Control.Error reduce nested cases

ReddisSession.hs

instance ISessionManager RedisSessionManager where

    --------------------------------------------------------------------------
    --load grabs the session from redis.
    load mgr@(RedisSessionManager (Just _) _ _ _ _   _  ) = return mgr
    load mgr@(RedisSessionManager _        _ _ _ rng con) = do
      res <- runMaybeT $ do
        (Payload x) <- MaybeT $ getPayload mgr
        cs <- hoistMaybe . hush $ S.decode x
        liftIO $ runRedis con $ do
          l <- hush <$> hgetall (sessionKey $ rsCSRFToken cs)
          maybe (liftIO $ mkCookieSession rng)
            (\l' -> return $ cs {rsSession = HM.fromList $ map decodeTuple l'}) l
      maybe (liftIO $ loadDefSession mgr)
        (\s -> return $ mgr {session = Just s}) res