[Haskell-cafe] Asynchronous exception wormholes kill modularity

[Twan van Laarhoven]

Bas van Dijk wrote:
> ...
> However now comes the problem I would like to talk about. What if I
> want to use modifyMVar_ as part of a bigger atomic transaction. As in:
> 
> block $ do ...
>            modifyMVar_ m f
>            ...
> 
>>From a quick glanse at this code it looks like asynchronous exceptions
> can't be thrown to this transaction because we block them. However the
> unblock in modifyMVar_ opens an asynchronous exception "wormhole"
> right into our blocked computation. This destroys modularity.

Would it work if 'block' adds a layer of blocking and 'unblock' removes one 
layer of blocking? So

     block a = do
         modifyIORef blockLevel (+1)
         result <- a
         modifyIORef blockLevel (-1)
         return result

     unblock a = do
         modifyIORef blockLevel (-1)
         result <- a
         modifyIORef blockLevel (+1)
         return result

     canThrowExceptions = (<= 0) `liftM` readIORef blockLevel

Although it is probably a better idea to not use block/unblock at all in user code.


Twan