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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Exceptions
Date: Thu, 13 Dec 2007 08:40:34 -0500
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Date: 2007-12-13T08:40:34-05:00
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Maciej Sobczak <see.my.homepage@gmail.com> writes:

> On 11 Gru, 09:07, Stephen Leake <stephen_le...@stephe-leake.org>
> wrote:
>
>> For code I have control over, I agree with Bob, except I use a
>> different exception; raise Programmer_Error if you've detected a bug.
>
> I understand that this way you want to signal something much more
> severe than "mere" Constraint_Error or such.
> The problem is that Program_Error can be shut up by some null handler
> or it can be translated to some other exception and then lose its
> original weight.

The solution to "when others => null;" and the like is better
education.  Competent programmers need to know not to do that,
except in rare cases, where a big fat comment is in order.
(Unfortunately, the designers of Ada 83 made exactly this
mistake for task bodies!  And Ada 95 repeats it for
interrupt handlers.)

I don't consider Constraint_Error "mere", by the way.
It almost always represents a bug.

> Why not terminating the program right on the spot by simply:
>
> pragma Assert (False);
>
> ?

That doesn't terminate the program on the spot.
It raises Assertion_Failure.

I consider all of these:

    pragma Assert (False);
    Assert (False); -- Assert is a user-defined procedure
    raise Program_Error;
    raise Bug; -- Bug is a user-defined exception used project-wide

to be legitimate ways of reporting a self-detected bug.

There ought to be a standard predefined procedure for stopping the
program "on the spot".  Since there isn't, you can import "exit".

> (yes, there are compiler options to switch it off as well)

Indeed, it's turned off by default in GNAT (which I don't much like).

> Does anybody use this strategy in regular code?
> I happen to use it in test units and I find it very appropriate there.
> Any thoughts on using assertions, including Assert(False), in
> production code?

Nothing wrong with using assertions in production code.

- Bob