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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Help Me Please :)
Date: 2000/03/28
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References: <tgg0u8cb6a.fsf@mercury.rus.uni-stuttgart.de>
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Date: 2000-03-28T00:00:00+00:00
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In article <wccya74m18e.fsf@world.std.com>,
  Robert A Duff <bobduff@world.std.com> wrote:
> There's no hierarchy of exceptions.

I dislike complexification of exception handling. Many Ada
programmers greatly overuse exceptions, and in particular
define far too many separate exceptions. When it comes to
exception handling, the rule should be keep it very simple.

> (On the other hand, Ada *pretends* that it's OK to raise and
> handle Constraint_Error, but if you read RM-11.6, you'll find
> that's not quite true -- and I doubt if any Ada programmer
> other than Tucker Taft actually *understands* 11.6.  So in
> practise, 11.6 says, "Do not handle predefined exceptions.")

Now now, that's completely disconnected from the real world :-)
"In practice", 11.6 says nothing of the sort at all.

It is often useful to be able to handle constraint error.
For example here is the implementation of one of the plus
operators in Calendar in GNAT:

   function "+" (Left : Time; Right : Duration) return Time is
      pragma Unsuppress (Overflow_Check);
   begin
      return (Left + Time (Right));
   exception
      when Constraint_Error =>
         raise Time_Error;
   end "+";

If you don't understand 11.6, at least don't make claims about
it that are excessive :-) Sure, the formulation of 11.6 is
complex, but you have to approach this with a pragmatist's hat
on, not a theoretician's viewpoint.

In practice many many Ada 95 programs have CE handlers and are
in practice perfectly portable. Even if in some cases a
theoretician could prove that it was possible to have an
implementation of Ada 95 that conformed to the RM under
which the handler is "wrong", that is irrelevant if no such
implementations exist or are likely to exist. In other cases,
even the theoretician's will not find any fault with the CE
handler :-)

Yes, Bob, I know you like to have the hope that the Ada RM is
a formal definition which precisely defines what you can and
cannot do. It is nowhere near that for many reasons, and a
pragmatic viewpoint always has to be in effect.



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