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

[Ryan Ingram]

On 4/23/08, David Roundy <droundy at darcs.net> wrote:
> 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