Hello,
It seems that rank-2 types are sufficient to make the more polymorphic types:
----------------------------------------------------
{-# LANGUAGE Rank2Types #-}
import Control.Exception
data Mask = Mask (forall a. IO a -> IO a)
mask :: (Mask -> IO a) -> IO a
mask io = do
b <- blocked
if b
then io (Mask id)
else block $ io (Mask unblock)
restore :: Mask -> IO a -> IO a
restore (Mask f) a = f a
----------------------------------------------------------
This is useful in an example like this:
forkThen :: IO () -> IO a -> IO a
forkThen io k = mask $ \m ->
do tid <- forkIO (restore m io)
restore m k `catch` \e ->
do when (e == ThreadKilled) (killThread tid)
throwIO e
-Iavor
On Thu, Apr 8, 2010 at 1:23 AM, Simon Marlow
On 07/04/2010 18:54, Isaac Dupree wrote:
On 04/07/10 11:12, Simon Marlow wrote:
It's possible to mis-use the API, e.g.
getUnmask = mask return
...incidentally, unmask a = mask (\restore -> return restore) >>= (\restore -> restore a)
That doesn't work, as in it can't be used to unmask exceptions when they are masked. The 'restore' you get just restores the state to its current, i.e. masked, state.
mask :: ((IO a -> IO a) -> IO b) -> IO b
It needs to be :: ((forall a. IO a -> IO a) -> IO b) -> IO b so that you can use 'restore' on two different pieces of IO if you need to. (alas, this requires not just Rank2Types but RankNTypes. Also, it doesn't cure the loophole. But I think it's still essential.)
Sigh, yes I suppose that's true, but I've never encountered a case where I needed to call unmask more than once, let alone at different types, within the scope of a mask. Anyone else?
Cheers, Simon _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe