protocol _Reducer {
    associatedtype State
    associatedtype Action

    func reduce(state: inout State, action: Action)
}

protocol Reducer: _Reducer where State == Body.State, Action == Body.Action {
    associatedtype Body: Reducer

    var body: Body { get }
}

extension Reducer {
    func reduce(state: inout State, action: Action) {
        body.reduce(state: &state, action: action)
    }
}

struct ConcreteReducer<State, Action>: Reducer {
    let performReduce: (inout State, Action) -> Void
    var body: Self { self } // ???

    func reduce(state: inout State, action: Action) {
        performReduce(&state, action)
    }
}

// ERROR: 'Reducer' requires the types 'Action' and '
// (some Reducer).Action' be equivalent
struct EmptyRedcer<State, Action>: Reducer {
    var body: some Reducer {
        ConcreteReducer<State, Action> { state, action in

        }
    }
}