pipes: push from return value to downstream

push2down.hs

import Control.Monad (forever)
import Pipes 
import Pipes.Core
import qualified Pipes.Prelude

list2pipe :: (Monad m) => [a] -> Producer a m ()
list2pipe [] = return ()
list2pipe (x:xs) = yield x >> list2pipe xs

tuple3 :: (Monad m) => Consumer a m (a,a,a)
tuple3 = do x <- await
            y <- await
            z <- await
            return (x,y,z)


push2down :: (Monad m) => Consumer a m b -> Pipe a b m ()
push2down c = (c >>~ \_ -> Pipes.Prelude.drain) >>= yield

main = do 
  runEffect $ (list2pipe [1..100] >> return undefined) 
               >-> forever (push2down tuple3) 
               >-> Pipes.Prelude.print 

-- run result
-- > pipes:[wavewave@macbookair:~/repo/workspace/haskellstudy/pipes]$ ./test3
-- > (1,2,3)
-- > (4,5,6)
-- > (7,8,9)
-- > (10,11,12)
-- > (13,14,15)
-- > (16,17,18)
-- > (19,20,21)
-- > (22,23,24)
-- > (25,26,27)
-- > (28,29,30)
-- > (31,32,33)
-- > (34,35,36)
-- > (37,38,39)
-- > (40,41,42)
-- > (43,44,45)
-- > (46,47,48)
-- > (49,50,51)
-- > (52,53,54)
-- > (55,56,57)
-- > (58,59,60)
-- > (61,62,63)
-- > (64,65,66)
-- > (67,68,69)
-- > (70,71,72)
-- > (73,74,75)
-- > (76,77,78)
-- > (79,80,81)
-- > (82,83,84)
-- > (85,86,87)
-- > (88,89,90)
-- > (91,92,93)
-- > (94,95,96)
-- > (97,98,99)