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

[Simon Marlow]

On 12 December 2005 10:09, Joel Reymont wrote:

> While we are waiting for ghc 6.6, could the same effect be achieved
> by launching the computation that writes to t with forkIO?

Certainly.  registerTimeout is just an optimisation that avoids creating
a new thread each time.

Cheers,
	Simon

> On Nov 29, 2005, at 12:00 PM, Simon Marlow wrote:
> 
>> 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