[Haskell-cafe] Re: Asynchronous exception wormholes kill modularity

[Iavor Diatchki]

Hello,
I wonder if it might be possible to use just one primitive which atomically
changes the interrupt mask for a thread?  Here is an example of what I'm
thinking:

data MaskingState   = Unmasked
                   | MaskedInterruptible
                   | MaskedNonInterruptible

-- Atomically changes the interrupt mask for a thread, and returns the old
mask.
setMask            :: MaskingState -> IO MaskingState
setMask             = error "primitive?"

-- Change the mask for the duration of an IO action.
-- The action is passed the old mask.
scopedSetMask      :: MaskingState -> (MaskingState -> IO a) -> IO a
scopedSetMask m io  = do m1 <- setMask m
                        a  <- io m1
                        setMask m1
                        return a

-- Change the mask for the duration of an IO action.
scopedSetMask_     :: MaskingState -> IO a -> IO a
scopedSetMask_ m io = scopedSetMask m $ \_ ->
                       io
-- Simon's mask:
mask               :: ((IO a -> IO a) -> IO b) -> IO b
mask f              = scopedSetMask MaskedInterruptible $ \m ->
                       f (scopedSetMask_ m)


-Iavor


On Sat, Apr 10, 2010 at 11:42 AM, Iavor Diatchki <iavor.diatchki at gmail.com>
wrote:
> 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 <marlowsd at gmail.com> wrote:
>> 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
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>
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