Question

鉴于websockets example如何在Handler内部runDB进行runDB $ selectList ...相关操作（例如chatStream :: WebSocketsT Handler () chatStream = do users <- liftIO $ runDB $ selectList [] [] :: Handler [Entity User] -- This line is not working sendTextData ("Welcome to the chat server, please enter your name." :: Text) -- ....）

这是我想要做的一个非工作的例子：

Couldn't match type ‘HandlerT App IO [Entity User]’
               with ‘ReaderT
                       websockets-0.9.7.0:Network.WebSockets.Connection.Connection
                       Handler
                       t0’
Expected type: ReaderT
                 websockets-0.9.7.0:Network.WebSockets.Connection.Connection
                 Handler
                 t0
  Actual type: Handler [Entity User]
In a stmt of a 'do' block:
  users <- liftIO $ runDB $ selectList [] [] :: Handler [Entity User]

这是错误：

Error:(27, 13) Failed to resolve: com.google.android.gms:play-services:9.4.0
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Answer 1

答案是使用lift而非liftIO

users <- lift $ (runDB $ selectList [] [] :: Handler [Entity User])

运行由另一个Monad包裹的Monad操作

1 个答案: