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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Access keyword
Date: Wed, 30 Apr 2008 18:40:43 -0500
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Date: 2008-04-30T18:40:43-05:00
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"Adam Beneschan" <adam@irvine.com> wrote in message
news:2cc577f4-704a-4235-aad4-a29186309c02@k10g2000prm.googlegroups.com...
> On Apr 30, 12:15 am, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
...
> > Anonymous access types are used by Ada.Containers for iteration (among
> > others). "Rationale" by Barnes presents some nice examples with
> > downward closures as well.
>
> I suspect that Randy was talking only about anonymous access-to-object
> parameters.  Those aren't used by the language's definition of
> Ada.Containers, or at least I didn't spot any.
>
> Anonymous access-to-subprogram parameters (which Ada.Containers does
> use) are a different animal altogether.  They do serve a legitimate
> purpose; downward closures are important but couldn't be done
> satisfactorily in Ada 95.

I agree with this, although the syntax is rotten.
Anonymous-access-to-subprogram parameters have special semantics and cannot
be stored (you'll get an accessibility error if you try). So they act like
parameters that are subprograms rather than an access type. I think the
syntax is rotten because these are access types only in the sense that a
compiler will implement them this way.

I do include regular anonymous-access-to-subprogram types in the category of
useless features.

And I do plead guilty to not opposing the expansion of this misfeature in
the Amendment hard enough. I was temporarily swayed by arguments that they
would allow using far fewer useless type conversions. The problem is that in
practice it is not true: you still need type conversions unless you purge
all of your named access types from the program. But then you lose the
ability to have storage pools -- a nasty trade-off. Essentially, they don't
provide the feature that was advertised, and thus should never have been
approved.

Beyond that, I agree with Dmitry that Ada needs a real iterator construct;
passing a subprogram into another subprogram is much less understandable
than
     for Element in Container loop
         ...
     end loop;

And that would eliminate much of the need for "downward closures". (But not
all! I don't want to get into a silly argument about that...)

                              Randy.