René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Tuple iteration and merging","articleBody":"**Edit:** Remove indexing part\r\n**Edit 2:** Replace tuple mapping syntax with [normal mapped types](https://github.com/Microsoft/TypeScript/pull/26063)\r\n\r\n## Search Terms\r\n\r\n\u003c!-- List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily --\u003e\r\n\r\nConcatentate and merge tuples' entries.\r\n\r\n## Suggestion\r\n\r\n\u003c!-- A summary of what you'd like to see added or changed --\u003e\r\n\r\n1. I would like to be able to extract and spread in positions other than the last part of a tuple.\r\n1. I would like to merge a tuple into an intersection or concatenation of its members.\r\n\r\n## Use Cases\r\n\r\n\u003c!--\r\nWhat do you want to use this for?\r\nWhat shortcomings exist with current approaches?\r\n--\u003e\r\n\r\n- Converting a tuple to its intersection, such as with `Object.assign`'s assignee.\r\n- Typing `Array.prototype.concat` correctly for tuples.\r\n\r\n## Syntax\r\n\r\nThe syntax comes in a few new forms:\r\n\r\n- `[...A, B]` appends `B` to the tuple `A`. Similarly, `[...infer A, any] extends B ? A : []` drops the last item of the tuple and `[...any[], infer A] extends B ? A : never` extracts the first.\r\n- `{... ...T}` for an n-ary merge and `[... ...T]` an n-ary concatenation.\r\n\r\n## Examples\r\n\r\nHere's the types for each method I mentioned above.\r\n\r\n```ts\r\ninterface Array\u003cT\u003e {\r\n    concat\u003cU extends any[]\u003e(...others: U): [...this, ... ...{[I in keyof U]: (\r\n        U[I] extends any[] ? N : U[I] extends ArrayLike\u003cinfer R\u003e ? R[] : [U[I]]\r\n    )}];\r\n}\r\n\r\ninterface ObjectConstructor {\r\n    assign(target: {... ...typeof sources}, ...sources: object[]): typeof target;\r\n}\r\n```\r\n\r\n\u003c!-- Show how this would be used and what the behavior would be --\u003e\r\n\r\n## Checklist\r\n\r\nMy suggestion meets these guidelines:\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. new expression-level syntax)","author":{"url":"https://github.com/dead-claudia","@type":"Person","name":"dead-claudia"},"datePublished":"2018-07-30T06:57:29.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":14},"url":"https://github.com/26058/TypeScript/issues/26058"}