Generalize unzip, perhaps

[treeowl]

I don't know if this makes sense, but it should work at least for Of and (,). The constraints on f look scary, but they really boil down to f being a bifunctor that's a comonad in its second argument.

unzip :: (Monad m, Bifunctor f, Functor (f b), Comonad (f (a,b)))
      => Stream (f (a,b)) m r -> Stream (f a) (Stream (f b) m) r
unzip = loop where
  loop str = case str of
    Return r -> Return r
    Effect m -> Effect (liftM loop (lift m))
    Step f -> Step
                  . first fst
                  . extend (Effect . Step . first snd)
                  . fmap (Return . loop) $ f

I don't know if the extra laziness here will cause any performance problems. If it does, and if this idea is actually good for something, we can offer both versions. As with copy, we really only need the power of Extend, not of Comonad.

[andrewthad]

This is no longer the official repo for streaming. Please continue discussion of this issue here.