Re: 'Class'Class'Class'Class ad libitum

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

On 2016-09-13 18:10, Alejandro R. Mosteo wrote:
> 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?

Semantics you mean. That semantics is wrong. If T'Class'Class is legal, 
I don't know. It should not be otherwise than in the described above sense.

> 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.

Dispatching is a property of an operation's argument, not of a type. The 
same object may dispatch or not. It should not be reader's concern. 
Dispatching is merely a method of operation implementation, that is by 
composition of the operation body out of multiple bodies of other 
operations selected by the tag. It is a pure implementation detail.

> 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).

 From the original Ada POV the former is better. Initially Ada strove to 
have nominal typing. T'Class notation parts with that, it is structural 
equivalence, which is not that great.

> First time I wrote the later I expected to
> get an error or warning of the short "subtype is already class-wide".

A good catch by compiler.

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