Re: Making sense of predicates

[Shark8 <onewingedshark@gmail.com>]

On Monday, October 21, 2013 1:51:58 PM UTC-6, Dmitry A. Kazakov wrote:
> Considering this:
>    type T is tagged null record;
>    procedure P (X : T);
> 
>    type S is new T with null record;
>    procedure Q (X : T);
> 
>    subtype Subclass is T'Class with
>       Dynamic_Predicate => Subclass in S'Class;
> 
>       -- Should have been Static_Predicate, but illegal

Er, why not have:
  Function Is_S( Input : in T'class ) return boolean is
  ( Input in S'Class );
or just use S'class variables?


> 
> Well, predicates were not supposed to work in an intelligent way, so why
> not to add a kind that will?

I think they are supposed to work in an intelligent way, though perhaps they're not as nice as compile-time/static checks.

> There are number of cases where this would be very useful.

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.)