Re: task model

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sat, 17 Oct 2009 13:10:04 +0200, Georg Bauhaus wrote:

> "Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org> writes:
> 
>> Georg Bauhaus wrote:
>>
>> What's the point of re-raising? Exceptions don't propagate from tasks.
> 
> Uhm, I forgot to mention that a handler at some nesting level
> might be reporting, permitting more accurate diagnostic messages.
> It could then decide there is or is not a possible rescue,
> and accordingly re-raise, for example.

The problem is that there is no nesting of tasks in terms of control flow.
There is a parent child relation, but it is no nesting.

If an exception is propagated out of a task, the parent task is doing its
things asynchronously to the child. In order to propagate an exception in
the parent you have only two options:

1. an asynchronous transfer of control = in effect aborting/interrupting
the parent;
2. to synchronize with the parent making a rendezvous with it.

Neither looks much promising.

More generally speaking in Ada tasks are objects. As an object a failed
task may complain no earlier than upon its finalization if no other
contacts (rendezvous) happen. This is too late in all senses.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de