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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: A bad counterintuitive behaviour of Ada about OO
Date: Fri, 8 Aug 2014 17:26:18 -0500
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Xref: news.eternal-september.org comp.lang.ada:21591
Date: 2014-08-08T17:26:18-05:00
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"Maciej Sobczak" <see.my.homepage@gmail.com> wrote in message 
news:e1a445fa-b3b8-4b3d-a856-3e0ccbc737bf@googlegroups.com...
W dniu piatek, 8 sierpnia 2014 10:14:27 UTC+2 uzytkownik Dmitry A. Kazakov 
napisal:

>> I still do not understand the example.
>
>http://en.wikipedia.org/wiki/Covariant_return_type
>
>> Is the operation in question
>> primitive for both the factory and product hierarchy? That is MD.
>
>I don't consider it to be an example of MD, because the two types (factory 
>and product)
>do not play equal roles in the dispatch. Namely, the operation is expected 
>to dispatch on
>the factory type only.

That''s not how Ada looks at things (as I'm sure you know). The result type 
participates in resolution and can participate in dispatch. Ergo, if an 
operation is primitive on multiple tagged types, then it is by definition 
MD.

Almost all other languages do not have the result type participate in 
resolution (and thus it makes no sense for it to participate in dispatch). 
This is a fundemental difference of Ada that clearly is unchangable at this 
point (we're not going to break thousands of programs that use return type 
resolution to meet someone's idea of language purity).

                                             Randy.