Haskell by Example: Atomic Counters

original

import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM

main = do
    ops <- atomically $ newTVar 0
    forM_ [0..49] $ \_ -> do
        forkIO . forever $ do
            atomically $ modifyTVar ops (+1)
            threadDelay 100

    threadDelay 1000000
    opsFinal <- atomically $ readTVar ops
    putStrLn $ "ops: " ++ show opsFinal

$ runhaskell atomic-counters.hs
ops: 148407

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