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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Newbie question
Date: Thu, 12 Mar 2009 07:31:07 -0700 (PDT)
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Date: 2009-03-12T07:31:07-07:00
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On Mar 12, 3:16=A0pm, Olivier Scalbert <olivier.scalb...@algosyn.com>
wrote:
> Ludovic Brenta wrote:
> > Solution 1: a variant record:
>
> > type Square_T (Empty : Boolean) is record
> > =A0 =A0case Empty is
> > =A0 =A0 =A0 when True =3D> null;
> > =A0 =A0 =A0 when False =3D> Piece : Piece_T;
> > =A0 =A0end case;
> > end record;
>
> > type Board_T is array (1 .. 8, 1 .. 8) of Square_T;
>
> > Solution 2: merge the Boolean directly into Piece_T:
>
> > type Piece_T (Present : Boolean) is record
> > =A0 =A0case Present is
> > =A0 =A0 =A0 when True =3D>
> > =A0 =A0 =A0 =A0 =A0Color : Color_T;
> > =A0 =A0 =A0 =A0 =A0Piece_Type : Piece_Type_T;
> > =A0 =A0 =A0 when False =3D>
> > =A0 =A0 =A0 =A0 =A0null;
> > =A0 =A0end case;
> > end record;
>
> > 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. =A0I thin=
k
> > Solution 3 is the most error-prone, though.
>
> 3 solutions ! Thanks Ludovic.
>
> I like the first two solutions. With solution 3, you can have an empty
> square (None) with a color.

Come to think of it, you'll want to add default values to the
discriminants in solutions 1 and 2:

type Square_T (Empty : Boolean :=3D True) is record ...  end record;

or

type Piece_T (Present : Boolean :=3D False) is record ... end record;

This makes it possible to change the value of the discriminant square
by assigning an aggregate, e.g.

Board (5, 5) :=3D -- Solution 1
  Square_T'(Empty =3D> False, Piece =3D> (Color =3D> Black, Piece_Type =3D>
Pawn));
Board (5, 5) :=3D -- Solution 2
  Piece_T'(Present =3D> True, Color =3D> Black, Piece_Type =3D> Pawn);

I think I like solution 2 better, but then I'd rename Piece_T into
Square_T.

--
Ludovic Brenta.