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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Class-wide types algebra
Date: Mon, 12 Sep 2016 22:26:19 +0200
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Xref: news.eternal-september.org comp.lang.ada:31756
Date: 2016-09-12T22:26:19+02:00
List-Id: <comp.lang.ada>

Presently class-wide types are singletons. I would propose to extend 
that to call-wide expressions, namely conjunction in all places where 
T'Class is expected. e.g.

    procedure Foo (X : in out T'Class and R'Class); -- Not Ada

    task type Worker (X : not null access T'Class and R'Class) is

    if X is in (T'Class and R'Class) then

    Object : T'Class and R'Class renames (T'Class and R'Class) (X);

It should not be difficult, at first glance.

Rationale. Presently it is impossible to declare an operation requiring 
an instance derived from the type T that also implements interface R. We 
cannot do that without having a explicit root type

    type T_And_R is new T and R with ...;

But that will exclude existing types already implementing T. If we 
already have:

    type S is new T with ...;

A type derived from S with R added would not fall into T_And_R.

Generics do not help much because descendants of T may have 
discriminants which are all unknown in advance, so there is no chance to 
create instances derived types.

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