Re: Q: type ... is new String

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

Le Mon, 04 Jun 2012 10:03:12 +0200, Dmitry A. Kazakov  
<mailbox@dmitry-kazakov.de> a écrit:

> On Mon, 04 Jun 2012 09:48:07 +0200, Yannick Duchêne (Hibou57) wrote:
>
>> Le Mon, 04 Jun 2012 08:30:14 +0200, J-P. Rosen <rosen@adalog.fr> a  
>> écrit:
>>
>>> Le 04/06/2012 07:58, Yannick Duchêne (Hibou57) a écrit :
>>>>    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); -- ?

A path is intently a string is all platform I know, and that's not just a  
representation trick. A punch card is not the same, and that would indeed  
be an error to derive it from a string. You may find a file path in a text  
file, you may not find a punch card in a text file. A string and a file's  
path belongs to the same class.

-- 
“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