Re: Q: type ... is new String

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Mon, 04 Jun 2012 09:48:07 +0200, Yannick Duch�ne (Hibou57) wrote:

> Le Mon, 04 Jun 2012 08:30:14 +0200, J-P. Rosen <rosen@adalog.fr> a �crit:
> 
>> Le 04/06/2012 07:58, Yannick Duch�ne (Hibou57) a �crit :
>>>    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.
>> Why not:
>> type Path_Type is array (Index_Type range <>) of Character;
>>
>> ?
>> Reminder: There is nothing special to type String, you'll still have
>> literals etc.
> 
> Good idea. I did not consider this one, may be due to the natural tendency  
> of deriving from an already existing type with similar meaning.

It is not natural, it is sloppy. You (1) derive when the result type shall
be in the same class [with class-wide objects]. You (2) clone or use other
means to create an independent type when there shall be no class [no
class-wide objects]. It is a conscious design choice between 1 and 2.

> While I'm OK with this idea and feel it good, I would not apply it,  
> because I would prefer to derive from String. Ada lacks a way to narrow  
> the allowed range of a derived array type.

subtype Punch_Card is String (1..80); -- ?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de