OCaml Testing Strategies

When to Use This Skill

Invoke this skill when:

• Setting up tests for an OCaml project
• Writing unit tests with Alcotest
• Using Eio mock infrastructure
• Discussing testing patterns and strategies

General Testing Setup

Basic Test Structure

Tests use Alcotest. See templates/test_template.ml for the basic structure.

let test_basic () =
  Alcotest.(check int) "same ints" 42 (21 + 21)

let suite = [
  "basic", `Quick, test_basic;
]

let () = Alcotest.run "MyLibrary" [
  "suite1", suite;
]

Test Dependencies

Add to dune-project package stanza:

(depends
  ; ... other deps ...
  (alcotest (and :with-test (>= 1.7.0)))
  (eio_main :with-test)  ; if using Eio
)

Test dune file

(test
 (name test_mylib)
 (libraries mylib alcotest))

For Eio-based libraries:

(test
 (name test_mylib)
 (libraries mylib alcotest eio_main eio.mock))

Testing Eio-Based Libraries

Using Eio Mock Infrastructure

If your library uses Eio, prefer using Eio's mock infrastructure for testing. This provides:

• Deterministic tests
• No actual filesystem/network operations
• Better error injection and edge case testing
• Faster test execution

Fetching Eio Sources for Reference

To understand mock APIs, fetch Eio sources:

mkdir -p third_party
cd third_party && opam source eio && cd ..

Key files to study:

• lib_eio/mock/ - Mock implementations
• tests/ - Example test patterns

Mock Modules

Common mock modules:

• Eio_mock.Backend - Mock backend for testing
• Eio_mock.Clock - Deterministic clock
• Eio_mock.Flow - Mock flows/streams
• Eio_mock.Net - Mock network
• Eio_mock.Fs - Mock filesystem

Example: Testing with Mocks

See templates/test_eio_mock.ml for a complete example.

let test_with_mock_clock () =
  Eio_mock.Backend.run @@ fun () ->
  let clock = Eio_mock.Clock.make () in
  (* Your test using the mock clock *)
  Eio_mock.Clock.advance clock 1.0;
  Alcotest.(check bool) "time advanced" true true

let () = Alcotest.run "MyLibrary" [
  "eio-mocks", [
    "mock clock", `Quick, test_with_mock_clock;
  ];
]

Integration Tests

For tests that need real I/O (use sparingly):

let test_real_io () =
  Eio_main.run @@ fun env ->
  (* Use env#fs, env#clock, etc. *)
  ()

Testing Strategies by Library Type

Pure Libraries (No I/O)

• Use simple Alcotest cases
• Focus on property-based testing if appropriate
• No special infrastructure needed

Libraries with Filesystem I/O

• Use Eio_mock.Fs for mocked filesystem
• Study Eio test patterns in third_party

Libraries with Network I/O

• Use Eio_mock.Net for mocked networking
• Test connection failures, timeouts
• No actual network calls in unit tests

Libraries with Time/Concurrency

• Use Eio_mock.Clock for deterministic time
• Advance time explicitly in tests
• Test races and timeouts reliably

Test Organization

project/
├── lib/
│   └── mylib.ml
├── test/
│   ├── dune
│   ├── test_mylib.ml          # Unit tests (with mocks)
│   └── test_integration.ml    # Integration tests (if needed)
└── third_party/               # Fetched sources for reference
    └── eio.*/

Best Practices

1. Prefer mocks over real I/O - Tests should be fast and deterministic
2. Fetch dependency sources - Understand mock APIs by reading source
3. Keep integration tests minimal - Most tests should use mocks
4. Test edge cases - Use mocks to inject errors and edge conditions
5. Clean up resources - Even in tests, clean up temp files/state

Running Tests

# Run all tests
dune runtest

# Run tests verbosely
dune runtest --verbose

# Run specific test
dune exec -- test/test_mylib.exe