Simple STM example
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module Main where
import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM
main = do shared <- atomically $ newTVar 0
before <- atomRead shared
putStrLn $ "Before: " ++ show before
forkIO $ 25 `timesDo` (dispVar shared >> milliSleep 20)
forkIO $ 10 `timesDo` (appV ((+) 2) shared >> milliSleep 50)
forkIO $ 20 `timesDo` (appV pred shared >> milliSleep 25)
milliSleep 800
after <- atomRead shared
putStrLn $ "After: " ++ show after
where timesDo = replicateM_
milliSleep = threadDelay . (*) 1000
atomRead = atomically . readTVar
dispVar x = atomRead x >>= print
appV fn x = atomically $ readTVar x >>= writeTVar x . fn