Attempt to solve stream reordering problem with Streamly

stream-reorderer-streamly.hs

{-# LANGUAGE FlexibleContexts #-}
module Main where

import           Data.Monoid
import qualified Streamly            as S
import qualified Streamly.Prelude    as S

-- Wrong: this should be
-- Int -> S.StreamT m a -> S.StreamT m a
reorderStreamly :: Monad m
                => Int -> S.StreamT m a -> S.StreamT m [a]
reorderStreamly n stream =
  S.scan accum (mempty, []) fst
  $ fmap Just stream
  <> pure Nothing
  where
    accum (_, buff) (Just x)
      | length buff == n - 1 = (x : buff, [])
      | otherwise            = (mempty, x : buff)
    accum (_, buff) Nothing  = (buff, [])

streamlyDemo :: Monad m => m [Int]
streamlyDemo = S.foldl (<>) [] id $ reorderStreamly 3 (S.each [1 .. 10])

main :: IO ()
main = do print $ streamlyDemo