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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: Newbie question
Date: Thu, 12 Mar 2009 07:36:22 -0700 (PDT)
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Date: 2009-03-12T07:36:22-07:00
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On Mar 12, 3:32=A0pm, stefan-lu...@see-the.signature wrote:
> On Thu, 12 Mar 2009, Olivier Scalbert wrote:
> > Ludovic Brenta wrote:
>
> > > 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 2a: Use an enumeration:
>
> type Color_T is (Black, White, None);
>
> type Piece_T(Color: Color_T) is
> =A0 record
> =A0 =A0 case Color is
> =A0 =A0 =A0 when White =3D>
> =A0 =A0 =A0 =A0 White_Piece: Piece_Type_T;
> =A0 =A0 =A0 when Black =3D>
> =A0 =A0 =A0 =A0 Black_Piece: Piece_Type_T;
> =A0 =A0 =A0 when None =3D>
> =A0 =A0 =A0 =A0 null;
> =A0 =A0 end case;
> =A0 end record;

The problem with this is that you'd then need two code paths for
Black_Piece and White_Piece, every time. To overcome this, here is
solution 2a1:

type Square_T (Color : Color_T :=3D None) is record
   case Color is
      when None =3D> null;
      when Black | White =3D> Piece_Type : Piece_Type_T;
   end case;
end record;

--
Ludovic Brenta.