package ppx_windtrap

Windtrap

One library for all your OCaml tests.

Unit tests, property-based tests, snapshot tests, expect tests, and code coverage -- in a single package with one API. No need to glue together Alcotest + QCheck + ppx_expect + Bisect_ppx + custom snapshot code.

open Windtrap

let () =
  run "My Project"
    [
      test "addition" (fun () ->
          equal int 5 (2 + 3));

      prop "reverse is involutive"
        (list int)
        (fun l -> List.rev (List.rev l) = l);

      test "json output" (fun () ->
          snapshot ~pos:__POS__ (to_json user));

      test "greeting" (fun () ->
          greet "World";
          expect "Hello, World!");
    ]

Install

opam install windtrap

For PPX inline tests and code coverage, also install:

opam install ppx_windtrap

dune setup

(test
 (name my_tests)
 (libraries windtrap))

For PPX inline tests:

(library
 (name mylib)
 (inline_tests)
 (preprocess (pps ppx_windtrap)))

Features

Assertions -- equal, not_equal, is_true, is_some, is_ok, raises, raises_match, no_raise, with structured diffs on failure.

Property testing -- prop generates random inputs using Testable types and shrinks counterexamples automatically. Same types used for assertions and generation.

Snapshot testing -- snapshot compares output against reference files stored in __snapshots__/. Update with -u or WINDTRAP_UPDATE=1.

Expect testing -- expect captures stdout and compares with whitespace normalization. capture returns the function result while checking output.

Inline expect tests -- let%expect_test and [%expect] via ppx_windtrap. Mismatches produce .corrected files compatible with dune promote.

Parameterized tests -- cases generates one test per input value from a list.

Fixtures -- bracket for per-test setup/teardown, fixture for lazy shared resources, group ~setup ~teardown for suite-level hooks.

Code coverage -- Built-in expression-level coverage. Run with dune runtest --instrument-with ppx_windtrap to get an inline coverage percentage after test results, windtrap coverage --per-file for a detailed text report, or windtrap coverage --json for machine-readable uncovered lines.

Test runner -- Filtering (-f), fail-fast (-x), output formats (verbose, compact, TAP, JUnit), snapshot updates (-u), GitHub Actions annotations.

CLI

./my_tests.exe [OPTIONS] [PATTERN]

  -f, --filter PATTERN   Filter tests by name
  -e, --exclude PATTERN  Exclude tests by name
  -x, --fail-fast        Stop on first failure
  --bail N               Stop after N failures
  --failed               Rerun only tests that failed last time
  -q, --quick            Skip slow tests
  -u, --update           Update snapshot files
  -l, --list             List tests without running
  -s, --stream           Stream output (don't capture)
  -v, --verbose          Verbose output (default: compact)
  --format FMT           compact, verbose, tap, junit
  --junit PATH           Write JUnit XML report
  --tag LABEL            Run only tests with this label (repeatable)
  --exclude-tag LABEL    Skip tests with this label (repeatable)
  --seed N               Reproducible property test seed
  --timeout N            Default timeout in seconds
  --prop-count N         Number of property test cases (default: 100)
  --color MODE           Color output: always, never, auto
  -V, --version          Show version

Examples

See examples/ for working code covering every feature.

License

ISC. Some files are under MIT or BSD-2-Clause due to derived code. See THIRD_PARTY_LICENSES.md for details.

Acknowledgments

Windtrap builds on ideas from several OCaml testing projects:

• Alcotest by Thomas Gazagnaire -- test structure and runner design
• Craig Ferguson's Alcotest PRs (#294, #247) -- API design and subcomponent diffing
• ppx_expect and ppx_inline_test by Jane Street -- expect test paradigm and dune integration
• QCheck2 by Simon Cruanes et al. -- generator design and integrated shrinking
• Bisect_ppx by Anton Bachin et al. -- coverage instrumentation and runtime
• mtime by The mtime programmers -- monotonic clock implementation