Re: S-expression I/O in Ada

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sun, 08 Aug 2010 17:08:50 -0400, Robert A Duff wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
> I actually have no opinion whether it's a good idea, because I
> don't like modular types in the first place.  ;-)

The implementation or the idea? Would you agree that objects with some
properties of modular integers have place in Ada programs which do not
interface C?

> I didn't mention "access" above, which are "elementary" but not
> "scalar".  Constrained access subtypes are a nightmare!

Yes, because the language should decide whether the constraint does
influence the behavior (all operations potentially), or is a kludge that
prohibits some values when assigned or passed. I understand the motivation
why Ada 83 chosen the second, but it does not make it right. Assignment is
an operation as any other.

>> Empty range does not have bounds.
> 
> Except that in Ada, they do.  An empty String is (1..0).
> It could also be 10..9 or even -100..-10_000_000, but
> that's not a good idea.

Yes, because it is wrong. Doing something wrong always will hit back.
 
>> Modular numbers are not ordered.
> 
> But they are -- they have "<", and "for" and so on.

"<" is wrong when non-transitive. I wished Ada clarified difference between
enumeration and total order.

> Perhaps they _should_ be unordered, but I won't agree or disagree,
> since I think in an ideal world they should be banished.

I think they could be fixed.

> By the way, one defense of modular types I've heard is that
> they are used in mathematics.  True.

>  But mathematicians do
> not use _implicit_ mod.  They say things like "X = Y (mod N)",
> which is pronounced "X is congruent to Y (modulo N)".
> Congruent, not equal.

The mathematical notation (mod N) is untyped. It applies to any natural
numbers and what is worse you have to add it at each point of the program
you use the type.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de