import Control.Concurrent
import Control.Concurrent.STM

threadA :: TChan Int -> TChan Int -> TChan () -> IO ()
threadA chan1 chan2 done = do
  ans <- atomically $ do
    writeTChan chan1 42
    readTChan chan2
  putStrLn $ "A: got answer " ++ show ans

threadB :: TChan Int -> TChan Int -> TChan () -> IO ()
threadB chan1 chan2 done = do
  (query, answer) <- atomically $ do
    q <- readTChan chan1
    let ans = 2 * q
    writeTChan chan2 ans
    return (q, ans)
  putStrLn $ "b: got query " ++ show query ++ ", answered " ++ show answer

main :: IO ()
main = do
  chan1 <- newTChanIO
  chan2 <- newTChanIO
  done <- newTChanIO
  _ <- forkIO $ threadA chan1 chan2 done
  _ <- forkIO $ threadB chan1 chan2 done
  () <- atomically $ readTChan done
  return ()