Here's a version that provides clean output with no delays. It uses a single-entry mailbox (the TMVar "output") to ensure the processing doesn't run too far ahead of the log. module Test where import System.Random import Control.Concurrent import Control.Concurrent.STM test :: IO () test = do tv <- atomically (newTVar 0) output <- atomically (newTMVar "Log begins") forkIO (writer output) forkIO (producer tv output) consumer tv output write :: TMVar String -> String -> STM () write output message = putTMVar output message producer tv o = do r <- randomRIO (1,10) atomically $ do v <- readTVar tv writeTVar tv (v+r) write o ("insert " ++ show r) producer tv o return () consumer tv o = do r <- randomRIO (1,10) atomically $ do v <- readTVar tv if (v < r) then retry else writeTVar tv (v-r) write o ("consume " ++ show r) consumer tv o return () writer :: TMVar String -> IO () writer o = do msg <- atomically $ takeTMVar o putStrLn msg writer o