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我实现了一个类似于此处描述的 Yesod 身份验证系统，通过查找硬编码在列表中的用户名和密码可以正常工作。

我现在尝试通过修改lookupUser和validPassword函数来修改此行为以在数据库中查找用户名和密码。对于初学者，只有lookupUser：

lookupUser :: Text -> Maybe Owner
lookupUser username = do 
    x <- runSqlite "db.sqlite3" . asSqlBackendReader $ selectFirst [ElementOwnerId ==. username] []
    case x of
        Nothing -> Nothing
        Just (Entity k v) -> Just $ Owner username (elementOwnerPw v)

asSqlBackendReader :: ReaderT SqlBackend m a -> ReaderT SqlBackend m a
asSqlBackendReader = id

但是我收到以下我不明白的错误：

    • Couldn't match type ‘Maybe’ with ‘IO’
        arising from a functional dependency between:
          constraint ‘MonadBaseControl IO Maybe’
            arising from a use of ‘runSqlite’
          instance ‘MonadBaseControl Maybe Maybe’ at <no location info>
    • In the first argument of ‘(.)’, namely ‘runSqlite "db.sqlite3"’
      In the expression: runSqlite "db.sqlite3" . asSqlBackendReader
      In a stmt of a 'do' block:
        x <- runSqlite "db.sqlite3" . asSqlBackendReader
               $ selectFirst [ElementOwnerId ==. username] []
    |
238 |     x <- runSqlite "db.sqlite3" . asSqlBackendReader $ selectFirst [ElementOwnerId ==. username] []
    |          ^^^^^^^^^^^^^^^^^^^^^^^^

runSqlite和selectFirst的签名：

runSqlite :: MonadCatchIO m => SqliteReader m a -> Pool Connection -> m a

selectFirst :: (PersistEntity val, PersistEntityBackend val ~ b) => [Filter val] -> [SelectOpt val] -> b m (Maybe (Entity val))
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