testing

---
name: testing
description: Testing strategies for OCaml libraries. Use when discussing tests, alcotest, eio mocks, test structure, or test-driven development in OCaml projects.
---

# OCaml Testing

## Test Directory Structure

Use `test/` directory with:
- `test.ml` - Main runner controlling initialization order
- `test_x.ml` - One file per module `x.ml` being tested, exports `suite`

```
lib/
├── foo.ml
└── bar.ml
test/
├── dune
├── test.ml          # Main runner
├── test_foo.ml      # suite : (string * unit Alcotest.test_case list) list
└── test_bar.ml
```

For single-module libraries, a single `test_foo.ml` as runner is acceptable.

## Dune Configuration

```dune
(test
 (name test)
 (libraries mylib alcotest logs logs.fmt fmt.tty))
```

## Main Runner Pattern (test.ml)

The main `test.ml` controls initialization order for side effects:

```ocaml
(* 1. Initialize RNG before any test module is loaded *)
let () = Crypto_rng_unix.use_default ()

(* 2. Set up logging *)
let () = Fmt_tty.setup_std_outputs ()
let () = Logs.set_reporter (Logs_fmt.reporter ())
let () = Logs.set_level (Some Logs.Debug)

(* 3. Run all test suites *)
let () = Alcotest.run "mylib" Test_foo.suite
```

For multiple modules:

```ocaml
let () = Crypto_rng_unix.use_default ()
let () = Alcotest.run "mylib" (Test_foo.suite @ Test_bar.suite)
```

## Module Test File Pattern (test_x.ml)

Each module exports a `suite` value. **Do not initialize RNG or run Alcotest here.**

```ocaml
(** Tests for Foo module. *)

let test_basic () =
  let result = Foo.process "input" in
  Alcotest.(check string) "expected output" "output" result

let test_empty () =
  let result = Foo.process "" in
  Alcotest.(check string) "empty input" "" result

let suite =
  [
    ( "process",
      [
        Alcotest.test_case "basic" `Quick test_basic;
        Alcotest.test_case "empty" `Quick test_empty;
      ] );
  ]
```

## Lazy State for Module-Level Values

If a test module needs RNG at load time, use lazy evaluation:

```ocaml
let key = lazy (Crypto_rng.generate 32)
let key () = Lazy.force key

let test_encrypt () =
  let ciphertext = Foo.encrypt ~key:(key ()) plaintext in
  ...
```

This defers RNG use until tests actually run, after `test.ml` initializes the RNG.

## Alcotest Patterns

### Custom Testables

```ocaml
let result_testable ok_t =
  Alcotest.result ok_t Alcotest.string

let my_type_testable =
  Alcotest.testable My_type.pp My_type.equal
```

### Common Checks

```ocaml
Alcotest.(check int) "count" 42 actual
Alcotest.(check string) "name" expected actual
Alcotest.(check bool) "flag" true actual
Alcotest.(check (list int)) "items" [1;2;3] actual
Alcotest.(check (option string)) "maybe" (Some "x") actual
Alcotest.(check (result int string)) "result" (Ok 42) actual
```

### Testing Exceptions

```ocaml
let test_raises () =
  Alcotest.check_raises "should fail" (Invalid_argument "bad")
    (fun () -> Foo.parse "bad")
```

## Initialization Order

Always initialize in `test.ml` before `Alcotest.run`:

1. **RNG** - `Crypto_rng_unix.use_default ()` or `Mirage_crypto_rng_unix.use_default ()`
2. **Logging** - `Logs.set_reporter` and `Logs.set_level`
3. **Other global state** - Environment setup, temp directories

This ensures deterministic test ordering and proper side-effect sequencing.

## Test Logging

Set up logging in tests using the standard `Logs` library:

```ocaml
let () = Fmt_tty.setup_std_outputs ()
let () = Logs.set_reporter (Logs_fmt.reporter ())
let () = Logs.set_level (Some Logs.Debug)
```

**Default behaviour**: Logs at Debug level. Alcotest captures output to a file by default, so verbose logging doesn't clutter the terminal. Output is shown only when tests fail.

### Per-Source Overrides

For per-source control, set levels after the reporter:

```ocaml
let () = Logs.Src.set_level Conpool.src (Some Logs.Debug)
let () = Logs.Src.set_level Requests.src (Some Logs.Warning)
```

### Why Default to Debug?

1. **Alcotest captures output**: Verbose logs don't clutter terminal
2. **Shown on failure**: When a test fails, you see all the debug info
3. **No re-running needed**: Debug output is already captured

## Core Philosophy

1. **Unit Tests First**: Prioritize unit tests for individual modules and functions.
2. **1:1 Test Coverage**: Every module in `lib/` should have a corresponding test module in `test/`.
3. **Isolated Tests**: Each test should be independent and not rely on external state.
4. **Clear Test Names**: Test names should describe what they test, not how.
5. **Test Inclusion**: All test suites must be included in the main test runner.

## Naming Conventions

- **Test suite names**: lowercase, single words (e.g., `"users"`, `"commands"`, `"process"`)
- **Test case names**: lowercase with underscores, concise but descriptive (e.g., `"basic"`, `"empty_input"`, `"parse_error"`)

## Writing Good Tests

**Function Coverage**: Test all public functions exposed in `.mli` files, including success, error, and edge cases.

**Test Data**: Use helper functions to create test data:

```ocaml
let make_user ?(name = "test") ?(id = 1) () = User.v ~name ~id
```

**Property-Based Testing**: For complex logic, consider property-based testing with QCheck:

```ocaml
let test_roundtrip =
  QCheck.Test.make ~count:1000
    ~name:"encode then decode is identity"
    QCheck.string
    (fun s -> Codec.decode (Codec.encode s) = s)
```

## End-to-End Testing with Cram

Cram tests verify CLI executable behavior.

**Use Cram Directories**: Every Cram test should be a directory ending in `.t` (e.g., `my_feature.t/`).

**Create Actual Test Files**: Avoid embedding code within `run.t` using heredocs. Create real source files within the test directory.

```
test/
└── my_feature.t/
    ├── run.t           # The cram test script
    ├── input.txt       # Test input file
    └── expected.json   # Expected output
```

## Running Tests

```bash
# Run all tests
dune test

# Run tests and watch for changes
dune test -w

# Run a specific test
dune exec test/test.exe -- test "suite_name"

# Run tests with coverage
dune test --instrument-with bisect_ppx
bisect-ppx-report summary
```

• SKILL.md 6 KB
• templates/test_eio_mock.ml 2 KB
• templates/test_template.ml 1 KB