Re: Self-referential types

["Robert I. Eachus" <eachus@mitre.org>]

Matthew Heaney wrote:
  
> For a limited type T, then inside the declaration of the type, the
> expression T'Access refers to the "current instance" of the type.
> 
> This is something a lot of programmers haven't learned yet.
> 
> This is the basis for programming with access discriminants, which is
> how you do MI in Ada95.

   I have to butt in here.  It may be how you do MI in Ada, and it is
one way, but I find it ugly.  If you treat a private access type as the
object, then you have to do a bit more work inside the package, but the
exterior looks much cleaner, and the users don't have to use 'Access at
all.  (Often you don't need it inside the package either.)

   The linked list example becomes:

   generic
     type Element is private; --or limited private
   package Lists is
      type List is private;
      procedure Append(L: in out List; E: in Element);
      procedure Prepend(L: in out List; E: in Element);
      procedure Pop(L: in out List; E: out Element);
      function Is_Empty(L: List) return Boolean;
      function First(L: List) return Element;
      function Last(L: List) return Element;
      -- etc
   private
      type Real_List_Element;
      type List is access all List;
      type Real_List_Element is record
        E: Element;
        -- if Element is limited private, you need to declare another
access
        -- type and use it here.
        Next, Previous: List;
      end record;
   end Lists;
       
-- 

                                        Robert I. Eachus

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