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From: "Alejandro R. Mosteo" <alejandro@mosteo.com>
Newsgroups: comp.lang.ada
Subject: Re: 'Class'Class'Class'Class ad libitum
Date: Tue, 13 Sep 2016 18:10:12 +0200
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Date: 2016-09-13T18:10:12+02:00
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On 13/09/16 14:37, Dmitry A. Kazakov wrote:
> On 13/09/2016 12:43, Alejandro R. Mosteo wrote:
>> I get why this is legal:
>>
>>    type X is tagged null record;
>>
>>    subtype Y is X'Class;
>>
>>    subtype Z is X'Class'Class'Class'Class;
>>
>> But can you think of any legitimate reason to use it besides obfuscation?
>
> Only the latter is wrong. X'Class'Class cannot be same as X'Class.
>
> X is a set of values with operations on X
>
> X'Class is a set of values of X and values of all types derived from X
>
> X'Class'Class should be a set of values of X'Class and values of all
> types derived from X'Class. We cannot derive from X'Class in Ada, so
> class of X'Class holds only X'Class. But that does not make them same.
> Like X is not same as X'Class even if there is no descendants of X.
> Class-wide operations of X would be primitive operations of X'Class'Class.

I see that from a strict theoretic POV. However, that's not the 
interpretation that current Ada grammar does, right?

Actually I arrived at this because I am somewhat wary of the fact that 
using a subtype like Y in my example hides the fact to the reader that 
the type is class-wide and hence dispatching by default. Furthermore, 
you can legally write

procedure Blah (This : Other_Package.Y); -- or
procedure Blah (This : Other_Package.Y'Class);

and AFAIK that's totally equivalent but the former is less informative 
(and alas, shorter to write). First time I wrote the later I expected to 
get an error or warning of the short "subtype is already class-wide".

Álex.