[Haskell-cafe] Printing of asynchronous exceptions to stderr

[Mitar]

Hi!

On Wed, Dec 1, 2010 at 10:50 AM, Simon Marlow <marlowsd at gmail.com> wrote:
>> 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.
>
> Surely if you don't care about these other exceptions, then the right thing
> to do is just to propagate them?  Why can't they be dealt with in the same
> way as user interrupt?

The problem is that Haskell does not support getting back to
(continue) normal execution flow as there was no exception. So for
user interrupt I want to deal with in a way to interrupt execution,
cleanup, print to user something, etc. But for other exceptions I want
to behave as nothing happened. This is why those cannot be dealt with
the same as user interrupt.

On way to "behave as nothing happened" is to mask them (and only them)
and the other way is to "eat" (and ignore) them. Probably the first
would be better if it would be possible. So the second one is the one
I have chosen.

The third one, to restore execution flow is sadly not possible in
Haskell. Would it be possible to add this feature? To have some
function which operates on Exception value and can get you back to
where the exception was thrown?

>> uninterruptible :: IO a ->  IO a
>> uninterruptible a = *CENSORED*
>
> That is a very scary function indeed.  It just discards all exceptions and
> re-starts the operation.  I definitely do not condone its use.

Yes, this one is extreme. For example, this would be more in the line
of above arguments:

uninterruptible :: IO a -> IO a
uninterruptible a = mask_ $ a `catches` [
    Handler (\(e :: AsyncException) -> case e of UserInterrupt ->
throw e ; _ -> uninterruptible a),
    Handler (\(_ :: SomeException) -> uninterruptible a)
  ]

Mitar