Question 1

How do I exclude test files from Bisect_ppx coverage in a ReScript project?

Accepted Answer

Add a PPX flag in bsconfig.json with --exclude-files and a regex pattern, e.g., ["bisect_ppx/ppx", "--exclude-files", ".*_test\.res$$"] for ReScript files. Multiple patterns can be specified for fine control.

Question 2

Bisect_ppx or ocaml-coverage: which is better for OCaml?

Accepted Answer

Bisect_ppx offers expression-level precision and broader build system support, making it superior for detailed analysis, while ocaml-coverage might suffice for basic line coverage in simpler setups. Bisect_ppx is also the only option with ReScript compatibility.

Question 3

How to set up Bisect_ppx with Dune from scratch?

Accepted Answer

Add bisect_ppx as a dev dependency in your opam file, mark libraries with (instrumentation (backend bisect_ppx)) in dune files, run tests with dune runtest --instrument-with bisect_ppx --force, and generate reports using bisect-ppx-report html.

Question 4

Why is Bisect_ppx installation slow for ReScript?

Accepted Answer

If pre-built binaries aren't available for your system, it falls back to building from source with esy, which can take several minutes. Installing esy globally (npm install -g esy) may be required, adding to setup time.

Question 5

Can Bisect_ppx send reports to Codecov from CircleCI?

Accepted Answer

Yes, use bisect-ppx-report send-to Codecov in your CircleCI script. For manual sends, generate a JSON report with bisect-ppx-report coveralls and set --service-name circleci and --service-job-id $CIRCLE_BUILD_NUM, then upload it.

Question 6

How to handle coverage in browser tests with Js_of_ocaml?

Accepted Answer

Call Bisect.Runtime.get_coverage_data() at test end to retrieve binary data as a string option, extract it from the browser, and save it as a .coverage file. Then use bisect-ppx-report html to generate the report locally.

Question 7

What's a common pitfall with Bisect_ppx and esy?

Accepted Answer

Forgetting to use esy commands (like esy dune) instead of direct dune calls, and not cleaning up .coverage files before runs, which can lead to inaccurate reports. The esy starter repo provides specific steps to avoid this.

Bisect_ppx

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