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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Exceptions
Date: Thu, 13 Dec 2007 13:29:17 -0600
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Date: 2007-12-13T13:29:17-06:00
List-Id: <comp.lang.ada>

"Maciej Sobczak" <see.my.homepage@gmail.com> wrote in message
news:9f16235c-260a-460b-873c-2183c154cc0c@e25g2000prg.googlegroups.com...
> On 13 Gru, 14:40, Robert A Duff <bobd...@shell01.TheWorld.com> wrote:
...
> > I don't consider Constraint_Error "mere", by the way.
> > It almost always represents a bug.
>
> What about a situation when a user entered an input data that was
> outside of legal (and documented) range? Or that caused our of range
> value further down the pipe?
> This is not necessarily a bug, just an indication that the value is
> out of range. The exception can help to come back to the input loop
> and repeat the user scenario, abandoning all the state that was
> created in the meanime. This way the program does not enter into any
> abnormal state.

An *unhandled* Constraint_Error represents a bug - in this case, a lack of
appropriate input validation. One can handle it very locally for that
purpose, although I prefer to input values into maximum size objects and
check the range with a membership before converting it. (That way, that
there is in fact validation is clearer.)

Even the propagation of Constraint_Error represents a bug.

Exception contracts would help with detecting that, but only if the rules
were strong enough that it is possible to write reasonable code that is
exception-free. Having Java-style "unchecked exceptions" defeats the
purpose.

                                        Randy.

P.S. Sorry about the old responses just appearing now; something happened to
the news server while I was answering those on Monday, and it didn't work
again until now.