Re: Division by zero

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

"Lurker" <nowhere@nothing.com> writes:

> "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
> news:wccfyvm78k0.fsf@shell01.TheWorld.com...
> 
> > Now I have a variable Gizmos, and I want to print out the
> > numbers of Gizmos per Widget (when that's meaningful).
> > In *portable* code, I might want to write:
> >
> >     if Widgets = 0 then
> >         Put_Line("Unknown");
> >     else
> >         Put_Line(Image(Gizmos/Widgets));
> >     end if;
> >
> > If divide by zero were illegal at compile time, even in unreachable
> > code, the above would be annoying illegal on Unix.
> 
> Interesting example. I guess I'm taking it a bit on a tangent now
> but how does that square off with RM 11.6.5 which says in part:
> 
> ... The exception need be raised by the implementation only if, in the
> absence of raising it, the value of this undefined result would have some
> effect on the external interactions of the program ...
> 
> Yes, I know it's just an implementation permission and so doesn't define
> the language. Yet, the idea seems to be that if there is no possible
> observable
> effect then it's ok. Shouldn't the same logic apply in your example as well?

11.6(5) is all about run time effects.  If divide by zero were illegal
(which means at compile time), then 11.6 would not apply.  11.6 is also
(as you said) all about implementation permissions -- i.e. not
portable.  I would hope a legality rule in this area would be portable.

- Bob