that the final test
    script must be linked with bisect_ppx.runtime

    (but not instrumented):

     (name my_tester)
     (modes js)
     (libraries bisect_ppx.runtime))
  2. If the tests will run on Node, call this function
    at the end of testing to write bisect0123456789.coverage:

    Bisect.Runtime.write_coverage_data ()

    If the tests will run in the browser, call

    Bisect.Runtime.get_coverage_data ()

    to get binary coverage data in a string option. Upload this string or
    otherwise extract it from the browser to create a .coverage file.

  3. Build the usual Js_of_ocaml target, including the instrumented code under
    test, then run the reporter to generate the coverage report in

    dune build my_tester.bc.js --instrument-with bisect_ppx
    node _build/default/my_tester.bc.js   # or in the browser
    bisect-ppx-report html

Ocamlfind, Ocamlbuild, and OASIS

  • Ocamlbuild
    and OASIS
    instructions can be found at

  • With Ocamlfind, you must have your build script issue the right commands, to
    instrument the code under test, but not the tester:

    ocamlfind opt -package bisect_ppx -c src/
    ocamlfind opt -c test/
    ocamlfind opt -linkpkg -package bisect_ppx src/source.cmx test/test.cmx

    Running the tester will then produce bisect0123456789.coverage files,
    which you can process with bisect-ppx-report.

Sending to Coveralls and Codecov

bisect-ppx-report can send reports to Coveralls
and Codecov directly from Travis, CircleCI,
and GitHub Actions. To do this, run

bisect-ppx-report send-to Coveralls


bisect-ppx-report send-to Codecov

When sending specifically from GitHub Actions to Coveralls, use

- run: bisect-ppx-report send-to Coveralls
    PULL_REQUEST_NUMBER: ${{ github.event.number }}

Put these commands in your CI script in the same place you would run
bisect-ppx-report html locally. See
for example CI scripts and current status of these integrations.

If you'd like Bisect_ppx to support other CI and/or coverage services, please
send a pull request!

As a workaround for missing CI/coverage integrations, and for development,
bisect-ppx-report can also generate a JSON report in Coveralls format, which
can be uploaded to a service of your choice using a separate command. For
example, to send manually from Travis to Coveralls:

bisect-ppx-report \
  coveralls coverage.json \
  --service-name travis-ci \
  --service-job-id $TRAVIS_JOB_ID
curl -L -F json_file=@./coverage.json

For other CI services, replace --service-name and --service-job-id as

| CI service | --service-name | --service-job-id |
| ---------- | ---------------- | -------------------- |
| Travis | travis-ci | $TRAVIS_JOB_ID |
| CircleCI | circleci | $CIRCLE_BUILD_NUM |
| Semaphore | semaphore | $REVISION |
| Jenkins | jenkins | $BUILD_ID |
| Codeship | codeship | $CI_BUILD_NUMBER |
| GitHub Actions | github | $GITHUB_RUN_NUMBER |

Note that Coveralls-style reports are less precise than the HTML reports
generated by Bisect_ppx, because Coveralls considers entire lines as visited or
not visited. There can be many expressions on a single line, and the HTML
report separately considers each expression as visited or not visited.

Controlling coverage with [@coverage off]

You can tag expressions with [@coverage off], and neither they, nor their
subexpressions, will be instrumented by Bisect_ppx.

Likewise, you can tag module-level let-declarations with [@@coverage off],
and they won't be instrumented.

You can also turn off instrumentation for blocks of declarations inside a
module with [@@@coverage off] and [@@@coverage on].

Finally, you can exclude an entire file by putting [@@@coverage exclude_file]
into its top-level module. However, whenever possible, it is recommended to
exclude files by not instrumenting with Bisect_ppx to begin with.

Other topics

See advanced usage for:

  • Exhaustiveness checking.

  • Excluding generated files from coverage.

  • SIGTERM handling.

  • Environment variables.

Cornell CS3110 offers a Bisect_ppx tutorial, featuring a video.

Bisect_ppx users

A small sample of projects using Bisect_ppx:


Bug reports and pull requests are warmly welcome. Bisect_ppx is developed on
GitHub, so please open an issue.

After cloning the repo, try these Makefile targets:

  • make test for unit tests.

  • make usage for build system integration tests, except ReScript.

  • make -C test/js full-test for ReScript. This requires npm and esy.

If you'd like to build an npm package, run npm pack. You can install the
resulting .tgz file in another project with npm install. This requires esy,
as the Bisect binaries will not be pre-built. The npm package will use esy to
build them automatically.

14 Mar 2022
with-test & = "0.16.0"
>= "0.21.0" & < "0.26.0"
>= "4.03.0"
>= "2.7.0"
>= "1.0.0"
Reverse Dependencies