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=-1.3 required=5.0 tests=BAYES_00,INVALID_MSGID,
	T_FILL_THIS_FORM_SHORT autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,29fe9a340e0d180d
X-Google-Attributes: gid103376,public
From: mheaney@ni.net (Matthew Heaney)
Subject: Re: Depending on passing mechanism
Date: 1997/10/18
Message-ID: <mheaney-ya023680001810971451240001@news.ni.net>#1/1
X-Deja-AN: 282348478
References: <x2hgal3jxa.fsf@moon.inf.fu-berlin.de> <dewar.876842858@merv>
 <hbaker-1410971205270001@10.0.2.1> <622b4t$nhe$1@gonzo.sun3.iaf.nl>
 <hbaker-1510970700110001@10.0.2.1>
 <Pine.BSF.3.96.971016131014.2530A-100000@shell5.ba.best.com>
 <hbaker-1710970933080001@10.0.2.1>
 <Pine.BSF.3.96.971018110638.11035B-100000@shell5.ba.best.com>
Organization: Estormza Software
Newsgroups: comp.lang.ada
Date: 1997-10-18T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <Pine.BSF.3.96.971018110638.11035B-100000@shell5.ba.best.com>,
Brian Rogoff <bpr@shell5.ba.best.com> wrote:

Brian, to Henry:

>You post elsewhere that Ada's flaws are well known everywhere but inside 
>the Ada community. I think the opposite is true, lots of "flaws" in Ada, 
>including those mentioned in your papers, are discussed on c.l.a., but the 
>knowledge of Ada elsewhere is simplistic (ADA is kitchen sink language 
>designed by DOD committee) and I've rarely seen useful criticisms from 
>"outside", except for those of Ada-83 tasking. I still haven't found a 
>widely useful language without serious flaws. The main competitor for Ada 
>in its class is C++, and I prefer Ada. 

Matt, to Brain and Henry:

OK, I think we can all agree that aliasing can break a limited abstraction. 
But in Ada 95 the programmer can implement the full view of a limited
private type as a by-reference type, either by 

1. Implementing the type as a record marked as limited, or

2. Privately implementing the type as tagged.


In code, something like

private

   type LT is
      limited record
         O : <your favorite copy-in, copy-out type here>
      end record;

end;


or 


private
 
   type LT is
      tagged record
         O : <your fave scalar type here>
      end record;

end;


We could even combine these ideas:

private

   type LT is 
      tagged limited record
         O : T;
      end record;

end;


My question is, Why is this considered a "serious flaw" in Ada?  Don't the
implementation techniques listed above solve the aliasing problem?

Perhaps the area of contention is the fact that the language doesn't
require a limited private type to be implemented using a by-reference type
(ie Baker's limited bank account is implemented as a scalar, fixed point
type, which must be passed copy-in, copy-out).  Is that the case?  Can we
solve that issue, by requiring compiler warnings, or a suitable invokation
of pragma Restrictions?

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271