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

```ts

//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
```