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From: nospam@thanks.com.au (Don Harrison)
Subject: Re: Interface/Implementation (was Re: Design by Contract)
Date: 1997/09/02
Message-ID: <EFv7yL.yr@syd.csa.com.au>#1/1
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Sender: news@syd.csa.com.au
References: <EFqDAG.2zn@ecf.toronto.edu>
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Organization: CSC Australia, Sydney
Newsgroups: comp.object,comp.software-eng,comp.lang.ada,comp.lang.eiffel
Date: 1997-09-02T00:00:00+00:00
List-Id: <comp.lang.ada>


Patrick Doyle wrote:

:In article <JSA.97Aug29191413@alexandria.organon.com>,
:Jon S Anthony <jsa@alexandria.organon.com> wrote:
:>In article <EForsv.Fqo@ecf.toronto.edu> doylep@ecf.toronto.edu (Patrick Doyle) writes:
:>
:>> >More to the point: not _relevant_ in the Ada case as this is handled
:>> >by a different orthogonal capability 

Agree. Multiple, polymorphic implementations are provided by deferred classes
and abstract types in Eiffel and Ada respectively. As Jon says, this is an 
orthogonal concept to interfaces.

:>> >(instead of conflating it with module as in Eiffel).

This is a different issue.


:>> using a deferred class is more powerful than a simple interface
:>> file, and can accomplish all the same things, so why split
:>> this into two concepts?

Because they serve different purposes. (See my previous post).

:  The Deja News suggestion is a good idea.  I'll take a look there
:when I get a moment.

You could, but you may get more confused. :(

Try thinking along the lines of

  a) Ada interfaces correspond to Eiffel short forms, and
  b) Ada abstract types correspond to Eiffel deferred classes.

In terms of b), the Ada (corrections welcome):

package P is
  type T is abstract tagged private;
  procedure Do_Something (X: T) is abstract;      -- deferred/abstract
private
  type T is tagged record 
    Y: Integer;
  end record;
end;


is basically equivalent to the Eiffel:

deferred class T
  feature
    do_something is deferred end                  -- deferred/abstract
  feature {NONE}
    y: INTEGER
end.


Deriving effective (concrete) descendants from these, we might have the Ada: 

with P;
package Q is
  type New_T is new P.T;
  procedure Do_Something (X: T);
end;

package body Q is
  procedure Do_Something (X: T) is begin .. end;      -- effective
end;


and the corresponding Eiffel:

deferred class NEW_T
  inherit T
  feature
    do_something is do .. end                         -- effective
end.


:>>   And, as far as the class-as-module approach in Eiffel goes, I
:>> once thought this was a bad idea too, but I'm not so sure any
:>> more.  Do you have some arguments against it?
:>
:>Tons.  See DejaNews...  

Whether or not they convinced anyone is another matter, however.


Don.                     (Reverse to reply)
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Don Harrison             au.com.csa.syd@donh