On 4/23/08, David Roundy
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.
Does the STM runtime do 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
If the STM runtime does the optimization you suggest, it's not different at all. But consider this computation:
-- non-primitive retryUntil: retryUntil v p = do x <- readVar v unless (p x) retry
broken2 = atomically $ do (v1, v2) <- expensive_computation :: STM (TVar Int, TVar Int) retryUntil v1 (> 50) x <- expensive_computation2 :: STM Int retryUntil v2 (> x)
If v1 succeeds and v2 fails, then v1 changes to some other value > 50, I am sure that the STM runtime as it stands now will re-run expensive_computation2. -- ryan