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Date: 7 Apr 93 13:49:59 GMT
From: pipex!uknet!warwick!zaphod.crihan.fr!univ-lyon1.fr!scsing.switch.ch!sicsu
n!disuns2.epfl.ch!lglsun!nebbe@uunet.uu.net  (Robb Nebbe)
Subject: Re: Classes vs Tagged Types - Terminology
Message-ID: <1993Apr7.153756@lglsun.epfl.ch>
List-Id: <comp.lang.ada>

In article <1993Apr6.164642.9487@cenatls.cena.dgac.fr>, bruniau@cenatls.cena.dg
ac.fr (Christophe Bruniau) writes:
: 
: In article <1993Apr5.155419.8598@evb.com>, jgg@evb.com (John Goodsen) writes:
: |> In <1993Mar25.155650.16244@inmet.camb.inmet.com> 
: |> stt@spock.camb.inmet.com (Tucker Taft) writes:
: |> 
: |> >
: |> >In Ada 9X, it is only class-wide types (named "T'Class") that provide
: |> >subclass matching.
: |> >                      ^^^^^^^^^^^^^^^^^
: |>                        Didn't you mean "tagged types" :-)
: |> 
: |> Can we take this as an indicator that you are about ready to
: |> abondon the "tagged type" terminology and adopt the more
: |> appropriate "class type" terminology?  It seems that even
: |> members of the 9X project are shying away from using "tagged
: |> type" these days, so why don't just finish the scenario and
: |> adopt the "class type" syntax and terminology ...
: |> 
: |> fingers crossed...
: |> 
: 
: Does anyone know the original reason why "tagged type" was introduced,
:  instead of something like "class type" ?
: 
: Christophe BRUNIAU     bruniau@cenatls.cena.dgac.fr

Yes. I posted the reasons I had found about two? weeks ago. It basically
comes down to an implementation issue.

It is possible to come up with a language that does not use tagged
and is not ambiguous. The problem with this approach is that it would
be far too difficult to implement.

Since tagged is there for implementation reasons (representation clauses
and declaring tagged types below the library level are the stumbling 
blocks) I don't think you could come up with a better term than tagged.

However it is for this very reason that you will get nowhere it you try
and push tagged type as being a feature. The OO features being added are
class-wide types (T'Class where T is a tagged type) and type extension
(between tagged types) as well as hierachical library units.

Robb