# Testing

For this library the types _are_ the feature — narrowing, `exhaustive()`
returns, the `Matcher<T>` contract — so a runtime-only test loop would verify
the wrong thing. The contributor-facing commands are in
[CONTRIBUTING.md](../CONTRIBUTING.md); this file records why the loop is shaped
the way it is.

## Type-driven development

The rules — the loop and the pairing rule — are in
[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).
What follows is why the loop is type-driven and what was rejected.

#### Decision (2026-09)

Test-driven development is _type_-driven here: the compile-time expectation is
written before the runtime assertion, and both before the implementation.

#### Why

- A runtime-only test can pass while the type is wrong, and a type-level library
  would then ship a broken feature that its tests bless.
- The type error is a more precise spec than a failing assertion, because it
  states the exact expected type before the logic exists.

#### Rejected

- Runtime-first (classic red/green): it verifies the value, not the contract,
  and the contract is the product.
- Testing the type only: it would not catch handler wiring, `exhaustive()`
  throwing, or the `otherwise` fallback (see `src/index.test.ts`).

## Enforcement

Type-first is also enforced structurally: `npm test` runs `check:tsc` before the
test runner, so a wrong type can never be papered over by a passing assertion.
Per [AGENTS.md § Never do](../AGENTS.md#never-do), reach green honestly — fix the
types so both the type check and the runtime assertion pass, never suppress the
ones you can't make pass.

The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
listed in
[CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).
`c8` uses V8 coverage, so the `--strip-types` source is instrumented without a
build step. The runner relies on the `.ts` import-extension convention (see
[tooling.md](./tooling.md#source-imports-use-ts-extensions)).

## Known issues

- The type-aware linter misidentifies `expectTypeOf()` as a floating promise,
  so `src/index.test.ts` carries a file-level `oxlint-disable
typescript/no-floating-promises` with an explanatory comment. This is a known
  false positive, not a rule we want off project-wide (see
  [tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).