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
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