gistfile1.hs

type Stream a = MVar (Item a)
data Item a = Item a (Stream a)
data Channel a = Channel (MVar (Stream a)) (MVar (Stream a))

newChannel :: IO (Channel a)
newChannel = do
    hole <- newEmptyMVar
    readVar <- newMVar hole
    writeVar <- newMVar hole
    return $ Channel readVar writeVar

writeChannel :: Channel a -> a -> IO ()
writeChannel (Channel _ writeVar) val = do
    newHole <- newEmptyMVar
    oldHole <- takeMVar writeVar
    putMVar writeVar newHole
    putMVar oldHole (Item val newHole)

readChannel :: Channel a -> IO a
readChannel (Channel readVar _) = do
    stream <- takeMVar readVar
    Item val new <- takeMVar stream
    putMVar readVar new
    return val