Re: [Haskell-cafe] Stronger STM primitives needed? Or am I just doing it wrong?

On Wed, Apr 23, 2008 at 12:12:15PM -0700, Ryan Ingram wrote:

On 4/23/08, Jan-Willem Maessen wrote:

...

I've been trying to decide whether either of these is implementable in terms of `orElse`, in such a way that we immediately check the predicate upon retry before doing anything else. I can't quite make up my mind whether this is possible or not.

I do not think it is possible; consider this case:

broken = atomically $ do v <- expensive_computation :: STM (TVar Int) retryUntil v (> 50)

Given that you don't know which tvar to use until the end of the expensive computation, I don't see how you can lift "orElse" to the make that tvar be the first thing checked when the transaction is rerun.

I'm confused as to how your retryUntil gains you anything. If any of the TVars used in the expensive_computation change while waiting for a retry, then the expensive_computation will need to be done again. If none of them change, then we can skip the expensive_computation. How does retryUntil help us with this? i.e. how does your broken function using retryUntil differ from the following? broken = atomically $ do v <- expensive_computation :: STM (TVar Int) vv <- readTVar v unless (vv > 50) retry -- David Roundy Department of Physics Oregon State University