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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Making sense of predicates
Date: Mon, 21 Oct 2013 21:51:58 +0200
Organization: cbb software GmbH
Message-ID: <myuy19txq4vq.6xym4vrt6irl.dlg@40tude.net>
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Date: 2013-10-21T21:51:58+02:00
List-Id: <comp.lang.ada>

Considering this:

   type T is tagged null record;
   procedure P (X : T);

   type S is new T with null record;
   procedure Q (X : T);

   subtype Subclass is T'Class with
      Dynamic_Predicate => Subclass in S'Class;
      -- Should have been Static_Predicate, but illegal

Now:

   X : Subclass := S'(null record);
begin
   X.P;
   X.Q; -- Illegal

Well, predicates were not supposed to work in an intelligent way, so why
not to add a kind that will?

There are number of cases where this would be very useful. E.g. smart
pointers:

   type T1 is tagged ...;
   type T1_Ptr is access all T1'Class;
   type H1 is new Ada.Finalization.Controlled with record -- Handle type
       Ptr : T1_Ptr;
   end record;

   type T2 is new T1 with ...;

Presently it is impossible to derive H2 from H1 constrained to point to
T2'Class. (Neither it were possible with implicit dereferencing)

With the above, one would create a subtype of H1 constrained to T2'Class
and then derive from it (better in one step, of course).

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