# 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

New behavior follows **type-driven development** (in Edwin Brady's sense):
_treat the type as the plan for a program, and use the compiler and type checker
as your assistant, guiding you to a complete program that satisfies the type_
([idris-lang.org](https://www.idris-lang.org/)). Here that plan is the
`expectTypeOf` assertion, written first. The loop is **type → red → green →
refactor**:

1. **Type** — write the compile-time expectation first
   (`expectTypeOf(...).toEqualTypeOf<…>()`) and let `npm run check:tsc` fail on
   the _type_. The type error is the spec you want to hit before the runtime
   logic exists.
2. **Red** — add the matching runtime assertion (`assert.*`) so
   `npm run test:unit` now fails on behavior.
3. **Green** — implement in `src/*.ts` until both the type check and the test
   pass.
4. **Refactor** — with the type system and the tests as the safety net, then
   `npm run verify` as the definition-of-done gate.

This is why every test in the suite pairs an `expectTypeOf(...)` with an
`assert.*` — keep them together.

#### 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.

## Test tiers

- `npm run test:unit` — the test runner alone, for the fast local loop.
- `npm test` — `check:tsc` + the unit suite; this is what pre-push and the
  baseline check run.
- `npm run test:ci` — adds c8 coverage; used by CI. `c8` uses V8 coverage, so
  the `--strip-types` source is instrumented without a build step.

The runner is `node --test --strip-types "src/**/*.test.ts"`. It relies on the
`.ts` import-extension convention (see [tooling.md](./tooling.md#source-imports-use-ts-extensions)).

#### Known issue

- 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)).