Re: Making sense of predicates

[Shark8 <onewingedshark@gmail.com>]

On Tuesday, October 22, 2013 1:35:02 AM UTC-6, Jacob Sparre Andersen wrote:
> Shark8 writes:
> 
> > Agreed; I was disappointed the following doesn't work:
> >
> >  Subtype Digit_Character is Character range '0'..'9';
> >  Subtype Part_Number is String
> >    with Static_Predicate => (for all C in Part_Number => C in Digit_Character);
> >
> > As it would be nice to basically subtype out [further-constrain] the
> > element-type just as we can do to the index (e.g. "Subtype
> > Phone_Digits is String(1..7);", which is equivalent to "subtype
> > phone_range is Positive range 1..7; Subtype Phone_Digits is
> > String(Phone_Range);", where 'phone_range' is an anonymous subtype.)
> 
> If you did it right, it would work:
> 
> package Parts is
>    subtype Digit_Character is Character range '0' .. '9';
>    subtype Part_Number is String
>      with Dynamic_Predicate => (for all C of Part_Number
>                                   => C in Digit_Character);
> end Parts;

I think you misunderstand my point: there's no real reason [at least that I can tell (apart from syntax, possibly)] that an array-type cannot be subtyped in order to place additional [static] constraint upon the base-type.

IOW, why is the predicate required to be dynamic when, for example, "subtype Number_String is Array(Positive range <>) of Digit_Character" or "subtype short_String is String(1..255)" is not a dynamic-predicate?