Re: Subtype predicates

[Shark8 <onewingedshark@gmail.com>]

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.