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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: stopping a loop iteration without exiting it
Date: Fri, 5 Jan 2018 01:08:28 +0200
Organization: Tidorum Ltd
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Xref: reader02.eternal-september.org comp.lang.ada:49759
Date: 2018-01-05T01:08:28+02:00
List-Id: <comp.lang.ada>

On 18-01-04 23:17 , Dmitry A. Kazakov wrote:

    [snip]

> I thought about really important cases, but could not invent syntax:
>
>    if X in T'Class then
>       declare
>          Same_X : T'Class renames T'Class (X);
>       begin
>          ...
>       end;
>    end if;

A syntax suggestion, drawing on analogy with exception handlers:

    case X is
    when Same_X : T'Class =>
       ...
    when Same_X : U'Class =>  -- other named classes
       ...
    when others =>  -- always required
       ...
    end case;

This would be legal only if no two classes in the "whens" are related 
with "in", so that at most one "when S'Class" matches X.

>    if P /= null then
>       declare
>          X : P_Type renames P.all;
>       begin
>          ...
>       end;
>    end if;

Similarly, applying an implicit dereference to the selecting expression 
when it has an access type:

    case P is
    when X : P_Type =>
       ...
    when null =>  -- or "when others =>"
       ...
    end case;

A combination might be possible when P is an access-to-classwide:

    case P is
    when X : P_Type'Class =>
       ...
    when X : Q_Type'Class =>   -- other named classes
       ...
    when null =>  -- optional, can be included in "others".
       ...
    when others =>  -- not null, but some other class
       ...
    end case;

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