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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: S-expression I/O in Ada
Newsgroups: comp.lang.ada
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Date: Sun, 8 Aug 2010 22:39:11 +0200
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Date: 2010-08-08T22:39:11+02:00
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On Sun, 08 Aug 2010 16:03:22 -0400, Robert A Duff wrote:

> For scalars, the primitive operations mostly don't know the
> subtype bounds -- they are operations of the type.  So this
> wouldn't fit into Ada well.

But array subtypes silently modify their operations or, if you want,
parametrize them using the dope. A modular subtype would have a dope as
well.

>> ...then they could be used as indices.
> 
> The problem I was alluding to is that if you have
> an unconstrained array type, sooner or later you might
> have an empty one.  So you want bounds 0..-1, but -1
> wraps around.

Empty range does not have bounds. What are bounds of

   type Empty is mod 0; -- Illegal in Ada, but must be legal

> Same thing with a "for" loop that goes from T'First up to
> something.

If ranges (contiguous sets of indices) were proper types you would be able
to construct them in many different ways. E.g.

   function ":" (From : Index; Length : Universal_Integer)
      return Index'Range

   L := 0
   ...
   for I in T'First :L loop

instead of what you can find in every Ada program:

   for I in T'First..T'First + L - 1 loop

or for enumerations:

   for I in T'First..T'Val (T'Pos (T'First) + L - 1) loop

which does not work anyway...

> If 2..10 wrapped around, then you'd want range 2..1, which
> has the same problem.

2..1 actually is 2,3,4,5,6,7,8,9,1

Modular numbers are not ordered.

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