Could not deduce (MonadIO Maybe) arising from a use of ‘throwString’ on P110

[paulspencerwilliams]

The provided findUserIdBySessionId function fails to compile

findUserIdBySessionId :: Redis r m => D.SessionId -> m (Maybe D.UserId)
findUserIdBySessionId sId = do
  result <- withConn $ R.get (encodeUtf8 sId)
  return $ case result of
    Right (Just uIdStr) -> readMay . unpack . decodeUtf8 $ uIdStr
    err                 -> throwString $ "Unexpected redis error: " <> show err

due to

    • Could not deduce (MonadIO Maybe)
        arising from a use of ‘throwString’
      from the context: Redis r m
        bound by the type signature for:
                   findUserIdBySessionId :: forall r (m :: * -> *).
                                            Redis r m =>
                                            D.SessionId -> m (Maybe D.UserId)
        at src/Adapter/Redis/Auth.hs:37:1-71
    • In the expression:
        throwString $ "Unexpected redis error: " <> show err
      In a case alternative:
          err -> throwString $ "Unexpected redis error: " <> show err
      In the second argument of ‘($)’, namely
        ‘case result of
           Right (Just uIdStr) -> readMay . unpack . decodeUtf8 $ uIdStr
           err -> throwString $ "Unexpected redis error: " <> show err’
   |
42 |     err                 -> throwString $ "Unexpected redis error: " <> show err

Pushing the return $ into the Right clause resolves this:

findUserIdBySessionId :: Redis r m => D.SessionId -> m (Maybe D.UserId)
findUserIdBySessionId sId = do
  result <- withConn $ R.get (encodeUtf8 sId)
  case result of
    Right (Just uIdStr) -> return $ readMay . unpack . decodeUtf8 $ uIdStr
    err                 -> throwString $ "Unexpected redis error: " <> show err