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From: Jacob Sparre Andersen <jacob@jacob-sparre.dk>
Newsgroups: comp.lang.ada
Subject: Re: Making sense of predicates
Date: Tue, 22 Oct 2013 09:35:02 +0200
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2013-10-22T09:35:02+02:00
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Shark8 <onewingedshark@gmail.com> 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;

with Parts;

procedure Use_Parts is
   Work : Parts.Part_Number := "1234";
   Fail : Parts.Part_Number := "fail";
begin
   null;
end Use_Parts;

% ./use_parts

raised SYSTEM.ASSERTIONS.ASSERT_FAILURE : Dynamic_Predicate failed at use_parts.adb:6
%

Greetings,

Jacob
-- 
Photos from the Faroe Islands:
       http://billeder.sparre-andersen.dk/The_Faroe_Islands/