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Newsgroups: comp.lang.ada
Date: Wed, 24 Aug 2016 09:45:51 -0700 (PDT)
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Subject: Re: Subtype predicates
From: Shark8 <onewingedshark@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:31548
Date: 2016-08-24T09:45:51-07:00
List-Id: <comp.lang.ada>

On Wednesday, August 24, 2016 at 7:06:10 AM UTC-6, AdaMagica wrote:
> Am Dienstag, 23. August 2016 22:14:30 UTC+2 schrieb Anh Vo:
> > type Human_Life_Span_Type is range 0 .. 130 
> >   with Default_Value => Human_Life_Span_Type'First;
> 
> No, I think it's the rule that the name of the entity declared in a declaration (here Human_Life_Span_Type) may not be used again in the same declaration. (You cannot use something that is to be defined recursively in the definition.)
> 
> I just cannot find the RM chapter and verse.

I think it's different for aspects -- similar that Rosen's Trick the name of the declaration is referent to the instance of the type. So we could say something like:

   -- A string of upper-case letters followed by a single digit.
   Subtype ID_String is new String
   with Dynamic_Predicate => 
     (For all Index in ID_String'Range => 
       (if Index /= ID_String'Last
        then ID_String(Index) in 'A'..'Z'
        else ID_String(Index) in '0'..'9'
       )
     );


And since [some] aspects are essentially attribute-specification clauses (eg "Size => 8" = "For T'Size use 8;") there is some sense in thinking it is/should-be legit.


OTOH, you could certainly consider something like ID_String's predicate as being part of the definition.