René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Mapping a generic type should again result in a generic type","articleBody":"# Suggestion\r\n\r\n## 🔍 Search Terms\r\n\r\n* mapped type\r\n* type mapping\r\n* generics\r\n* generic function\r\n* unknown \r\n\r\n## ✅ Viability Checklist\r\n\r\nMy suggestion meets these guidelines:\r\n\r\n* [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code\r\n* [x] This wouldn't change the runtime behavior of existing JavaScript code\r\n* [x] This could be implemented without emitting different JS based on the types of the expressions\r\n* [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)\r\n* [x] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals).\r\n\r\n## ⭐ Suggestion\r\n\r\nPerforming type mapping on a generic type should result in a new generic type with that mapping applied, rather than a concrete type with `unknown` in place of generics.\r\n\r\n## 📃 Motivating Example\r\n\r\nConsider the following utility type `Async\u003cT\u003e`. It takes an arbitrary function signature and returns the signature of an equivalent asynchronous function.\r\n\r\n```ts\r\ntype Async\u003cT extends (...args: any) =\u003e any\u003e = T extends (...args: infer Args) =\u003e infer Result\r\n  ? (...args: Args) =\u003e Promise\u003cResult\u003e\r\n  : never;\r\n\r\ntype SimpleFunction = (arg: number) =\u003e string;\r\ntype AsyncSimpleFunction = Async\u003cSimpleFunction\u003e;\r\n// result: (arg: number) =\u003e Promise\u003cstring\u003e ✅\r\n```\r\n\r\nNow let's give it a generic function:\r\n\r\n```ts\r\ntype GenericFunction1 = \u003cT\u003e(arg: T) =\u003e string;\r\ntype AsyncGenericFunction1 = Async\u003cGenericFunction1\u003e;\r\n// expected: \u003cT\u003e(arg: T) =\u003e Promise\u003cstring\u003e\r\n// actual: (arg: unknown) =\u003e Promise\u003cstring\u003e ⛔️\r\n\r\ntype GenericFunction2 = \u003cT\u003e(arg: number) =\u003e T;\r\ntype AsyncGenericFunction2 = Async\u003cGenericFunction2\u003e;\r\n// expected: \u003cT\u003e(arg: number) =\u003e Promise\u003cT\u003e\r\n// actual: (arg: number) =\u003e Promise\u003cunknown\u003e ⛔️\r\n```\r\n\r\nAs this example shows, we cannot use the helper type `Async\u003cT\u003e` with a generic function signature. If we try, we get a non-generic function signature with `unknown` wherever a generic argument was used.\r\n\r\n## 💻 Use Cases\r\n\r\nWe maintain a plugin system. Plugins are developed against a carefully designed API. For every type in the plugin API, there is a corresponding internal type. Plugin values and internal values are identical at runtime, but their types differ. For instance, the same concept may be represented by a class internally, but by a branded interface towards the plugins.\r\n\r\nTo convert between an internal type and its corresponding plugin type, we use a recursive mapped type. This words great, *unless* the value is (or contains) a generic function.","author":{"url":"https://github.com/DanielSWolf","@type":"Person","name":"DanielSWolf"},"datePublished":"2022-02-25T11:11:25.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/48036/TypeScript/issues/48036"}