Ability to replace return type of function type

Search Terms

• replace return type
• parameters generic

Suggestion

I would like to be able to replace just the return type of a function with a utility type, like WithReturn<F, R>.

I'm not 100% sure this is needed, but it seems like using (...args: Parameters<F>) => R is not sufficient because any generics in F are lost.

Use Cases

My use case is a utility function that captures the arguments to a function to allow it to be called later.

Examples

export const defer = <F extends (...args: any) => any>(f: F) => ((
  ...args: Parameters<F>
) => ({
  function: f,
  args,
})) /* as any as F */;

const map = <T, R>(array: Array<T>, f: (i: T) => R) => array.map(f);
const deferredMap = defer(map);

let r = deferredMap([1], (i: number) => 2); // error now: Type 'unknown' is not assignable to type 'number'.(2345)
r.args; // error if defer returns F

Right now there's an error because the generics to map() are lost when using Parameters.

Playground: https://www.typescriptlang.org/play/?ssl=13&ssc=1&pln=1&pc=1#code/KYDwDg9gTgLgBAYwgOwM7wCbAGbCnAXjgB4AxOUGYZDVOACgDpmBDKAc1QC44XkBPAJSEAfLwEj62HqWEEx9egCg4cZozaceABTYsAtsCpRUZEUrkKA3irjYArsgQwAlih7YANLc2pvAX0FhAHoAKl46Pn4IuHJQ4IBuJSUkNHh9FjBCEgAVTzgAJUk2KBZ+HgBBKFL+YhyRfOkGFx4cy0L2krLGDLApQSTU9DgsXGrgDABZTOzRvHpegeSAGyM4fCI58anM+gBtAEYAXXz6Frhke30AIzx2gCYlqA0OVCSgA

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Bắt đầu bằng cách tái hiện hành vi mất generic trong TypeScript Playground được liên kết, tập trung vào Parameters và hành vi WithReturn<F, R> được đề xuất. Được xem là hoàn tất khi một utility có thể giữ lại các tham số generic của một kiểu hàm trong khi chỉ thay thế kiểu trả về, mà không thay đổi đầu ra khi chạy của JavaScript.