Ability to parametrize generic function type

Suggestion

I would like to propose something along the lines of $Call utility type in Flow so it is possible to represents the result of calling a generic function with a concrete parameters.

Alternatively could be an utility type that hoists parameters from generic function type into type itself e.g.

HoistTypeParams<<I, O>(inn:I[], f:(p:I) => O) => O[]> // => type DerivedType<I, O> = (inn:I[], f:(p:I) => O) => O[]

Or yet another alternative might be to add some way to parametrize through infer e.g. I would have expected following to work:

type $Call<T extends (args: any[]) => any, Args extends unknown[]> =
  T extends (...args:Args) => infer O ? O : never

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✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

📃 Motivating Example

Today it seems impossible capture return type of the generic function, even when concrete type of the input is known. Both built-in ReturnType and infer based solutions turn generics into a unknowns.

Here is concrete example:

type Map = <I, O> (list:I[], f:(i:I) => O) => O[]

ReturnType<Map> // unknown[]
Map extends (...args:infer Args) => infer O ? O : never // unknown[]
Call<Map, [number[], (i:number) => string]> // unknown[]

type Call<F, Args extends unknown[]> = F extends (...args:Args) => infer O ? O : never

As per https://github.com/microsoft/TypeScript/issues/33185#issuecomment-526969274 it is possible to accomplish this at the implementation level, however I do not believe it is possible to do it at the type level.

💻 Use Cases

Libraries that generate more complex interfaces from a simple ones, something like RPC client for a service with generics becomes impossible here to express:

type Service<T, Path extends PropertyKey[]=[]> = {
  [K in keyof T]: T[K] extends (...args:infer I) => infer O ? Method<I, O, [...Path, K]> : Service<T[K], [...Path, K]>
}

interface Method<I extends unknown[], O, Path extends PropertyKey[]> {
  <Args extends I> (...args:Args): Promise<O>
  meta: {  path: Path }
}


declare function service <T>(impl:T):Service<T>
const s = service({
  map <I, O>(inn:I[], f:(inn:I) => O):O[] {
    return inn.map(f)
  }
})

s.map([1, 2, 3], (n:number):string => String(number))
//               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// Argument of type '(n: number) => string' is not assignable to parameter of type '(inn: unknown) => unknown'.
//  Types of parameters 'n' and 'inn' are incompatible.
//    Type 'unknown' is not assignable to type 'number'.(2345)

Inizia esaminando gli esempi motivazionali per ReturnType, infer e l’utilità Call proposta, insieme all’issue #33185 indicata. Definisci come i parametri di una funzione generica diventano concreti e verifica che gli esempi di Map e service producano i tipi di ritorno previsti senza modificare il comportamento a runtime.