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,FREEMAIL_FROM
	autolearn=ham autolearn_force=no version=3.4.4
X-Google-Thread: 103376,c406e0c4a6eb74ed
X-Google-Attributes: gid103376,public
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news2.google.com!proxad.net!news.tele.dk!news.tele.dk!small.news.tele.dk!not-for-mail
Sender: malo@0x53586c34.boanxx18.adsl-dhcp.tele.dk
Newsgroups: comp.lang.ada
Subject: Re: Formal and informal type systems?
References: <49dc98cf.0408110556.18ae7df@posting.google.com>
 <m3wtz7e4hd.fsf@rigel.goldenthreadtech.com>
 <413e2fbd$0$30586$626a14ce@news.free.fr> <wvbrsm9tjg7e.fsf@sun.com>
 <m3n000bv64.fsf@rigel.goldenthreadtech.com>
 <DjH%c.10540$w%6.8640@newsread1.news.pas.earthlink.net>
 <m3656obc1w.fsf@rigel.goldenthreadtech.com>
 <plbrfqoln6.fsf_-_@sparre.crs4.it>
 <m3mzza7ohc.fsf@0x53586c1f.boanxx18.adsl-dhcp.tele.dk>
 <mailman.121.1096365964.390.comp.lang.ada@ada-france.org>
 <cjbgcu$fh0$1@a1-hrz.uni-duisburg.de>
 <m3r7om8bsl.fsf@0x53586c34.boanxx18.adsl-dhcp.tele.dk>
 <mailman.126.1096415274.390.comp.lang.ada@ada-france.org>
From: Mark Lorenzen <mark.lorenzen@ofir.dk>
Date: 29 Sep 2004 11:11:40 +0200
Message-ID: <m3acv9e8xf.fsf@0x53586c34.boanxx18.adsl-dhcp.tele.dk>
User-Agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Organization: TDC Totalloesninger
NNTP-Posting-Host: 83.88.108.52
X-Trace: 1096449101 dtext02.news.tele.dk 170 83.88.108.52:38860
X-Complaints-To: abuse@post.tele.dk
Xref: g2news1.google.com comp.lang.ada:4366
Date: 2004-09-29T11:11:40+02:00
List-Id: <comp.lang.ada>

Stephen Leake <stephen_leake@acm.org> writes:

> Mark Lorenzen <mark.lorenzen@ofir.dk> writes:
> 

[some examples removed]

> > 
> > The pattern matching could also be extended to case statements such as:
> > 
> >    procedure Do_Something (I : Optional_Integer) is
> >    begin
> >       case I is
> > 	 when Optional_Integer'(Defined => False)       => null;
> > 	 when Optional_Integer'(Defined => True, Value) =>
> > 	    Do_Something_More(Value);
> >       end case;
> >    end Do_Something;
> 
> I'm sorry, but I have no idea what semantics you intend this new
> syntax to represent. I don't know ML, so I can't guess. Please treat
> this as a chance to entice me to learn more about ML.
> 
> How, exactly, is your Do_Something different from:
> 
> procedure Do_This (I : Integer) is
> begin
>    case I is
>     when False       => null;
>     when True =>
>        Do_Something_More(Value);
>    end case;
> end Do_Something;

I assume that you meant:

    procedure Do_This (I : Optional_Integer) is
    begin
       case I.Defined is
        when False       => null;
        when True =>
           Do_Something_More(Value);
       end case;
    end Do_Something;

It not different, but instead of referring to the discriminant, my
example matches the possible ways that I can be constructed. I another
answer to your article, Georg Bauhaus showed the definition of a
function called 'even' that matched it's argument against 3 possible
pattaerns.

> 
> In particular, where is "Value" coming from? It appears to be a global
> variable.

No it is bound in the match

	 when Optional_Integer'(Defined => True, Value) => ...

Here Value is bound to I.Value. Maybe this was a bad name to use in
the pattern, so we could also write:

	 when Optional_Integer'(True, V) => ...

Which would match a value of type Optional_Integer, where the value of
I.Defined is true and V is assigned the value of I.Value. V should
probably be delared before it can be used in a pattern.

> 
> You mention "pattern matching", but I have no clue what dataspace you
> are matching against.
> 
> -- 
> -- Stephe