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

[Tomasz Zielonka]

On Tue, Nov 29, 2005 at 12:00:03PM -0000, 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

Wouldn't it be
    readOrTimeout :: TVar (Maybe a) -> Int -> IO (Maybe a)
                                              ^^
?

Alternatively, it would be nice to have a new STM primitive:

    wailUntil :: ClockTime -> STM ()

so you would wait until some time-point passes, not for a number of
time-units (waiting for a number of time-units wouldn't work because of
retries). I think it could be efficiently implemented, wouldn't it?

Best regards
Tomasz

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