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From: mario@cs.man.ac.uk (Mario Wolczko)
Newsgroups: comp.object,comp.lang.eiffel
Subject: Re: Unification: Class=Type=Module
Message-ID: <2462@m1.cs.man.ac.uk>
Date: 27 Apr 91 14:04:47 GMT
References: <1991Apr23.142800.12215@bony1.bony.com>
Sender: news@cs.man.ac.uk
Reply-To: mario@cs.man.ac.uk (Mario Wolczko)
Followup-To: comp.object
Organization: Department of Computer Science, University of Manchester
Lines: 37

In article <1991Apr23.142800.12215@bony1.bony.com>, richieb@bony1.bony.com (Richard Bielak) writes:
> 
> One of the things I liked about "Object Oriented Software Construction"
> by B. Meyer, was the idea that a class is both a type and a module.
> This idea clears up the confusion as to whether inheritance extends or
> specializes.
...
> I also like the "cleanness" and simplicity of this idea. 

I also like this idea, but unfortunately no existing object-oriented
language (to the best of my knowledge) supports it properly.  Eiffel,
Smalltalk and CLOS all go only half way; C++ is a little better, but
still short of the mark.

For a class to be a module it should have a clear separation between
interface and implementation, such that any compatible
reimplementation can be substituted without affecting "users" of that
class.  That's what module means: you can swap compatible modules with
impunity. 

When the "user" is a client class, most if not all OO languages
succeed in hiding implementation, but when the "user" is a subclass,
they fail miserably.  For example, all of Smalltalk, Eiffel and CLOS
allow direct access to all inherited instance variables, so that no
part of the internal representation can be hidden.  In contrast, C++
lets you declare an instance variable (member) to be private.

In none of these languages can you isolate a class from the choice of
superclasses made by its parent(s).  

Mario Wolczko

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