From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Is it possible to make a possibly self-referential type using
 containers?
Date: Sat, 30 Jul 2016 22:36:22 -0700
Organization: Also freenews.netfront.net; news.tornevall.net;
 news.eternal-september.org
Message-ID: <nnk2os$cbi$1@dont-email.me>
References: <18dfc647-ef22-4052-b6ad-ce3516124085@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Injection-Date: Sun, 31 Jul 2016 05:36:30 -0000 (UTC)
Injection-Info: mx02.eternal-september.org;
 posting-host="d7c030f56102b58a2c16dea977db88bb";
	logging-data="12658"; mail-complaints-to="abuse@eternal-september.org";
	posting-account="U2FsdGVkX1/SJ3auAwBbJEhmUXc1QRYn+vp6fJ0OLlI="
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.2.0
In-Reply-To: <18dfc647-ef22-4052-b6ad-ce3516124085@googlegroups.com>
Cancel-Lock: sha1:Ol+I5tmItFxJx69SrTSM+IZW3mc=
Xref: news.eternal-september.org comp.lang.ada:31228
Date: 2016-07-30T22:36:22-07:00
List-Id: <comp.lang.ada>

On 07/30/2016 05:31 PM, Shark8 wrote:
> Consider something like Forth, IDL, and ASN.1 where there recursion plays a big part in the underlying ideas: the former in its definition of "Word"*, and in the latter two in the defining of types**.
> 
> So, given that the most straightforward way to implement these would be in a self-referencing type it would be nice to be able to use containers for this, perhaps like:

There was a discussion of this on here about a year ago in the context of having
a private type that is self-referential using a container. See

https://groups.google.com/d/msg/comp.lang.ada/SO9olMGfF4o/aGrCFMsVCwAJ

The advantage of using a container is that one doesn't have to deal with the
error-prone aspects of using access types and doing memory management.

I presented a solution there that used a tagged type, an indefinite container of
the 'Class of that type, and a type extension to achieve this:

with Ada.Containers.Indefinite_Vectors;

package P is
   type T (<>) is private;
private
   type Root is tagged null record;

   package Lists is new Ada.Containers.Vectors
      (Index_Type => Positive, Element_Type => Root'Class);

   type T is new Root with record
      List : Lists.Vector;
   end T;
end P;

The intention here is that only values of type T will be stored in such a
vector, so Root and the type extension for T are noise. You also have to
explicitly convert anything retrieved from the list to type T, which is more
noise. But it beats messing with access values.

-- 
Jeff Carter
"I was either in love, or I had smallpox."
Take the Money and Run
137