Re: How to best make a custom range?

[Shark8 <onewingedshark@gmail.com>]

On Tuesday, November 5, 2019 at 7:02:37 AM UTC-7, Andrew Shvets wrote:
> This is what I have now.
> 
> subtype DNA_Chars is Character range ‘A’ .. ‘Z’;
> subtype DNA_Symbols is Character with Static_Predicate => DNA_Symbols in ‘ ‘;
> subtype DNA_Char is Character with Static_Predicate => DNA_Char in DNA_Chars | DNA_Symbols;
> 
> And this is the warning that I get (one of many, but I can’t copy and paste):
> 
> warning: in instantiation at a-nudira.adb:54
> type “Result_Subtype” has predicates, attribute “First” not allowed

OK, the problem here is that the Ada language *does* discriminate between subtypes with predicates and those without -- mostly because there are arguments about how such attributes should behave. Things like if we have:
   TYPE Digits is range 0..9;
   SUBTYPE Odds is Digits with Static_Predicate => Odds in 1|3|5|7|9;
what should Odds'Pred(1) be? 0? Constraint_Error?
what about Odds'Pos(1) should it be 0, the first item of the subtype? Or 1, the position in the parent-type?