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