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=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,c1fe4bc1dd51fc87
X-Google-Attributes: gid103376,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news2.google.com!eweka.nl!lightspeed.eweka.nl!62.179.104.142.MISMATCH!amsnews11.chello.com!nuzba.szn.dk!news.jacob-sparre.dk!pnx.dk!not-for-mail
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: User-defined type attributes
Date: Fri, 14 Mar 2008 22:04:53 -0500
Organization: Jacob's private Usenet server
Message-ID: <frffdo$2jo$1@jacob-sparre.dk>
References: 
 <d9065e94-8586-4889-b5c7-a8ca9e8f2248@d21g2000prf.googlegroups.com>
  <p0joauyj6p7$.z2nuhn0t1c78.dlg@40tude.net>
 <30f629fd-69d3-4d40-995f-9933f52b8dad@s37g2000prg.googlegroups.com>
 <965suhjl0bxt$.74se2ylyksin.dlg@40tude.net> <frcppv$de8$1@jacob-sparre.dk>
 <pqup5kf1u34t$.19kwms4bw12hs.dlg@40tude.net>
NNTP-Posting-Host: static-69-95-181-76.mad.choiceone.net
X-Trace: jacob-sparre.dk 1205551352 2680 69.95.181.76 (15 Mar 2008 03:22:32
 GMT)
X-Complaints-To: news@jacob-sparre.dk
NNTP-Posting-Date: Sat, 15 Mar 2008 03:22:32 +0000 (UTC)
X-Priority: 3
X-MSMail-Priority: Normal
X-Newsreader: Microsoft Outlook Express 6.00.2800.1914
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1914
Xref: g2news1.google.com comp.lang.ada:20407
Date: 2008-03-14T22:04:53-05:00
List-Id: <comp.lang.ada>

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:pqup5kf1u34t$.19kwms4bw12hs.dlg@40tude.net...
> On Thu, 13 Mar 2008 20:46:41 -0500, Randy Brukardt wrote:
>
> > "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> > news:965suhjl0bxt$.74se2ylyksin.dlg@40tude.net...
> > ...
> >> Why? The only thing you need is formal generic ADTs. Ada's generics
lack
> >> ADTs, which is the reason why you need to pass operations together with
the
> >> formal types. There exist formal built-in types like "private", "<>",
> >> "digits <>" etc, but there is no user-defined ones. Add these and all
> >> primitive operations will go with the actual type just per magic. It
would
> >> be IMO a natural step for anybody who wanted generics to evolve in an
> >> Ada-way. (Not for me, as you know, I prefer generics removed from the
> >> language.)
> >
> > Generic formal derived types are the way to bring an ADT into a generic.
> > After all, they do bring in all of the primitive operations of the ADT.
> >
> > The only problem is that they can't be used on unrelated types, but
that's
> > not very relevant if you are using tagged types (and you might as well
being
> > doing so if you're defining any sort of composite type).
>
> Right, but this also implies that there is no need to have generics at
all.
> A class-wide object does all what needed:
>
>    generic
>       type S is new T with private;
>    procedure Foo (X : in out S);
>
> can be replaced by:
>
>    procedure Foo (X : in out T'Class);

Not if you need other generic parameters along with the type. And this is a
way to do mixins, which otherwise can't be done in Ada 95 because type can't
derive from a class-wide type:

   generic
      type S is new T with private;
   package Foo is
      type New_S is new S with private;
      procedure New_Operation (Op : in out New_S);
   end Foo;

It's a nice way to add persistence, for instance. (Interfaces only allow you
to add, umm, interfaces, not code.)

                     Randy.