haskell - パイプの終わりを見つける方法

Question

以下のコードでは、どうすればよいですか

• stdoutCharConsumer入力ストリームからのすべてのデータを出力した後に改行を出力するように変更します

• 実装mixinし、mixin'

Pipes.Internalに入らずに？出来ますか？nextProducersの関数のようなものが必要です。

Pipes 4.1.0を使用しています

#!/usr/bin/env runhaskell
{-# OPTIONS_GHC -Wall #-}

import Pipes

digits, characters :: Monad m => Producer Char m ()
digits = each "0123456789"
characters = each "abcdefghijklmnopqrstuvwxyz"

interleave :: Monad m => Producer a m () -> Producer a m () -> Producer a m ()
interleave a b = do
  n <- lift $ next a
  case n of
    Left () -> b
    Right (x, a') -> do
      yield x
      interleave b a'

stdoutCharConsumer :: Consumer Char IO ()
stdoutCharConsumer = await >>= liftIO . putChar >> stdoutCharConsumer

-- first element of the mixin should go first
mixin :: Monad m => Producer b m () -> Pipe a b m ()
mixin = undefined

-- first element of the pipe should go first
mixin' :: Monad m => Producer b m () -> Pipe a b m ()
mixin' = undefined

main :: IO ()
main = do

    -- this prints "a0b1c2d3e4f5g6h7i8j9klmnopqrstuvwxyz"
    runEffect $ interleave characters digits >-> stdoutCharConsumer
    putStrLn ""

    -- this prints "0a1b2c3d4e5f6g7h8i9jklmnopqrstuvwxyz"
    runEffect $ interleave digits characters >-> stdoutCharConsumer
    putStrLn ""

    -- should print "0a1b2c3d4e5f6g7h8i9jklmnopqrstuvwxyz"
    runEffect $ characters >-> mixin digits >-> stdoutCharConsumer
    putStrLn ""

    -- should print "a1b2c3d4e5f6g7h8i9jklmnopqrstuvwxyz"
    runEffect $ digits >-> mixin characters >-> stdoutCharConsumer
    putStrLn ""

    -- should print "a1b2c3d4e5f6g7h8i9jklmnopqrstuvwxyz"
    runEffect $ characters >-> mixin' digits >-> stdoutCharConsumer
    putStrLn ""

    -- should print "0a1b2c3d4e5f6g7h8i9jklmnopqrstuvwxyz"
    runEffect $ digits >-> mixin' characters >-> stdoutCharConsumer
    putStrLn ""

UPD:プル/プッシュ ベースのストリームについて読んだ後、Pipes.Internal でも不可能だと思います。本当ですか？