want a type operator that converts a function type to the type of one of its parameters

[fc01]

//Type A ----> Type B

type A = (state: string, action: any) => string
type B = string


//SyntaxError
type GetB<K extends (state: S, action: any) => S, S ???? > = S 
type xxx = GetB<A>  


//right
const getB = <S>(f: (state: S, action: any) => S) => ({} as S)
let b = getB({} as A) //string

[jcalz]

I think a little prose would be helpful for people trying to understand what you're asking.

If I read you correctly, you want a type operator that converts a function type to the type of one of its parameters. That's not currently possible, although #6606 would presumably let you do this:

type Param1<T extends Function> =
  typeof (null! as <P=never>(f: (p1: P, ...rest: any[]) => any) => P)(null! as T);
type xxx = Param1<A>

So this is either a duplicate of #6606 or a new request that could be tracked in #16392, maybe? Not sure.

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.