However even if Haskell finalizers + MVars are impossible in NHC, I don't think Haskell finalizers + mutable state have to be. For example another mutable variable we could have would be a PVar which is always full and has functions [snip]
updatePVar (PVar ioRef) updateFn = do [stop any new finalizers running] a <- readIORef ioRef writeIORef (updateFn a) [reenable finalizers] return a
Just for the record, I think that if we were to pursue this approach, then the right primitive to add is: -- | -- Execute argument atomically with respect to finalizers. -- Nested calls to blockFinalizers are allowed. -- -- That is, while executing the argument, no finalizers will start -- to execute. -- (Finalizers that are already executing may continue to execute.) blockFinalizers :: IO a -> IO a [The last sentence is to take care of cases like finalizers calling blockFinalizers.] One would then write George's example like this:
updatePVar (PVar ioRef) updateFn = blockFinalizers $ modIOVar ioRef updateFn
This is what I was referring to the other day when I suggested that a mechanism in the style of interrupt disable might be easier to implement than MVars. -- Alastair