It just occurred to me, for what it's worth, that this problem is easily
solved using the `chunksOf`
from the still clumsy `Streaming.Pipes` module
https://github.com/michaelt/streaming-utils/blob/master/Streaming/Pipes.hs#L341
Then you can use one of the functor-general zipping functions from the
`Streaming` module e.g.
zips :: (Monad m, Functor f, Functor g) => Stream f m r -> Stream g m r
-> Stream (Compose f g) m r
zipsWith:: (Monad m, Functor h) => (forall x y. f x -> g y -> h (x, y)) ->
Stream f m r -> Stream g m r -> Stream h m r
Of course these zips could as well be implemented in
`Control.Monad.Trans.Free`. Then you just write something that in
Data.List style would
look like, say, `mapM_ action $ zip filepaths $ chunksOf n producer`
> module Main (main) where
>
> import Pipes
> import qualified Pipes.Prelude as P
> import Streaming
> import qualified Streaming.Prelude as S
> import qualified Streaming.Pipes as SP
> import qualified System.IO as IO
>
> main = run $ maps action $ zips filepaths $ SP.chunksOf 2 $
> text_producer 3
> where
>
> text_producer :: Int -> Producer String IO ()
> text_producer n = P.stdinLn >-> P.take n
>
> filepaths :: Stream (Of FilePath) IO ()
> filepaths = S.each $ map (\n -> show n ++ ".txt") [1..] --
> ["1.txt","2.txt"..]
>
> action :: Compose (Of FilePath) (Producer String IO) r -> IO r
> action (Compose (file :> p)) = IO.withFile file IO.WriteMode $ \h ->
> runEffect $ p >-> P.toHandle h
>
-- >>> main
-- hello<Enter>
-- world<Enter>
-- goodbye<Enter>
-- >>> :! cat 1.txt
-- hello
-- world
-- >>> :! cat 2.txt
-- goodbye
-- >>>
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