import Data.IORef (IORef, newIORef, atomicModifyIORef, readIORef)
import System.IO.Unsafe (unsafePerformIO)
import Control.Concurrent (threadDelay)
import Control.Parallel (par)
import Control.DeepSeq (force, NFData)

delayed_inc :: Int -> Int
delayed_inc x = (unsafePerformIO $
  do
    putStrLn "Incrementing variable (waiting 2 seconds)..."
    threadDelay 2000000
    putStrLn "Done incrementing variable."
    ) `seq` (x + 1)

f :: Int -> (Int, ())
f x = (delayed_inc x, ())

atomicModifyIORefPar :: (NFData a) => IORef a -> (a -> (a, b)) -> IO b
atomicModifyIORefPar ioref f =
  let g olddata = (let (newdata, result) = f olddata in (newdata, (result, newdata)))
  in do
    (result, newdata) <- atomicModifyIORef ioref g
    force newdata `par` return result

main :: IO ()
main =
  do
    ioref <- newIORef 20
    putStrLn "Modifying IORef..."
    atomicModifyIORefPar ioref f
    putStrLn "Waiting 5 seconds..."
    threadDelay 5000000
    putStrLn "Reading variable..."
    newiorefval <- readIORef ioref
    putStrLn "Showing variable..."
    putStrLn $ show newiorefval
    threadDelay 5000000