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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Seeking for papers about tagged types vs access to subprograms
Date: Mon, 13 May 2013 21:14:42 -0500
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2013-05-13T21:14:42-05:00
List-Id: <comp.lang.ada>

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message 
news:wccmwrzu9oj.fsf@shell01.TheWorld.com...
> Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:
>
>> I'm not sure I see the point in having an object for a closed file,
>
> Yeah, I was about to post basically the same thing.  A closed file
> is pretty useless.  It's like an uninitialized variable -- you can't
> do anything with it.

Yes, and of course Ada allows uninitialized variables, too. Clearly the same 
sorts of reasons apply.

We briefly tried that model when designing Claw, but it really didn't work, 
because the underlying handle can be closed by some other operation and in 
that case the object becomes closed (we called it "invalid") without any 
explicit action in your code. For a windowing system, that other operation 
can be the guy with the mouse, who can easily screw up all of your carefully 
thought out code.

Another issue is that doing that means that you are limited to Ada's scopes 
for managing objects, which means that you're limited in how you can set up 
and tear down structures. (You can always get around those problems by 
essentially making your clients use new/unchecked_deallocation for 
everything -- but I don't consider that a step forward. I'd rather have 
"invalid" objects than force people to use access types.)

We used that model throughout Claw, and that included in Claw.Directories, 
so of course it carried over into the original proposal for Ada.Directories 
(which was heavily based on Claw.Directories). So "Search_Type" originally 
had a "Is_Valid" routine, which eventually got folded into "More_Entries". 
(Which demonstrates an intermediate approach that often works -- have a 
default initial state which is well-defined but not useful.)

                                        Randy.