Re: OT: A bit of Sudoku

[Shark8 <OneWingedShark@gmail.com>]

On 05-Jun-14 13:00, J-P. Rosen wrote:
> I know that not everybody likes this idea, but to me exceptions are a
> powerful programming structure, not limited to handling errors.

Indeed, you can [ab]use them to get out of otherwise sticky situations, 
like the recursion problem when defining your "=" function:

   -- Nullable access-string type.
   Type LString is Access String;

   -- Definition of "=" returns true when both sides are NULL,
   -- returns false when only one side is NULL, and behaves as
   -- normal string-equality when neither side is NULL.
   Function "=" (Left, Right: IN LString) Return Boolean is

      Function Is_Equal(Left : LString; Right : String) Return Boolean is
      begin
         Return Right = Left.All;
      exception
         When CONSTRAINT_ERROR => Return False;
      end Is_Equal;

   Begin
      Return Is_Equal(Left, Right.All);
   Exception
      When CONSTRAINT_ERROR =>
         begin
            -- If this raises CONSTRAINT_ERROR, then both were NULL.
            Return Is_Equal(Right,Left.All);
         Exception
            When CONSTRAINT_ERROR => Return True;
         end;
   End "=";