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

[Joel Reymont]

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

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

--
http://wagerlabs.com/