Hi Serguey Thank you for reply. I tried with IORef but I am missing a function which modify it. In this case every thread just write value 10 to variable n. incr_count :: MVar () -> IORef Int -> IO () incr_count m n = ( forM_ [ 1 .. 10000 ] $ \_ -> writeIORef n 10 ) >> putMVar m () main :: IO () main = do count <- newIORef 0 list <- forM [1..10] $ \_ -> newEmptyMVar forM_ list $ \var -> forkIO . incr_count var $ count forM_ list $ \var -> takeMVar var val <- readIORef count print val ghci>:t atomicModifyIORef atomicModifyIORef :: IORef a -> (a -> (a, b)) -> IO b ghci>:t readIORef readIORef :: IORef a -> IO a ghci>:t writeIORef writeIORef :: IORef a -> a -> IO () I have atomicModifyIORef but it puts it into IO. I am missing some thing like this ghci>:t modifyMVar_ modifyMVar_ :: MVar a -> (a -> IO a) -> IO () modifyIORef_:: IORef a -> ( a -> IO a ) -> IO () Mukesh Tiwari On Tue, Dec 18, 2012 at 11:50 PM, Serguey Zefirov <sergueyz@gmail.com>wrote:
Hello All I have two questions. 1. I wrote this code to create 10 simultaneous threads. Could some one please tell me if this is correct or not ?
incr_count :: MVar () -> MVar Int -> IO () incr_count m n = ( forM_ [ 1..10000 ] $ \_ -> modifyMVar_ n ( return . (
2012/12/18 mukesh tiwari <mukeshtiwari.iiitm@gmail.com>: +
10 ) ) ) >> putMVar m ()
main :: IO() main = do count <- newMVar 0 list <- forM [1..10] $ \_ -> newEmptyMVar forM_ list $ \var -> forkIO . incr_count var $ count forM_ list $ \var -> takeMVar var val <- takeMVar count print val
It is pretty much correct (some comments would be nice, though).
2. I am trying to create race condition which makes the variable in inconsistent state. Could some one please tell me how to achieve this ? I look at IORef but it does not have function like modifyMVar_.
MVars are "atomic" in the sense that they have empty state and mutator can wait until variable will have a value. So you have to operate with something else, either IORef or with readMVar instead of takeMVar or modifyMVar_.
Mukesh Tiwari
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