On Wed, Apr 23, 2003 at 03:51:25PM +0200, Nick Name wrote:
Hi all, does someone know why this program works
------------------------ import Concurrent
loopM = sequence_ . repeat
main = do v <- newMVar 0 forkIO (loopM (modifyMVar_ v (return . (1+)))) -- forkIO (loopM (modifyMVar_ v (return . (1+)))) loopM (readMVar v >>= print) -------------------------
but uncommenting the commented line makes the program fail with "stack overflow" ?
Thanks for help
Vincenzo
It definitely seems a bug to me. Though I'm not sure if it's in your code or in Concurrent ;) However, the following code _does_ work, which makes me suspect it has something to do with the locking done inside of an MVar. import Control.Concurrent loopM = sequence_ . repeat main = do v <- newMVar 0 forkIO $ loopM $ do modifyMVar_ v $ return . (1+) forkIO $ loopM $ do modifyMVar_ v $ return . (1+) loopM $ do readMVar v >>= print --withMVar v print Swapping the last 2 lines (that is, their comments) makes it crash again. And I guess this actually _is_ the code you should use: withMVar is a "safe wrapper for operating on the contents of an MVar", which is exactly what you're doing. Does anyone else know what exactly is going on? Happy hacking, Remi -- Nobody can be exactly like me. Even I have trouble doing it.