Possible Monad instance for Source?

gistfile1.hs

sourceJoin :: Resource m => Source m (Source m a) -> Source m a
sourceJoin s = Source $ do
  ps <- prepareSource s
  innerSource <- newRef Nothing
  let pull = do inner <- readRef innerSource
                case inner of
                  Just x -> do st <- sourcePull x
                               case st of
                                 Closed -> do pullFrom Nothing
                                 Open vs -> return $ Open vs
                  Nothing -> do st <- sourcePull ps
                                case st of
                                  Open xs -> do inner' <- prepareSource xs
                                                pullFrom (Just inner')
                                  Closed -> return Closed
      pullFrom x = writeRef innerSource x >> pull
      close = do maybe (return ()) sourceClose =<< readRef innerSource
                 sourceClose ps
  return $ PreparedSource {
      sourcePull = pull
    , sourceClose = close
    }

sourceReturn :: Resource m => a -> Source m a
sourceReturn x = sourceState True
                             (\s -> case s of
                                 False -> return (False, Closed)
                                 True -> return (False, Open x))