Re: about inheritance of subtypes and entities (such as constants) related to a type in the same package

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

On 2018-06-01 16:01, Dan'l Miller wrote:
> On Friday, June 1, 2018 at 2:56:55 AM UTC-5, Dmitry A. Kazakov wrote:
>> On 2018-05-31 11:10 PM, Dan'l Miller wrote:
>>> On Thursday, May 31, 2018 at 2:59:48 PM UTC-5, Dmitry A. Kazakov wrote:
>>>> On 2018-05-31 20:53, Dan'l Miller wrote:
>>>>> On Thursday, May 31, 2018 at 12:37:07 PM UTC-5, Dmitry A. Kazakov wrote:
>>>>>> On 2018-05-31 04:29 PM, Dan'l Miller wrote:
>>>>>>> Conversely, subtyping in Ada does none of that type-extension stuff:  no
>>>>>>>     additional data/cardinality-of-the-members-of-the-set, no additional
>>>>>>     > routines/behavior, no arbitrary refinement of behavior of overridden/copied
>>>>>>>     routines/operations.  Instead, subtyping in Ada is strictly subjecting the
>>>>>>>     parent type to an additional membership-narrowing predicate, such as subset
>>>>>>     > in set theory.  Randy likewise spoke of membership on other branches of this
>>>>>>     > thread.
>>>>>>
>>>>>> Untrue. Ada 83 subtyping extends the supertype (base type), because it
>>>>>> exports operations defined on the subtype to the base type. Ada 83
>>>>>> subtyping introduces an equivalent type, which is both sub- and
>>>>>> supertype of the base.
>>>>>
>>>>> If Ada83 untagged subtyping is type extension, please provide even a single characteristic that was extended/added/supplemented instead of contracted/subtracted/elided by the predicate.
>>>>
>>>> Elementary:
>>>>
>>>>       subtype Extension is Integer;
>>>>       procedure Extended_Operation (X : Extension) is null;
>>>>       X : Integer;
>>>>       Y : Extension;
>>>> begin
>>>>       Extended_Operation (X); -- See #1
>>>>       Extended_Operation (Y); -- See #2
>>>
>>> There is exactly one procedure Extended_Operation, which despite its cleverly-deceptive name is not a demonstration of type extension:  We start with having a single procedure that takes X as a parameter and that is precisely where we end:  no new procedure for Y, no additional storage for Y beyond Y's Xness, no new outcome in the execution of that procedure when passed a Y (except enforcement of the [unfortunately-missing] predicate as a post-condition).
>>
>> Interface was extended. Implementation details as to how many bodies
>> there will be used are irrelevant.
>>
>>> Or in fewer words:  nothing about Y is X-plus-more.  There is no "more" there.
>>> (not even a predicate to enforce, which would have made this example a tad juicier).
>>>
>>>> The subtype Extension extends Integer with Extended_Operation, which has
>>>> two separate meanings:
>>>>
>>>> 1. The base type Integer has now a new operation:
>>>>
>>>>       new Integer interface = old Integer interface + Extended_Operation
>>>>
>>>> 2. The subtype Extension extends inherited Integer's set of operations:
>>>>
>>>>       Extended interface = old Integer interface + Extended_Operation
>>>
>>> Invoking the same subprogram
>>
>> Same to what? The interface of Integer as defined by the standard does
>> not have Extended_Operation. How is it same?
> 
> (How clever that you snipped my already-existing answer to your question at precisely the word where it becomes the inconvenient answer.  The elided original answer is restored as the next quotation below.)
> 
> Extended_Operation is merely invoked twice.  Extended_Operation is •the same• as itself, regardless if passed X as an argument or passed Y as an argument.

But X and Y are declared of different subtypes. How can it be same?

> If Extended_Operation were named GiveTheKidABath instead, it would be obvious to all readers that giving two kids {X, Y} a bath, does not extend Y by growth and does not extend the size of the family by birthing more babies.  Nothing whatsoever was extended by invoking Extended_Operation against both X and Y.  Despite its cleverly-deceptive name (and despite invoking that subprogram twice), there is no extension whatsoever in Y beyond Y's Xness.  If Y was declared with a predicate, then Y would actually demonstrate some contraction of Xness.  Contraction (smaller domain) is not extension (bigger size or bigger mission) either.

Again, Integer does not have Extended_Operation. Try this:

    procedure Test is
       X : Integer;
    begin
       Extended_Operation (X);
    end Test;

does it compile? No.

    procedure Test is
       subtype Extension is Integer;
       procedure Extended_Operation (X : Extension) is null;
       X : Integer;
    begin
       Extended_Operation (X);
    end Test;

Does it compile now? Yes.

What happened with the Integer type?

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