Swift 通用協議問題？

Question

我正在測試一個簡單的 Swift Redux 實現。 有人可以解釋為什么調用store.dispatch(.test)會導致：

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

為什么不能將 AppAction 轉換為 Action？，盡管 AppAction 實現了 Action 協議。

中間件接受(S, Action, (Action) -> Void) ，我將它dispatch(_ action: A)作為第三個參數傳遞。 這是一種((Action) -> Void)但它不能接受它。

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

感謝@martin-r 提供線索。 在閱讀@rob-napier 答案后：“但是 function 參數以相反的順序工作。（字符串）-> Void 是您發布的鏈接中的（任何）-> Void 的超類型”，我將代碼重寫為下面的片段。 它可能會節省某人處理相同問題的時間。

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)
      }
   }
}