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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Depth First Search of a Char_Matrix?
Date: Sat, 27 Apr 2013 20:31:49 +0100
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Date: 2013-04-27T20:31:49+01:00
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Shark8 <onewingedshark@gmail.com> writes:

> On Saturday, April 27, 2013 11:25:57 AM UTC-6, Jeffrey Carter wrote:
>> 
>> There is absolutely no reason to use access types (especially not
>> anonymous access types) or addresses for this problem.
>
> Sure, but there's no real reason not to: the illustration was simply a
> DFS on something that might-not-exist and so null/some-object map to
> that perfectly fine. (It's an error to ask "what color is the piece at
> X,Y?" when the element X,Y contains no piece; this situation must be
> addressed somewhere, and access/null works.)

As a general rule of thumb it is a bad idea to use access types unless
you Absolutely Have To.

To solve this problem, a discriminated record would have done just fine:

   type Cell (Occupied : Boolean := False) is record
      case Occupied is
         when True =>
            Piece : Piece_Colour;
            Alive : Boolean;
         when False =>
            null;
      end case;
   end record;

And anonymous access types are worse, see the recent problems with
anonymous task access types.