On 11-02-01 02:58 PM, Warren Harris wrote:
I have an application that forks a thread to run an activity on a timer. (The activity happens to be Berkeley DB checkpointing, but that's actually beside the point here.) The problem is that when the application wants to quit, I would like my main thread to be able to tell the timer thread to shut down in a timely way. However, I don't see a primitive in haskell that allows me to both wait for a timeout, or a notification. (If I were to do this in java, I would use wait/notify.)
Use an MVar for signalling; use a two-valued data type to represent time-to-work or time-to-die. For extra correctness, use a second MVar to be notified that the child thread is really done --- because otherwise there would be the race condition of the child thread still in the middle of critical I/O when the program quits. import Control.Concurrent import Control.Exception(finally) data DoOrDie = Do | Die main = do v <- newEmptyMVar finale <- newEmptyMVar forkIO (timeloop v `finally` putMVar finale ()) putStrLn "press enter to quit" getLine putMVar v Die takeMVar finale timeloop v = run where run = do forkIO (threadDelay 15000000 >> putMVar v Do) m <- takeMVar v case m of Do -> putStrLn "checkpoint" >> run Die -> putStrLn "checkmate"