从 Monad 中提升一个值 - Yesod

Question

我想在用户未登录时被重定向到登录页面时显示一条消息。

资源授权：

isAuthorized AdminR _ = isAdmin
isAuthorized _ _ = return Authorized

和

    isAdmin = do
            mu <- maybeAuthId
            return $ case mu of
                    Just "Foo" -> Authorized
                    Just _ -> Unauthorized "You are NOT a Admin !"
                    Nothing -> do
                                    setMessage "You have to Login "
                                    return AuthenticationRequired

错误：

Couldn't match type `m0 AuthResult' with `AuthResult'
Expected type: m0 () -> m0 AuthResult -> AuthResult
  Actual type: m0 () -> m0 AuthResult -> m0 AuthResult
In a stmt of a 'do' block: setMessage "You have to Login "
In the expression:
  do { setMessage "You have to Login ";
       return AuthenticationRequired }
In a case alternative:
    Nothing
      -> do { setMessage "You have to Login ";
              return AuthenticationRequired }

那么，如何从 Monad 中提取 AuthResult ？

Answer 1

由于您在案例结束时运行 monad 操作，因此您无需返回案例。

isAdmin = do
   mu <- maybeAuthId
   case mu of
        Just "Foo" -> Authorized
        Just _ -> Unauthorized "You are NOT a Admin !"
        Nothing -> do
             setMessage "You have to Login "
             return AuthenticationRequired