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.

Beginne damit, das Verhalten des Verlusts generischer Typen im verlinkten TypeScript Playground zu reproduzieren, mit Fokus auf Parameters und das vorgeschlagene Verhalten von WithReturn<F, R>. Die Aufgabe ist erledigt, wenn ein Utility die generischen Parameter eines Funktionstyps beibehalten und nur dessen Rückgabetyp ersetzen kann, ohne die JavaScript-Laufzeitausgabe zu ändern.