快速通用协议问题？

Question

我正在测试一个简单的Swift Redux实现。有人可以解释为什么打电话吗store.dispatch(.test)原因：

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

尽管AppAction实现了Action协议，为什么无法将AppAction强制转换为Action？

中间件接受(S, Action, (Action) -> Void)，我将其传递给dispatch(_ action: A)作为第三个参数。这是((Action) -> Void)的一种，但它不能接受。

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

感谢@ martin-r提供线索。在阅读@ rob-napier答案后：“但是函数参数的工作顺序相反。（字符串）->虚空是（任何）->虚空的超类型”，我将代码重写为以下代码段。这可能会节省某人处理相同问题的时间。