[Haskell-cafe] STM, orElse and timed read from a channel

Simon Marlow simonmar at microsoft.com
Tue Nov 29 07:00:03 EST 2005

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


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. 

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