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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Suggestion] Mapped types with inner generic arguments and constraints","articleBody":"# Proposal\r\nI'd like to suggest a feature where in mapped types we could unwrap some of the properties.  I'm trying to solve a problem where I want to **unpromise** a type, and this requires access to the inner generic argument type.  This is useful in creating redux style reducers that are strongly typed.\r\n\r\n## Example\r\n```ts\r\n//i'd like access to the inner generic arguments, without having to pass TPayload\r\ntype Unpromise\u003cTPayload, TPromise extend Promise\u003cTPayload\u003e\u003e = TPayload;\r\n\r\n//and then use that also as a type guard for mapped types\r\ntype Reducer\u003cTActions\u003e = {\r\n    [K in keyof TActions where TActions[K] extends Promise\u003cany\u003e]: (payload: Unpromise\u003cTActions[K]\u003e)=\u003evoid;\r\n    [K in keyof TActions where TActions[K] !extends Promise\u003cany\u003e]: (payload: TActions[K] )=\u003evoid;\r\n}\r\n```\r\n### Complete Example\r\n```ts\r\n//redux style actions class\r\nclass Actions{\r\n    get(): Promise\u003cstring\u003e;\r\n}\r\n//redux style reducer\r\nclass MyReducer{\r\n  get(payload:string){...}\r\n}\r\n\r\n//unpromise type\r\ntype Unpromise\u003cTPayload, TPromise extend Promise\u003cTPayload\u003e\u003e = TPayload;\r\n\r\n//example derived usage with generic constraints\r\ntype Reducer\u003cTActions\u003e = {\r\n    [K in keyof TActions where TActions[K] extends Promise\u003cany\u003e]: (payload: Unpromise\u003cTActions[K]\u003e)=\u003evoid;\r\n    [K in keyof TActions where TActions[K] !extends Promise\u003cany\u003e]: (payload: TActions[K] )=\u003evoid;\r\n}\r\n\r\n//redux style reducer wireup\r\nfunction handleActions\u003cTActions\u003e(actions: TActions, reducer: Reducer\u003cTActions\u003e){\r\n...\r\n}\r\n\r\n//this would check that MyReducer has methods similar to Actions taking in the resolved promise payload as an argument\r\nhandleActions(new Actions(), new MyReducer());\r\n```\r\n\r\nrelated to #12424 ","author":{"url":"https://github.com/richardspence","@type":"Person","name":"richardspence"},"datePublished":"2017-07-24T19:13:51.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/17375/TypeScript/issues/17375"}