getAuthEntity失败“无法匹配预期类型”

我正在尝试向Yesod添加硬编码身份验证。我刚刚简单地修改了Yesod脚手架,并按照文档添加了一个硬编码用户( http://hackage.haskell.org/package/yesod-auth-1.6.3/docs/Yesod-Auth-Hardcoded.html ).所以我有以下代码:

instance YesodAuth App where
    type AuthId App = Either UserId Text

    -- Where to send a user after successful login
    loginDest :: App -> Route App
    loginDest _ = HomeR
    -- Where to send a user after logout
    logoutDest :: App -> Route App
    logoutDest _ = HomeR
    -- Override the above two destinations when a Referer: header is present
    redirectToReferer :: App -> Bool
    redirectToReferer _ = True

    authPlugins _ = [authHardcoded]

    authenticate Creds{..} =
        return
        (case credsPlugin of
            "hardcoded" ->
                case lookupUser credsIdent of
                    Nothing -> UserError InvalidLogin
                    Just m  -> Authenticated (Right (manUserName m)))


-- | Access function to determine if a user is logged in.
isAuthenticated :: Handler AuthResult
isAuthenticated = do
    muid <- maybeAuthId
    return $ case muid of
        Nothing -> Unauthorized "You must login to access this page"
        Just _ -> Authorized


...        

instance YesodAuthPersist App where
    type AuthEntity App = Either User SiteManager

    getAuthEntity (Left uid) =
      do x <- liftHandler $ runDB (get uid)
         return (fmap Left x)
    getAuthEntity (Right username) = return (fmap Right (lookupUser username))
...
instance PathPiece (Either UserId Text) where
    fromPathPiece = readMaybe . unpack
    toPathPiece = pack . show

lookupUser :: Text -> Maybe SiteManager
lookupUser username = find (\m -> manUserName m == username) siteManagers

instance YesodAuthHardcoded App where
    validatePassword u = return . validPassword u
    doesUserNameExist  = return . isJust . lookupUser

validPassword :: Text -> Text -> Bool
validPassword u p =
    case find (\m -> manUserName m == u && manPassWord m == p) siteManagers of
        Just _ -> True
        _      -> False

因此,getAuthEntity似乎已经得到了正确的实现。然而,现在当我尝试使用getAuthEntity获取用户时,如下所示:

getProfileR :: Handler Html
getProfileR = do
    uid <- getAuthEntity   
    defaultLayout $ do
        setTitle . toHtml $ ("hola" :: Text )
        $(widgetFile "profile")

它只会失败,并出现以下错误:

    • Couldn't match expected type ‘HandlerFor App a0’
                  with actual type ‘AuthId (HandlerSite m0)
                                    -> m0 (Maybe (AuthEntity (HandlerSite m0)))’
   |
12 |     uid <- getAuthEntity
   |            ^^^^^^^^^^^^^

我完全不知道会出什么问题。提前感谢您的帮助。