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,e4042699db4db63b
X-Google-Attributes: gid103376,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news1.google.com!newsfeed2.dallas1.level3.net!news.level3.com!newsfeed-00.mathworks.com!nntp.TheWorld.com!not-for-mail
From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Newbie question
Date: Thu, 12 Mar 2009 11:07:00 -0400
Organization: The World Public Access UNIX, Brookline, MA
Message-ID: <wcc1vt2vkyz.fsf@shell01.TheWorld.com>
References: <49b90e2d$0$2847$ba620e4c@news.skynet.be>
 <0ff2f139-06cf-4fa5-8c0d-f97c43a33186@s20g2000yqh.googlegroups.com>
NNTP-Posting-Host: shell01.theworld.com
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Trace: pcls4.std.com 1236870421 26032 192.74.137.71 (12 Mar 2009 15:07:01
 GMT)
X-Complaints-To: abuse@TheWorld.com
NNTP-Posting-Date: Thu, 12 Mar 2009 15:07:01 +0000 (UTC)
User-Agent: Gnus/5.1008 (Gnus v5.10.8) Emacs/21.3 (irix)
Cancel-Lock: sha1:f/7r4btTQunnCtV/Csiaz73dgUE=
Xref: g2news1.google.com comp.lang.ada:4078
Date: 2009-03-12T11:07:00-04:00
List-Id: <comp.lang.ada>

Ludovic Brenta <ludovic@ludovic-brenta.org> writes:

> Solution 3: introduce a new value for Piece_Type_T:
>
> type Piece_Type_T is (None, Pawn, Knight, Bishop, Rook, Queen, King);
>
> I don't know which is the best solution; perhaps you'd like to try
> them all and see how well they integrate with the algorithms.  I think
> Solution 3 is the most error-prone, though.

A variation on 3 is not error prone:

    type Optional_Piece_Kind is 
        (None, Pawn, Knight, Bishop, Rook, Queen, King);
    subtype Piece_Kind is Optional_Piece_Kind range
        Optional_Piece_Kind'Succ(None)..Optional_Piece_Kind'Last;
    
    type Square(Kind: Optional_Piece_Kind is := None) is
        record
            case Kind is
                when None => null;
                when Piece_Kind => Color: ...;
            end case;
        end record;

    type Rank is range 1..8;
    type File is new Character range 'A'..'H';
    -- Maybe I've got this backwards???
    type Board is array (Rank, File) of Square;

As somebody said, you will want a default value for the discriminant, in
all these variations.  Having a default has the odd property that it
allows you to change the discriminant by a whole-record assignment.
This oddity often confuses beginners to Ada.

- Bob