From: Eric Schulte <schulte.eric@...>
Date: 2008-10-16T10:58:00+09:00
Subject: Re: Sample for Metaprogramming

Chad Perrin <perrin@apotheon.com> writes:

> On Thu, Oct 16, 2008 at 04:53:04AM +0900, Sebastian Hungerecker wrote:
>> Chad Perrin wrote:
>> > I'm aware of the existence of CLOS, but don't really know anything about
>> > its specifics of implementation and use.  How does it handle the kind of
>> > functionality that, in Ruby, consists of sending messages to objects to
>> > invoke their methods?
>> 
>> It doesn't, really. CLOS methods are overloadable functions basically, i.e. 
>> you can define multiple methods of the same name, but with different argument 
>> types, and it'll pick the right one to call depending on the type(s) of the 
>> object(s) you pass as arguments. There's no concept of a receiver though.
>
> So, basically, instead of real methods, CLOS just uses . . . what?
> Parametric polymorphism?  This isn't meant as a challenge or complaint,
> but rather just a question so I'll understand it better:
>

The following begins with a good overview of the rational behind the use
of generic functions in CLOS instead of using a message passing system
as used in ruby.

http://www.dreamsongs.com/Files/ECOOP.pdf

linked to from

http://www.dreamsongs.com/CLOS.html

It makes the good point that generic functions are a natural solution in
any lisp.  The paper states...

|      Furthermore, in Common Lisp, arithmetic operators are already
| generic in a certain sense. The expression
| 
|      (+ x y)
| 
| does not imply that x and y are of any particular type, nor does it
| imply that they are of the same type. For example, x might be an
| integer and y a complex number, and the + operation is required to
| perform the correct coercions and produce an appropriate result.
| 
|      Because the Object System is a system for Common Lisp, it is
| important that generic functions be first-class objects and that the
| concept of generic functions be an extension and a generalization of
| the concept of Common Lisp functions. In this way, the Object System
| is a natural and smooth extension of Common Lisp.

Also, CLOS contains mechanisms for implementing a message-passing based
object system.  Again from the paper...

|  Despite the advantages of generic functions, there may at times be
|  reasons for preferring a message-passing style; such a style can be
|  implemented by means of generic functions or by means of the Common
|  Lisp Object System Meta-Object Protocol.

I would very much like to see any implementation of a ruby-like message
based object system using CLOS.

>
> What about CLOS makes it "object oriented"?
>

The fact that it provides for the creation of objects which are
instances of classes, and provides for inheritance between classes. :)

-- Eric