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

[David Roundy]

On Wed, Apr 23, 2008 at 12:12:15PM -0700, Ryan Ingram wrote:
> On 4/23/08, Jan-Willem Maessen <jmaessen at alum.mit.edu> 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