threadDelay is IO-only; there's no way to use threadDelay in an STM transaction. For example, if you want to wait for a TVar to go from Nothing to Just x with a timeout, you could do this: readOrTimeout :: TVar (Maybe a) -> Int -> STM (Maybe a) readOrTimeout t secs = do timeout <- registerTimeout secs let check_timeout = do b <- readTVar timeout if b then return Nothing else retry check_t = do m <- readTVar t case m of Nothing -> retry Just x -> return x atomically $ check_timeout `orElse` check_t Cheers, Simon On 29 November 2005 10:11, Joel Reymont wrote:
Simon,
How is this easier than just calling threadDelay?
Ideally, I would be looking for something like reading from a TVar with a timeout. So that you either get a Nothing (timeout) or the value from the TVar. Can I implement it using the GHC timeout thread?
Thanks, Joel
On Nov 29, 2005, at 9:20 AM, Simon Marlow wrote:
Interestingly, GHC already has a timeout thread - the I/O manager thread handles threadDelay too. It wouldn't be too hard to adapt it to do STM timeouts too, with a function like
registerTimeout :: Int -> STM (TVar Bool)
and you wait for your timeout by waiting for the TVar to contain True.