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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Generic private type declaration
Date: Tue, 29 Nov 2016 12:42:19 +0100
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Date: 2016-11-29T12:42:19+01:00
List-Id: <comp.lang.ada>

On 28/11/2016 22:32, Randy Brukardt wrote:
> So, yes, Dmitry, the language could strengthen contracts this
> way -- if one didn't care about usability [or compatibility]).

Rather one should make contracts more complete. If the formal type "X is 
private" may or may not allow type cloning by "Y is new X" of the actual 
type, that must be a part of the contract. E.g. tagged types should not 
match or, better cloning hierarchies tagged types must be allowed.

The principal design rule should be legality to be determined 
exclusively by visible specifications.

For generic formals it would mean resolving all conflicting declarations 
in the formal part. For example:

generic
    type T is private;
package
    function "+" (Left, Right : T) return T; -- Must be illegal!

Reason: T might have "+" visible in the instantiation context. So the 
package declaration must be:

generic
    type T is private;
    without function "+" (Left, Right : T) return T;
       -- I promise there will be none
package
    function "+" (Left, Right : T) return T; -- It is OK now

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