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Subject: Re: Ada Annoyances
From: AdaMagica <christ-usch.grein@t-online.de>
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Xref: news.eternal-september.org comp.lang.ada:47070
Date: 2017-06-23T02:33:07-07:00
List-Id: <comp.lang.ada>

Am Donnerstag, 22. Juni 2017 23:52:42 UTC+2 schrieb Dmitry A. Kazakov:
> There is many ways to achieve the same [wrong] effect. E.g. without any 
> dispatch:
>
>     procedure Foo (P : Parent) is
>     begin
>        Bar (Some_Descendant_Of_Parent (P));
>     end Foo;
>
This code seems illegal to me, at least for tagged types:
1. If Foo is a primitive operation, the body cannot know the descendants.
2. The type conversion is illegal - here, an extension aggregate is needed.

Niklas' example is a completely different thing.

  type Parent is tagged record .... end record;
  procedure Foo (P : Parent);  -- 1
  procedure Bar (P : Parent);  -- 2

  type Child is new Parent with record ... end record;
  -- Child inherits Foo from Parent.     -- 1'
  overriding procedure Bar (C : Child);  -- 3

  C: Child;

  Foo (C);  -- calls 1', i.e Foo (Parent (C));

  procedure Foo (P: Parent) is
  begin
    Bar (P);                  -- Bar 2
    Bar (Parent'Class (P));   -- Bar 3
  end Foo;

Whether this behaviour is wrong or not, is a different story. There are some contrived cases which only work correctly with redispatch.

I do aggree that the declaration of Foo at (1) cannot show whether the body has a redispatch or not - and that's bad. It's the chore of the writer of this code that it works correctly. The user (caller) simply has to believe the writer.