CODEWITHSUNDEEP
haskellyesod
amitaibu
amitaibu

Reputation: 1116

Run Monad operations wrapped by another Monad

Given the websockets example how can I have Handler related operations such as runDB inside (e.g. runDB $ selectList ...)

Here's a non working example of what I want to do:

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)
     -- ....

Here's the error:

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]

Upvotes: 2

Views: 85

Answers (1)

amitaibu
amitaibu

Reputation: 1116

The answer is to use lift and not liftIO

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

Upvotes: 1

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