try4.hs

-- can I write this using STM? I know the print part is not pure function.
-- STM can help with variable read/write, but it can't help you with IO
-- action.  since there is really no variable in haskell, it's similar to try1
-- where it is implemented using locks (MVar). STM TVar is implemented using
-- transactional memory. STM can only ensure TVar and T* access is safe in
-- atomically block, it can't do anything about printing.

import System.IO
import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM
import Text.Printf
import System.Random

fuzz :: IO ()
fuzz = do
  seconds <- getStdRandom random :: IO Double
  threadDelay $ round (realToFrac (1000 * seconds))

incrementAndPrint :: TVar Integer -> IO ()
incrementAndPrint counter = do
  fuzz
  newCount <- atomically $ do
    modifyTVar' counter (+ 1)
    readTVar counter
  fuzz
  printf "The count is %d\n" newCount
  fuzz
  putStrLn "---------------"

main :: IO ()
main = do
  putStrLn "Starting up"
  counter <- newTVarIO 0
  replicateM_ 10 (forkIO $ incrementAndPrint counter)
  putStrLn "Finishing up"