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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Run-time accessibility checks (was: Construction initialization
 problem)
Date: Wed, 10 Dec 2008 18:55:41 -0600
Organization: Jacob Sparre Andersen
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Date: 2008-12-10T18:55:41-06:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:txbc0ucekvix.ntpy85qsan7h$.dlg@40tude.net...
> On Fri, 5 Dec 2008 19:42:40 -0600, Randy Brukardt wrote:
...
>> I think that I might be able to fix the problem in the context of the
>> containers only, and for dereference of the objects only, but it is not
>> clear that the fix is worth the effort.
>
> It certainly does not. IMO Ada needs "setters" with the syntax sugar of an
> assignment, it does not need outward access types. The whole idea is just
> not Ada.

Well, I've thought about that, but I don't see a decent way to describe 
read/write semantics other than with an access type. Specifically, I was 
trying to figure out a user-defined dereference operation. One would presume 
that we would use an operator function for that, but what would it look 
like? The obvious answer of:

    function "all" (Obj : in Some_Type) return access Some_Other_Type;

has the accessibility issues. And something like:

   function "all" (Obj : in Some_Type) return Some_Other_Type;

means that you have (usually) copy semantics and in any case no assignment 
into. That doesn't really fix anything.

You certainly can't have a procedure ":=" to do assignment in because of 
discrimant-dependent components. That's never going to change for Ada, so 
please forget that.

I suppose you could have a component setter, but that requires a procedure 
with two names:

    procedure <component>.":=" (Target : in out Some_Tagged_Type; Source : 
in Some_Other_Type);

which would then be used as:

    Target.<component> := Source;

But that looks like a giant change to the Ada models, almost certainly out 
of scope for this current round. And it certainly means no useful iterators. 
And it also isn't very flexible -- where does the Cursor of a container go?? 
And so on.

                                Randy.