Apply function generics as part of type definition

[jmacmahon]

Search Terms

generic typeof functions

Suggestion

The problem is with the code as below:

const id = <T>(val: T) => val
type IdType = typeof id
// type IdType = <T>(val: T) => T

type Ret = ReturnType<typeof id>
// type Ret = unknown

It would be nice to be able to do something like so:

const id = <T>(val: T) => val
type IdType<T> = (typeof id)<T>
// type IdType<T> = (val: T) => T

// Or by extension:
type Ret<T> = ReturnType<(typeof id)<T>>
// type Ret<T> = T

Use Cases

This would be useful to use with the vriad/zod library, to implement different field validators depending on a parameter, like so:

// the dateSchema param could be a schema for an ISO 8601 string or a Date object, or even some custom date-like type
const fooSchema = <T>(dateSchema: z.ZodSchema<T>) => z.object({
  name: z.string(),
  date: dateSchema
})
type Foo<T> = z.infer<ReturnType<(typeof fooSchema)<T>>>
// expected result: type Foo<T> = { name: string, date: T }

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.

[Gozala]

I am understanding this proposal correctly that IdType here

const id = <T>(val: T) => val
type IdType = typeof id

would be type IdType<T> = (val:T) => T as opposed to type IdType = <T>(val:T) => T ? Or is idea here that you you should be able to qualify generic function type like type IdType = <T>(val:T) => T via <T> suffix e.g. type IdNum = IdType<number> // (val:number) => number ?

[jmacmahon]

I meant the latter, so like you say, type IdNum = IdType<number> // (val:number) => number.

I think the former would be incorrect, because the parameter T is not present when declaring type IdType.

[ahejlsberg]

This feature is now implemented in #47607.