Re: Real OO

[ncohen@watson.ibm.com (Norman H. Cohen)]

When Don Harrison asked how to achieve selective export in Ada, I
proposed a technique, but I never asked why one would want selective
export.

Selective export does indeed help document the modular structure of a
program, and make the program easier for a reader to understand, but it
seems to me that the documentation is in the wrong place.  In general,
the writer of a program component should be as unaware as possible of the
context in which that component will be used, especially if loose
coupling and reusability are valued.

I don't find Bertrand Meyer's example of selective export in
_Object-Oriented_Software_Construction_ particularly compelling.  That
example would be written in Ada as follows: 

   generic
      type Element_Type is private;
   package Lists is
      type List_Type is private;
      ...
   private
      type Cell_Type;
      type Cell_Pointer_Type is access Cell_Type;
      type Cell_Type is
         record
            Data : Element_Type;
            Link : Cell_Pointer_Type;
         end record;
      type List_Type is
         record
            Length : Natural;
            Cells  : Cell_Pointer_Type;
         end record;
   end Lists;

However, because of the Eiffel constraint that module = class = type,
Meyer cannot define List_Type in the same class as Cell_Type.  Therefore,
he puts his equivalent of Cell_Type in a separate class which is
selectively exported only to the List_Type class.  (There is no need for
an explicit Cell_Pointer_Type in Eiffel because of Eiffel's implicit
reference semantics; the Link component belongs to Cell_Type.)

Perhaps Don will decry this as "quasi-encapsulation", but the whole point
of the selective export in Eiffel was to emphasize that the Eiffel class
corresponding to Cell_Pointer_Type was to be used only in the module
defining the Eiffel analog of List_Type.  Nesting multiple type
definitions (suitably hidden) inside a single Ada package accomplishes
the same thing.

Because of the ability to define more than one type in the same module in
Ada, selective export is of less interest to an Ada programmer than to an
Eiffel programmer.

--
Norman H. Cohen    ncohen@watson.ibm.com