[Haskell-cafe] Generalized monadic exception handling with monad-peel

[Bas van Dijk]

On Wed, Nov 3, 2010 at 10:59 AM, Anders Kaseorg <andersk at mit.edu> wrote:
> I just released the monad-peel library to Hackage.

This is great news!

My regions library uses 'bracket' from MonadCatchIO in the runRegionT
function. Due to the recent discussion on MonadCatchIO I realized that
users could hack around the safety guarantees of the library by
inserting an Error monad somewhere in the stack of monad transformers
of a RegionT. Then they could do something like:

runRegionT $ do
  h <- openFile "file.txt" ReadOnly
  throwError "die!"

which then won't close h! This obviously breaks the safety guarantees
of my regions library.

I will update the regions library and related packages with your
monad-peel. Thanks!

Something else: In the soon to be released base-4.3 the block and
unblock functions will be deprecated in favor of mask. It would be
great if you can also add these new functions to
Control.Exception.Peel:

import qualified Control.Exception as CE (mask, mask_, uninterruptibleMask)

#if MIN_VERSION_base(4,3,0)
-- |Generalized version of 'CE.mask'. Note, any monadic side
-- effects in @m@ of the \"restore\" computation will be discarded; it
--  is run only for its side effects in @IO at .
mask :: MonadPeelIO m => ((forall a. m a -> m a) -> m b) -> m b
mask f = do
  k <- peelIO
  join $ liftIO $ CE.mask $ \restore -> k $ f $ liftIOOp_ restore

-- |Generalized version of 'CE.mask_'.
mask_ :: MonadPeelIO m => m a -> m a
mask_ = liftIOOp_ CE.mask_

-- |Generalized version of 'CE.uninterruptibleMask'. Note, any monadic side
-- effects in @m@ of the \"restore\" computation will be discarded; it
--  is run only for its side effects in @IO at .
uninterruptibleMask :: MonadPeelIO m => ((forall a. m a -> m a) -> m b) -> m b
uninterruptibleMask f = do
  k <- peelIO
  join $ liftIO $ CE.uninterruptibleMask $ \restore -> k $ f $ liftIOOp_ restore
#endif

You have to review these definitions carefully; I'm not yet fully
comfortable with peelIO.

Finally, would you also like to add wrapped functions for alloca and
friends from all the Foreign modules?

Regards,

Bas