Sink that forks input to two other sinks

SinkFork.hs

{-# LANGUAGE ScopedTypeVariables, BangPatterns #-}
{-# OPTIONS -Wall #-}
import Data.Conduit
import qualified Data.Conduit.List as CL
import qualified Data.Conduit.Binary as CB

sinkFork::Resource m => Sink a m b -> Sink a m c -> Sink a m (b,c)
sinkFork mlsink0 mrsink0 = do
  SinkData (push0 Nothing) (close0 Nothing)
    where push0 st input = do
            newst <- case st of
              Nothing -> do
                lsink <- sinkPush mlsink0 input
                rsink <- sinkPush mrsink0 input
                return $ Just (lsink, rsink)
              Just (Processing lpush lclose, Processing rpush rclose) -> do
                lsink <- lpush input
                rsink <- rpush input
                return $ Just (lsink, rsink)
              Just (Processing lpush lclose, rsink@(Done rr ro)) -> do
                lsink <- lpush input
                return $ Just (lsink, rsink)
              Just (lsink@(Done ll lo), Processing rpush rclose) -> do
                rsink <- rpush input
                return $ Just (lsink, rsink)
              Just (lsink@(Done ll lo), rsink@(Done rr ro)) -> do
                return $ Just (lsink, rsink)

            case newst of
              Just (Processing lpush lclose, Processing rpush rclose) -> do
                 return $ Processing (push0 newst) (close0 newst)
              Just (Processing lpush lclose, Done rr !ro) -> do
                 return $ Processing (push0 newst) (close0 newst)
              Just (Done ll !lo, Processing rpush rclose) -> do
                 return $ Processing (push0 newst) (close0 newst)
              Just (Done ll !lo, Done rr !ro) -> do
                 return $ Done Nothing (lo,ro) -- what gets returned here?
              Nothing -> error "newst Nothing should not happen"
              
          close0 (Just (Processing lpush lclose, Processing rpush rclose)) = do 
              l <- lclose
              r <- rclose
              return (l,r)
          close0 (Just (Processing lpush lclose, Done rr !r)) = do 
              l <-  lclose
              return (l,r)
          close0 (Just (Done ll !l, Processing rpush rclose)) = do 
              r <- rclose
              return (l,r)
          close0 (Just (Done ll !l, Done rr !r)) = do 
              return (l,r)
          close0 Nothing = do
               l <- sinkClose mlsink0 
               r <- sinkClose mrsink0 
               return (l,r)