tmu
a25ac6d1e2
Type-driven tests: every expectTypeOf pairs an assert, covering the header matrix (exhaustive vs partial patterns, strict R vs widened returns, string and numeric keys, _ fallback wiring). Negative cases use Parameters<typeof factory>[0] assignability because expect-type's .not.toBeCallableWith collapses to never on these generic Simplify<>-wrapped signatures. Same documented expectTypeOf floating-promise false positive as index.test.ts, so the same file-level disable applies; testing.md known-issue updated to list both files.
47 lines
2.0 KiB
Markdown
47 lines
2.0 KiB
Markdown
# Testing
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For this library the types _are_ the feature, so a runtime-only test loop would
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verify the wrong thing. The 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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The rules — the loop and the pairing rule — are in
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[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).
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What follows is why and what was rejected.
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#### Decision (2026-09)
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The compile-time expectation is written before the runtime assertion, and both
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before the implementation.
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#### Why
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- A runtime-only test can pass while the type is wrong, so a type-level library
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would ship a broken feature 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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The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are in
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[CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).
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`c8` uses V8 coverage, so the `--strip-types` source is instrumented without a
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build step, and the runner relies on the `.ts` import-extension convention (see
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[tooling.md](./tooling.md#source-imports-use-ts-extensions)).
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## Known issues
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- The type-aware linter misidentifies `expectTypeOf()` as a floating promise, so
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test files that use it (`src/index.test.ts`, `src/primitive.test.ts`) carry a
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file-level `oxlint-disable
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typescript/no-floating-promises` with an explanatory comment. It is a known
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false positive, not a rule worth disabling 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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