io-streams partitionEithers

Question

I have a worker function

worker :: a -> Either b c

and I want to apply it to a stream of a and produce 2 streams of b and c and process those streams further. For example, I want to accumulate c in a Map (essentially to fold the stream) and output b to stderr .

How can I achieve this with io-streams ? It seems I cannot call connect twice. So I have to put it before the partitioning, so the partitioning will operate on OutputStream in a "contravariant" way:

contrapartitionEithers
  :: OutputStream b -> OutputStream c -> IO (OutputStream (Either b c))

Is it implementable? If no, how can do the task at hand? If yes, is is somehow "dual" to System.IO.Streams.zip ?

System.IO.Streams.zip
  :: InputStream a -> InputStream b -> IO (InputStream (a, b))

Answer 1

Sometimes the right question contains the answer. So it's possible indeed:

contrapartitionEithers
  :: OutputStream b -> OutputStream c -> IO (OutputStream (Either b c))
contrapartitionEithers b c = makeOutputStream $ maybe
     (writeTo b Nothing >> writeTo c Nothing)
     (either (writeTo b . Just) (writeTo c . Just))