[Haskell-cafe] Printing of asynchronous exceptions to stderr

Mitar mmitar at gmail.com
Wed Dec 1 04:02:10 CET 2010


On Thu, Nov 18, 2010 at 2:19 PM, Simon Marlow <marlowsd at gmail.com> wrote:
> then it isn't uninterruptible, because the timeout can interrupt it.  If you
> can tolerate a timeout exception, then you can tolerate other kinds of async
> exception too.

Yes, but semantics are different. I want to tolerate some exception
because they are saying I should do this and this (for example user
interrupt, or timeout) but I do not want others, which somebody else
maybe created and I do not want to care about them.

> My main question is then, why do you want to use maskUninterruptible rather
> than just mask?  Can you give a concrete example?

I have written few of them. As I said: timeout and user exceptions.
Timeout is for interrupts with which me as a programmer want to limit
otherwise uninterruptible code, and user exceptions so that users can
limit otherwise uninterruptible code.

But in meanwhile I have find an elegant way to solve my problems (very
old one in fact). ;-)

I have defined such function:

uninterruptible :: IO a -> IO a
uninterruptible a = mask_ $ a `catch` (\(_ :: SomeException) ->
uninterruptible a)

with which I wrap takeMVar and other interruptible operations. As they
can only be interrupted or succeed they can simply be retried until
they succeed. In this way my code is not susceptible to possible dead
locks which could happen if I would use maskUninterruptible and for
example throw two exceptions between two threads, while they would
both be in maskUninterruptible section. In this way I can remain in
only masked state, exceptions can still be delivered, but my program
flow is not interrupted. Of course function about can be also easily
modified to allow user interrupts and timeouts, but not other
exceptions. And as I want to use this code only in my cleanup code I
do not really care about new exceptions being ignored.


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