tmu
dc7ce22fc5
Category files under development/ replace the decision prose that was scattered through README.md and CONTRIBUTING.md. Each decision is a block with Decision (YYYY-MM) / Why / Rejected / Known issue; the new library.md records the public API contract and its limitations. AGENTS.md and backlog.tasks now point at the new home.
77 lines
3.4 KiB
Markdown
77 lines
3.4 KiB
Markdown
# Testing
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For this library the types _are_ the feature — narrowing, `exhaustive()`
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returns, the `Matcher<T>` contract — so a runtime-only test loop would verify
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the wrong thing. The contributor-facing commands are in
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[CONTRIBUTING.md](../CONTRIBUTING.md); this file records why the loop is shaped
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the way it is.
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## Type-driven development
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New behavior follows **type-driven development** (in Edwin Brady's sense):
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_treat the type as the plan for a program, and use the compiler and type checker
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as your assistant, guiding you to a complete program that satisfies the type_
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([idris-lang.org](https://www.idris-lang.org/)). Here that plan is the
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`expectTypeOf` assertion, written first. The loop is **type → red → green →
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refactor**:
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1. **Type** — write the compile-time expectation first
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(`expectTypeOf(...).toEqualTypeOf<…>()`) and let `npm run check:tsc` fail on
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the _type_. The type error is the spec you want to hit before the runtime
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logic exists.
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2. **Red** — add the matching runtime assertion (`assert.*`) so
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`npm run test:unit` now fails on behavior.
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3. **Green** — implement in `src/*.ts` until both the type check and the test
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pass.
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4. **Refactor** — with the type system and the tests as the safety net, then
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`npm run verify` as the definition-of-done gate.
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This is why every test in the suite pairs an `expectTypeOf(...)` with an
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`assert.*` — keep them together.
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#### Decision (2026-09)
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Test-driven development is _type_-driven here: the compile-time expectation is
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written before the runtime assertion, and both before the implementation.
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#### Why
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- A runtime-only test can pass while the type is wrong, and a type-level library
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would then ship a broken feature that its tests bless.
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- The type error is a more precise spec than a failing assertion, because it
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states the exact expected type before the logic exists.
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#### Rejected
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- Runtime-first (classic red/green): it verifies the value, not the contract,
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and the contract is the product.
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- Testing the type only: it would not catch handler wiring, `exhaustive()`
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throwing, or the `otherwise` fallback (see `src/index.test.ts`).
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## Enforcement
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Type-first is also enforced structurally: `npm test` runs `check:tsc` before the
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test runner, so a wrong type can never be papered over by a passing assertion.
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Per [AGENTS.md § Never do](../AGENTS.md#never-do), reach green honestly — fix the
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types so both the type check and the runtime assertion pass, never suppress the
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ones you can't make pass.
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## Test tiers
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- `npm run test:unit` — the test runner alone, for the fast local loop.
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- `npm test` — `check:tsc` + the unit suite; this is what pre-push and the
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baseline check run.
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- `npm run test:ci` — adds c8 coverage; used by CI. `c8` uses V8 coverage, so
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the `--strip-types` source is instrumented without a build step.
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The runner is `node --test --strip-types "src/**/*.test.ts"`. It relies on the
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`.ts` import-extension convention (see [tooling.md](./tooling.md#source-imports-use-ts-extensions)).
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#### Known issue
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- The type-aware linter misidentifies `expectTypeOf()` as a floating promise,
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so `src/index.test.ts` carries a file-level `oxlint-disable
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typescript/no-floating-promises` with an explanatory comment. This is a known
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false positive, not a rule we want off project-wide (see
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[tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).
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