Thanks for the response. I spent some time thinking about leftovers and understand the Category issue now. Thanks for clearing that up. While trying to work conduits into a program I'm working on, I find myself wanting something more powerful: a resumable Conduit. For example, consider receiving a stream of messages over a network: data Message = Data ByteString | CompressedData ByteString | RestartCompressor When CompressedData is received, feed the bytes to a decompressor conduit. When RestartCompressor is received, close the first decompressor conduit and fire up a new one. Supporting restarts needs more than just Conduit i m o -> Conduit (Either x i) m (Either x o). It involves opening and closing a conduit within another conduit's operations. Here's a possible API for a resumable Conduit: newtype ResumableConduit i m o = -- hidden -- newResumableConduit :: Monad m => Conduit i m o -> ResumableConduit i m o -- | Feed the 'Source' through the conduit, and send any output from the -- conduit to the 'Sink'. When the 'Sink' returns, close the 'Source', but -- leave the 'ResumableConduit' open so more data can be passed through it. runResumableConduit :: Monad m => ResumableConduit i m o -> Source m i -> Sink o m r -> m (ResumableConduit i m o, r) -- | Tell the conduit there is no more input available, and send the remaining -- output (if any) to the 'Sink'. closeResumableConduit :: Monad m => ResumableConduit i m o -> Sink o m r -> m r Does anyone want to comment on this interface? Perhaps conduit could have a module called "Data.Conduit.Resumable" that contains ResumableSource, ResumableConduit, and ResumableSink. The conduit-resumablesink package by Andrew Miller [1] implements ResumableSink; it just needs to be updated for conduit 1.0. [1]: http://hackage.haskell.org/package/conduit-resumablesink