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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Proposed change to BC iterator parameters
Date: Thu, 11 Dec 2003 09:33:06 +0100
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On 10 Dec 2003 18:00:31 -0500, Robert A Duff
<bobduff@shell01.TheWorld.com> wrote:

>Here, I think you're talking about regular private types in
>packages (as opposed to generic formals).  One idea is to say:
>
>    type T is private;
>    function "="(X, Y: T) return Boolean is abstract;
>
>Then clients can't call predefined "=".
>
>(But predefined "=" reemerges in generics!  And records.)
>
>I've also done things like this (in the visible part):
>
>    type T is private;
>
>    function Predefined_Equal(X, Y: T) return Boolean renames "=";
>    -- For use only in the package body!
>
>    function "="(X, Y: T) return Boolean is abstract;
>
>Unfortunately, the Ada compiler does not understand the comment, "-- For
>use only in the package body!".  You say above, ``with the capability of
>defining "=" for either type.''  But it's not trivial to make that "="
>be the predefined one.

Sticking to implemetation inheritance is the actual problem. Why do
not we have true abstract interfaces? For example:

type Can_Do_Nothing is <interface-keyword>;

type Can_Only_Compare is <interface-keyword>;
function "=" (X, Y : Can_Only_Compare) return Boolean is abstract;

type Can_Copy_Assign_Compare is <interface-keyword>;
<some-syntactic-sugar-defining-abstract-copy-constructor>
<some-syntactic-sugar-defining-abstract-assignment>
function "=" (X, Y : Can_Copy_Assign_Compare)
   return Boolean is abstract;
...

Then an integer without predefined arithmetic could be an
implementation of Can_Copy_Assign_Compare via private inheritance from
Integer :

   type My_Number is new private Can_Copy_Assign_Compare;
private
   type My_Number is new Integer;
   
--
Regards,
Dmitry Kazakov
http://www.dmitry-kazakov.de