Swift 通用协议问题？

Question

I'm testing a simple Swift Redux implementation. Could somebody explain why calling store.dispatch(.test) causes:

Could not cast value of type '(Test.AppAction) -> ()' to '(Test.Action) -> ()'.

Why it is not possible to cast AppAction to Action?, although AppAction implements Action protocol.

Middleware accepts (S, Action, (Action) -> Void) , I pass it dispatch(_ action: A) as the 3rd param. which is a type of ((Action) -> Void) but it can't accept it.

protocol State {}
protocol Action {}

typealias Reducer<S: State, A: Action> = (S, A) -> S
typealias Dispatcher = (Action) -> Void
typealias Middleware<S: State> = (S, Action, @escaping Dispatcher) -> Void

protocol Store: ObservableObject {
   associatedtype S: State
   associatedtype A: Action

   func dispatch(action: A)
}

final class DefaultStore<S: State, A: Action>: ObservableObject {
   @Published private(set) var state: S

   private let reducer: Reducer<S, A>
   private let middlewares: [Middleware<S>]

   init(initialState: S, reducer: @escaping Reducer<S, A>, middlewares: [Middleware<S>] = []) {
      self.state = initialState
      self.reducer = reducer
      self.middlewares = middlewares
   }

   func dispatch(_ action: A) {
      state = reducer(state, action)

      middlewares.forEach { middleware in
         middleware(state, action, dispatch as! Dispatcher)
      }
   }
}

// START

struct AppState: State { }
enum AppAction: Action { // A test action to have smthg. to call
   case test
}

let appReducer: Reducer<AppState, AppAction> = { s, a in s }
let middleware: Middleware<AppState> = { s, a, dispatch in }

var store = DefaultStore(initialState: AppState(), reducer: appReducer, middlewares: [middleware])
store.dispatch(.test)

Answer 1

Thanks @martin-r for clues. After reading @rob-napier answer: "But function parameters work in the reverse order. (String)->Void is a supertype of (Any)->Void" from the links you posted, I rewrote the code to snippet bellow. It might save someone's time dealing with the same problem.

protocol State {}
protocol Action {}

typealias Reducer<S: State, A: Action> = (S, A) -> S
typealias Dispatcher<A: Action> = (A) -> Void
typealias Middleware<S: State, A: Action> = (S, A, (A) -> Void) -> Void

protocol Store: ObservableObject {
   associatedtype S: State
   associatedtype A: Action

   func dispatch(action: A)
}

final class DefaultStore<S: State, A: Action>: ObservableObject {
   @Published private(set) var state: S

   private let reducer: Reducer<S, A>
   private let middlewares: [Middleware<S, A>]

   init(initialState: S, reducer: @escaping Reducer<S, A>, middlewares: [Middleware<S, A>] = []) {
      self.state = initialState
      self.reducer = reducer
      self.middlewares = middlewares
   }

   func dispatch(_ action: A) {
      state = reducer(state, action)

      middlewares.forEach { middleware in
         middleware(state, action, dispatch)
      }
   }
}