Re: Q: type ... is new String

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Thu, 31 May 2012 09:37:22 +0200, Maciej Sobczak  
<see.my.homepage@gmail.com> a écrit:

> On 30 Maj, 18:33, tmo...@acm.org wrote:
>> Any comments on the effectiveness of using multiple types for different
>> 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 provision  
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 was  
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 := <platform dependent>;
    Maximum_Length : constant := <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 := "..................."; -- Will fail.


Since Ada 2012:

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

    subtype Index_Type is Natural range Minimum_Length .. Maximum_Length;

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

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

-- 
“Syntactic sugar causes cancer of the semi-colons.” [1]
“Structured Programming supports the law of the excluded muddle.” [1]
[1]: Epigrams on Programming — Alan J. — P. Yale University