type Selector<S> = (state: S) => any

export interface Reducer<T = any> {
  (state: T | undefined, action: any): T
}

type UnboxState<R extends Reducer> = R extends Reducer<infer U> ? U : never

declare function example<R extends Reducer, S extends Selector<UnboxState<R>>>(r: R, s: S) : void

example(
    (state, action) => ({
        name: 'Gio Va'
    }),
    state => 23
)

declare function example2<R extends Reducer, S extends Selector<UnboxState<R>>>(x: { r: R, s: S }) : void

example2({
    r: (state, action) => ({
        name: 'Gio Va'
    }),
    s: state => 23
})