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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
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 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: Q: type ... is new String
Date: Mon, 04 Jun 2012 07:58:06 +0200
Organization: Ada @ Home
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Date: 2012-06-04T07:58:06+02:00
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Le Thu, 31 May 2012 09:37:22 +0200, Maciej Sobczak  =

<see.my.homepage@gmail.com> a =C3=A9crit:

> On 30 Maj, 18:33, tmo...@acm.org wrote:
>> Any comments on the effectiveness of using multiple types for differe=
nt
>> categories of strings, rather than "String" all the time?
>
> Interestingly, this is a bit different from defining multiple number
> types instead of using standard types all the time.
> The difference is that number types carry bounds as additional
> information and that can be comfortably used to later define arrays,
> loop ranges, etc.
> Strings don't have this capability (unless, of course, you define
> constrained String (A .. B) types)

There is nothing like a range with text type, but there is still provisi=
on  =

for contracts (pre/post in Ada 2012). An invariant for a Path_Type would=
  =

be as much useful as a range for a numeric type would be.

Talking about range and this particular case, prior to Ada 2012, there w=
as  =

no way to narrow the range for a derive type of an array type, without  =

freezing its bounds. It's now possible with Ada 2012.


Prior to Ada 2012:

    Minimum_Length : constant :=3D <platform dependent>;
    Maximum_Length : constant :=3D <platform dependent>;

    subtype Index_Type is Natural range Minimum_Length .. Maximum_Length=
;

    type Path_Type is new String (Index_Type);
    -- Unfortunately, this freeze the lower and upper bound, and the
    -- declaration below fails.

    XYZ_Location : Path_Type :=3D "..................."; -- Will fail.


Since Ada 2012:

    Minimum_Length : constant :=3D <platform dependent>;
    Maximum_Length : constant :=3D <platform dependent>;

    subtype Index_Type is Natural range Minimum_Length .. Maximum_Length=
;

    subtype Path_Type is String
       with Static_Predicate =3D> Path_Type'Length in Index_Type;
       -- You may add additional contracts, specific to file paths,
       -- like conformance with a format.

    XYZ_Location : Path_Type :=3D "..................."; -- OK.

-- =

=E2=80=9CSyntactic sugar causes cancer of the semi-colons.=E2=80=9D [1]
=E2=80=9CStructured Programming supports the law of the excluded muddle.=
=E2=80=9D [1]
[1]: Epigrams on Programming =E2=80=94 Alan J. =E2=80=94 P. Yale Univers=
ity