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From: donh@syd.csa.com.au (Don Harrison)
Subject: Re: Eiffel and Java
Date: 1996/11/15
Message-ID: <E0w1An.F0v@syd.csa.com.au>#1/1
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sender: news@syd.csa.com.au
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references: <328A411C.388B@watson.ibm.com>
organization: CSC Australia, Sydney
reply-to: donh@syd.csa.com.au
newsgroups: comp.lang.eiffel,comp.lang.ada
Date: 1996-11-15T00:00:00+00:00
List-Id: <comp.lang.ada>


Norman H. Cohen writes:

:Ada DOES support separate interface and implementation inheritance:
:
:   package Interface is
:      type Interface_Type is abstract tagged ...;
:      ...
:   end Interface;
:
:   package Parent_Implementation is
:      type Parent_Implementation_Type is
:         new Interface.Interface_Type with ...;
:      ...
:   end Parent_Implementation;
:
:   package Child_Implementation is
:      type Child_Implementation_Type is 
:         new Interface.Interface_Type with private;
:      ...
:   private
:      type Child_Implementation_Type is 
:         new Parent_Implementation.Parent_Implementation_Type
:            with ...;
:   end Child_Implementation;
:
:The declaration of Child_Implementation_Type in the visible part of the
:package declares this type to be descended from Interface_Type, and this
:is all that a client of the Child_Implementation package knows about the
:type.  The full declaration of Child_Implementation_Type in the private
:part indicates that Child_Implementation_Type implements the
:Interface_Type interface by inheriting from a particular descendant of
:Interface_Type, namely Parent_Implementation_Type.

My error. I'm just going to simplify your example and put in some detail to 
see how you get hold of a 'covariant' operation. (Hope you don't mind). 

Assuming D_Type inherits from C_Type (and omitting various syntactic details),

   package A_Module is
      type A_Type is tagged ...;
      procedure op (a: A_Type; c: C_Type'Class);
      ...
   end A_Module;

   package B_Module is
      type B_Type is new A_Module.A_Type with private;
      procedure op (b: B_Type; c: C_Type'Class);   -- Overridden op
      procedure op (b: B_Type; d: D_Type'Class);   -- New, 'covariant' op
      ...
   private
      type B_Type is new A_Module.A_Type with ...;
   end B_Module;

   package body B_Module is
      procedure op (b: B_Type; c: C_Type'Class) is
      begin
        op (b, D_Type(c) );   -- No dispatching; Is this valid?
      exception
        when others => raise; -- chuck a wobbly!
      end;

      procedure op (b: B_Type; d: D_Type'Class) is ...
      ...
   end B_Module;

Is this how it works?


Don.
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Don Harrison             donh@syd.csa.com.au