Re: Structured exception information (task, ANEX E)

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Martin Krischik <krischik@users.sourceforge.net> writes:

> Maciej Sobczak schrieb:
>
>> Yes, I'm asking for "throwing objects", in the C++ parlance.
>> How to do this in Ada?
>> If I cannot - how to solve this design problem?
>
> I believe this was skipped in Ada because of ANEX E and task types.

I don't see any problems there.

>...You
> must understand that Ada exceptions quite often propagated across thread
> borders and sometimes - if ANEX E is implemented - over a network
> connections to another computer.
>
> I do not say this is impossible but it could be quite expensive
> performance wise.

Efficiency is irrelevant to language design.  You should never leave a
feature out merely because it's slow.  What matters is distributed
overhead, and I see none here.  Yes, if an exception propagates across a
network, you have to ship all the data across the network.  The
programmer controls how much data that is, so it's not a problem.
Besides, shipping a String is no more efficient than shipping properly
typed data.

Many other languages have solved this problem (even in the presence of
threads and distributed systems).  The obvious solution is that what Ada
calls "exception" should be a type (derived from Root_Exception), and
what Ada calls "exception occurrence" should be an object of that type.
This language design is much simpler than Ada's, because we don't need
a bunch of separate concepts, with their own special rules -- just reuse
the concepts of type and object.

The 200-character limit was a mistake, by the way.  It was based on
fictitious implementation difficulties.  Ada already has functions
that return results whose size is not known at the call site (e.g.
"return String").  The same memory-management techniques would apply to
arbitrary-sized exception occurrences.

- Bob