Re: [Haskell-cafe] Printing of asynchronous exceptions to stderr

18 Nov 2010


      On 18/11/2010 11:31, Mitar wrote:
...
Hi!
On Wed, Nov 17, 2010 at 12:00 PM, Simon Marlow  wrote:
...
That's hard to do, because the runtime system has no knowledge of exception
types, and I'm not sure I like the idea of baking that knowledge into the
RTS.
But currently it does have a knowledge of interruptible and
uninterruptible exceptions? So adding another which would ctrl-c
interrupt?
The RTS knows when a thread is blocked, which is when it becomes 
interruptible.  It doesn't need to know about different kinds of 
exeptions.  In fact, when an exception is thrown it might well be 
unevaluated.
...
...
The point of maskUninterruptible is for those
hoefully rare rare cases where (a) it's really inconvenient to deal with
async exceptions and (b) you have some external guarantee that the critical
section won't block.
But is it possible to make an uninterruptible code section which can
timeout?
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.
...
Because once you enter maskUninterruptible probably
System.Timeout does not work anymore? I see uses of
maskUninterruptible (or some derivation of it, preferably) if:
- user would still be able to interrupt it
- you could specify some timeout or some other condition after which
the code section would be interrupted, so you could try for example to
deal with some cleanup code (and you do not want interrupts there) but
if this cleanup code hangs you still want some way to kill it then
Currently, with this absolute/all approach maskUninterruptible is
really not useful much because it masks too much. But I would see a
lot more useful something which would still allow me (and only me, as
a programmer) to interrupt it or user (because user should know what
he/she does). And this is why I argue for some way of being able to
specify which interrupts you still allow: like mask everything
except... (user interrupts, my special exception meant for cleanup
code hanging conditions...).
My main question is then, why do you want to use maskUninterruptible 
rather than just mask?  Can you give a concrete example?

Cheers,
	Simon