docs: document the exhaustiveness vs. generics trade-off in match constructs#85
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WalkthroughREADME.md の ChangesREADME Type Safety ガイダンス更新
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Code Review
This pull request updates the README.md to clarify PHPStan's behavior regarding type narrowing and exhaustiveness checks, specifically comparing the use of instanceof in match (true) against the match() method and isOk()/isErr(). The reviewer suggested a minor phrasing improvement to use "preserves the type arguments" instead of "keeps T/E" for better terminology consistency with the rest of the text.
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| plain `Ok`, so `unwrap()` becomes `mixed`), so use `instanceof` in a | ||
| `match (true)` purely for exhaustiveness. When you also need the values, prefer | ||
| the `match()` method — it is exhaustive by construction (both arms are required) | ||
| and keeps `T`/`E` — or narrow with `isOk()`/`isErr()`. (`isOk()`/`isErr()` arms |
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直前の文で loses the type arguments と表現されているため、こちらでも keeps T/E ではなく preserves the type arguments（または keeps the type arguments）と表現する方が、用語の一貫性と正確性の観点からより適切です。
| and keeps `T`/`E` — or narrow with `isOk()`/`isErr()`. (`isOk()`/`isErr()` arms | |
| and preserves the type arguments — or narrow with `isOk()`/`isErr()`. (`isOk()`/`isErr()` arms |
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Pull request overview
Updates the README’s “Type Safety” section to more precisely explain the PHPStan trade-off between (1) enforced exhaustiveness checking when using match (true) + instanceof, and (2) preserving generic type arguments when using match() or isOk()/isErr().
Changes:
- Rewrites the “Sealed interface” paragraph to clarify that enforced exhaustiveness and preserved generics are currently mutually exclusive in PHPStan 2.2.2.
- Documents that the
match()method preservesT/Eand requires both success/error handlers. - Adds context that
Resultis practically fixed toOk|Err, somatch()is total for real cases.
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| `match (true)` purely for exhaustiveness. When you also need the values, the | ||
| `match()` method handles both cases and keeps `T`/`E` (it requires both an `ok` | ||
| and an `err` arm), or narrow with `isOk()`/`isErr()`. These two goals are a | ||
| trade-off in PHPStan 2.2.2: *enforced* exhaustiveness — where adding a new | ||
| `Result` variant would turn every unhandled site into an analysis error — comes | ||
| only from `instanceof` in a `match (true)`, which is exactly the form that drops | ||
| the type arguments. The `match()` method and `isOk()`/`isErr()` keep the generics | ||
| but are not checked against variant additions (`isOk()`/`isErr()` arms in a | ||
| `match (true)` also need a `default`). So per call site you currently pick one: | ||
| enforced exhaustiveness *or* preserved generics. (In practice `Result` is fixed | ||
| at `Ok|Err`, so the `match()` method covering both is total for all real cases.) |
概要
Type Safety セクションの Sealed interface の記述を、
match (true)における「厳密な網羅性チェック」と「ジェネリクス保持」のトレードオフを正確に説明する内容へ更新しました。背景（PHPStan 2.2.2 で実測）
3つ目の variant を sealed に追加して「未対応ハンドラが静的解析エラーになるか（=厳密な網羅性）」を検証した結果:
（variant 追加で自動エラー）
match (true)+instanceofOk→unwrap()がmixedisOk()/isErr()アームdefault必須で variant 追加を吸収）Ok<int>match()メソッドU|V厳密な（enforced）網羅性が得られるのは
match (true)+instanceofだけで、それは型引数を失うことが確定しました。match()メソッドは「現在の2アリティに対して total」ではあるものの、variant 追加に対する自動検出は行いません。変更内容
Resultは実際にはOk|Errの2値固定なので、match()メソッドが実用上は全ケースを賄える点も補足ドキュメントのみの変更です。