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Date: Tue, 25 Nov 2008 12:24:59 +0100
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Extending discriminant types
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Martin schrieb:

> You can't ever use 'Base on its own in Ada83.

The collection of multifocal arguments given
for or against 'Base, and/or GNAT, is, I guess,
multitargetting some other, ideas (that, frankly,
I find slightly elusive):

"the Base was a concept only not a usable attribute,
 which is also stated in book "Ada Problem  Solving and
 Program Design", 1990"  -- anon

However, 'Base can designate something very regular in
"Ada 83 mode",  it is not "just a concept", at
least in sofar as this would imply "not in the language".
(I don't have a copy of Mike Feldman's book, which anon
has been referencing in its enterity, so I can't check,
but I' guess that the author has not spun that many
arguments on loosly related issues around 'Base.)


As you point out, 'Base *is* an attribute (and nothing else)
in Ada 83, with a restriction on where and how to use it.
'Base can be applied to record type names in suitable context.

Other constructs of current and past editions of Ada
have similar restrictions, haven't they?

So here are two uses of 'Base.  The unit compiles fine,
it seems.

generic
   type N is (<>);
package U is

   type T (Len: Integer) is private ;
   type Count is new N range 0 .. N'Base'Last;  -- 'BASE

   function "=" ( Left, Right :  Count) return Boolean ;


private
   type T (Len: Integer) is record
      null;
   end record;

   X: T (Len => T'Base'Size) ; -- 'BASE, TOO
end U ;


A sequence of attributes is still part of Ada, so their
sequencing is not just Ada 83.