Re: Division by zero

["Lurker" <nowhere@nothing.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:79ahr0jozmqb$.10jzllrcjpmsv.dlg@40tude.net...

> > Well, yes. But can't it tell that division by zero is always wrong
> > regardless?
>
> No, it is not, because it has a defined effect: Constraint_Error. Formally
> speaking division by zero is not a contract violation and thus cannot be
> "wrong". Consider the following:
>
>    A : constant Integer := 0;
>    B : Integer := 1;
> begin
>    B := B / A;
>    Put_Line ("Hello!");
> exception
>    when others =>
>       Put_Line ("Good bye!");
>
> The above is a valid program, which should print "Good bye!". At the same
> time the following is *wrong*:
>
>    A : constant Integer := 0;
>    B : constant Integer := 1;
>    C : Integer;
> begin
>    C := B / A;
>
> B/A is a static expression, which has to have a value at compile-time.

OK, I see you point. But, pratically speaking, the chances are that
that construct was an error. Perhaps a warning or some such would
be enough. If whoever wrote that was sure that's what they meant - fine.
But I was always under the impression that the Ada philosophy was
to avoid accidental mistakes as much as possible. And division by
zero sure should qualify as a suspect at least - not something that
just happily gets compiled.

> >> Constraint_Error will be raised at runtime.
> >
> > It was. My question was - should it have been caught
> > at compile time instead?
>
> I don't think it should. A class of errors which cannot be detected at
> compile time cannot be compile-time errors. Or, in other words, if you
> cannot rely on the compiler, you should not pretend you can. (:-))

I'm afraid I don't follow your logic there. Do you really mean that
a constant declared as = 0 cannot be detected at compile time?

Yes, the effects of using it to divide something may or may not be ok.
But that would equally apply to dividing a literal 1 by it, wouldn't it?

(Again, I realise your point about static vs other expressions.
But c'mon, unless someone specifically allows and handles
cases like that, surely the safest bet would be to at least point
out that there is something fishy going on and a "moral equivalent"
of asking "are you sure"?
>
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de