Re: How to best make a custom range?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2019-11-05 15:02, 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

Because ordering attributes are ones that get broken either way. Outside 
generics the language threats them contravariant [the result is of the 
base subtype], which breaks ordering but keeps much of other semantics. 
When you pass a subtype as an actual parameter to a generic it suddenly 
becomes covariant [the result of the subtype], which is sometimes worse, 
sometimes quite impossible to implement. Ada plays safe here and just 
does not let you. In other cases you might not be so lucky. The language 
cannot deduce right semantics from the constraint. It is undecidable, 
incomputable etc. In short, do not do that.

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