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

On Wed, Apr 23, 2008 at 7:54 AM, Tim Harris (RESEARCH) wrote:

What do you think about a slight change:

readTVarWhen :: TVar a -> (a -> bool) -> STM a

This seems strictly less powerful than retryUntil:

readTVarWhen v p = retryUntil v p >> readTVar v

Consider the following transaction:

intV :: TVar Int boolV :: TVar Bool

interesting = atomically $ do retryUntil intV (> 50) retryUntil boolV id

Lets say that intV contains 100 and boolV contains False. Then this transaction retries. Now, if intV changes to 101, this transaction doesn't need to re-run; we can see immediately that no predicate changed. Using "readTVarWhen", this is less clear; the transaction log would hold a read on intV which would be more difficult to ignore. -- ryan