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

The proper way is just to wrap System.Timeout, which does some rather clever things with regards to exception semantics. The code for it is a joy to read, by the way. --S. On May 30, 2009, at 5:36 PM, Sebastian Sylvan wrote:

On Sat, May 30, 2009 at 10:32 PM, Cetin Sert <cetin.sert@gmail.com> wrote: 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)'

Right, I forgot about the "Either" bit so you'd have to make sure the action's result and the default has the same type (or modify it to return an Either).

-- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe