I have a streaming package question. Suppose I have a: Stream (Of (Int, a)) m () I would like to demultiplex it into Vector (Stream (Of a) m ()) using the Int as the index of an item into the Vector of output streams. How can I do this efficiently (constant memory and linear time)? Does the following work? import qualified Streaming.Prelude as SP import qualified Data.Vector as V type StreamOf a m r = Stream (Of a) m r demuxStream :: forall a m. MonadIO m => Int -> StreamOf (Int, a) m () -> m (Vector (StreamOf a m ())) demuxStream numSyms stream = let emptyStreams = V.replicate numSyms (pure ()) processItem v (iD, x) = V.modify (\vm -> VM.modify vm (>> SP.yield x) iD) v in SP.fold_ processItem emptyStreams id stream My guess is that it takes more than constant memory as it goes through the entire input stream before returning. Josh