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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada Successor Language
Date: Mon, 25 Jun 2018 22:32:08 +0300
Organization: Tidorum Ltd
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Xref: reader02.eternal-september.org comp.lang.ada:53311
Date: 2018-06-25T22:32:08+03:00
List-Id: <comp.lang.ada>

On 18-06-25 17:44 , J-P. Rosen wrote:
> Le 25/06/2018 à 16:21, Dmitry A. Kazakov a écrit :
>> Ada could allow T'Class for untagged T. T'Class would be an indefinite
>> type with values consisting of the actual type's tag and its value. When
>> T is by-value type, you pass T'Class to a subprogram as tag + value.
>> When T is by-reference type, you pass tag + reference.
>>
>> Upon dispatch, you strip the tag from the value or reference before
>> passing value/reference down.
>
> It was a requirement of Ada95 that OOP be strictly contained, and that
> the same typing system as Ada83 be kept for those who don't want OOP.

Neither Dmitry's suggestion for untagged class-wide programming nor my 
suggestion for untagged record extension conflicts with or replaces the 
Ada83 typing system. Rather, these suggestions would interpolate new 
typing features intermediate between the Ada83 typing system and Ada95 
tagged types.

If these features had been suggested for and included in Ada95, they 
might have helped some Ada83 designs evolve incrementally towards OOP, 
without leaping all the way to tagged types.

Whether the impact on Ada95 compiler complexity would have been too 
large to be worth it, is another question. Both suggestions seem to 
require new kinds of code generation for some calls, not needed for 
Ada83 (where there are no tags) nor for tagged types (where tags are 
embedded in objects).

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .