Thank you for your reply, I'd come up with the following: timed :: Int → IO a → b → IO (Either b a) timed max act def = do r ← new t ← forkIO $ do a ← act r ≔ Right a s ← forkIO $ do wait max e ← em r case e of True → do kill t r ≔ Left def False → return () takeMVar r --------------------- *Network.Port.Scan> timed 500 (wait 50000 >> return 0) 'x' Left 'x' *Network.Port.Scan> timed 500000 (wait 50000 >> return 0) 'x' Right 0 --------------------- before reading your reply: timed timeout act fallback = do res <- newEmptyMVar tid <- forkIO $ act >>= writeMVar res threadDelay timeout stillRunning <- isEmptyMVar res if stillRunning then killThread tid >> return fallback else takeMVar res --------------------- *Network.Port.Scan> timed2 500 (wait 50000 >> return 0) 'x' <interactive>:1:33: No instance for (Num Char) arising from the literal `0' at <interactive>:1:33 Possible fix: add an instance declaration for (Num Char) In the first argument of `return', namely `0' In the second argument of `(>>)', namely `return 0' In the second argument of `timed2', namely `(wait 50000 >> return 0)' Regards, Cetin Sert 2009/5/30 Sebastian Sylvan <sebastian.sylvan@gmail.com>
2009/5/30 Cetin Sert <cetin.sert@gmail.com>
Hi how could one implement a function in concurrent haskell that either returns 'a' successfully or due timeout 'b'?
timed :: Int → IO a → b → IO (Either a b) timed max act def = do
Something like (warning, untested code - no compiler atm).
timed timeout act fallback = do res <- newEmptyMVar tid <- forkIO $ act >>= writeMVar res threadDelay timeout stillRunning <- isEmptyMVar res if stillRunning then killThread tid >> return fallback else takeMVar res
-- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862