Re: Newbie question

[Robert A Duff <bobduff@shell01.TheWorld.com>]

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