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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: [Newbie] doubly constrained array, dumb question
Date: Tue, 27 Feb 2018 09:01:11 +0000
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Date: 2018-02-27T09:01:11+00:00
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"Randy Brukardt" <randy@rrsoftware.com> writes:

> "J-P. Rosen" <rosen@adalog.fr> wrote in message 
> news:p71rvj$vgh$1@gioia.aioe.org...
>> Le 26/02/2018 à 17:26, Mehdi Saada a écrit :
>>> Hello.
>>>
>>> I would like an constrained String subtype with narrower bounds that 
>>> Positive'Range.
>>> How can I do something like:
>>> subtype Possible_Length is NATURAL range 1..80;
>>> subtype T_Line is String (Possible_Length range <>);
>>> ? I know "range <>" isn't included in the definition of 
>>> "range_constraint". How can I express the same thing.
>>> So that I can get after, a dynamic string with:
>>> A: access T_LINE := new T_LINE'("BLABLABLA"); while checking for its 
>>> range.
>>>
>> You can't do that, because a subtype can't be both constrained and
>> unconstrainde at the same time...
>>
>> OTOH, you can define your own string type:
>> type Short_String is array (Possible_length range <>) of character;
>
> Or you could use a subtype with a dynamic predicate:
>
>     subtype Short_String is String
>        with Dynamic_Predicate => Short_String'First >= Possible_Length'First 
> and Short_String'Last <= Possible_Length'Last;
>
> This would be checked any time that you convert a string value into a 
> Short_String subtype (explicitly or implicitly), so it probably would give 
> the right effect. Note that unlike a real constraint, it wouldn't have any 
> effect on other subtypes, so:
>      Silly_Object : Short_String (1..100);
> would not raise Constraint_Error, but any attempt to assign into it would 
> raise Assertion_Error. (Assuming the Assertion_Policy is Check, it isn't for 
> GNAT by default.)

I tried this first:

   with Ada.Text_IO;
   procedure Short_String is
      pragma Assertion_Policy (Check);
      subtype Possible_Length is Integer range 0 .. 5;
      subtype Short_String is String
      with Dynamic_Predicate => Short_String'Length <= Possible_Length'Last;
   begin
      for J in 0 .. 6 loop
         declare
            S : Short_String (1 .. J) := (others => 'm');
            --  pragma Assert (S'Length <= Possible_Length'Last);
         begin
            S := (others => 'x');
            Ada.Text_IO.Put_Line (String (S) & S'Length'Img);
         end;
      end loop;
   end Short_String;

and then with your suggestion, and neither raised AE.

I put in an assertion as commented out, and it was triggered.

SPARK detectd the problem ("medium: predicate check might fail").