On Fri, Jan 07, 2005 at 03:31:10PM +0200, Einar Karttunen wrote:

Hello

What is the best way of doing an computation with a timeout?

A naive implementation using two threads is easy to create - but what is the preferred solution?

withTimeout :: forall a. Int -> IO a -> IO (Maybe a)

btw How would I do the same with the new STM abstraction?

My guess is it would be something like this, however you may want to do it differently to get better compositionality (withTimeout returns an IO action, not a STM action): import Control.Concurrent (forkIO, threadDelay) import Control.Concurrent.STM withTimeout :: Int -> STM a -> IO (Maybe a) withTimeout time fun = do mv <- atomically newEmptyTMVar tid <- forkIO $ do threadDelay time atomically (putTMVar mv ()) x <- atomically (fmap Just fun `orElse` (takeTMVar mv >> return Nothing)) killThread tid return x PS. STM is cool! :) Best regards, Tomasz

On Fri, 07 Jan 2005 15:31:10 +0200, Einar Karttunen wrote:

Hello

What is the best way of doing an computation with a timeout?

I like the approach taken in "Tackling the ackward squad": First a funcion which will "race" two IO computations against each other, returning the "winning" result. This is accomplished by simply spawning a thread for each computation which does nothing but evalute the IO computation and putting the result in an MVar. Then the MVar is read (this will lock until there is something in the MVar to read), when finally there is something to read it will obviously contain the result of the IO computation that completed first. Then both of the spawned threads are killed (one of them is already dead at this point) via throwTo. parIO :: IO a -> IO a -> IO a parIO a1 a2 = do m <- newEmptyMVar ; c1 <- forkIO (child m a1) ; c2 <- forkIO (child m a2) ; r <- takeMVar m ; throwTo c1 Kill ; throwTo c2 Kill ; return r where child m a = do r <- a putMVar m r Next we simply race the IO computation to be "timed out" against a thread which delays and then returns Nothing. timeout :: Int -> IO a -> IO (Maybe a) timeout n a = parIO a1 a2 where a1 = do r <-a return (Just r) a2 = do threadDelay n return Nothing /S -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862