From: mlucius@... (Mike Lucius)
Date: 2002-09-13T14:22:22+09:00
Subject: Re: Multimethods (was Re: Larry Wall's comments on Ruby)

Paul Brannan <pbrannan@atdesk.com> wrote in message news:<20020912103044.P25425@atdesk.com>...
> On Thu, Sep 12, 2002 at 02:59:22PM +0900, Mike Lucius wrote:
> > Multimethod dispatch (which is like C++ function overloading, but not
> > quite)is when there are two(or more) receivers.  It is exactly like
> > C++ function overloading except the method would presumably have
> > access to the internal state of both(all) instances dispatched upon.
> 
> With C++ function overloading the function to call is picked at
> compile-time.  Wouldn't the multimethod to call be picked at run-time
> (like virtual functions)?

Yes, you're right. OO Polymorphism is a limited way of doing safe
dynamic typing.

> 
> > i.e. it has the status of "method" for both classes.  C++ doesn't
> > support this idea directly, but Alexandrescu uses Template Programming
> > to make a fair approximation (using double-dispatch IIRC).
> >
> > Function overloading does the same thing, but functions are not

> > methods and this limits their usefulness.
> 
> What is the difference between a virtual member function and a method?

In C++ a plain old 'function' does not belong to any class.
A 'member function' is the C++ way of saying a method(i.e. it belongs
to a class).
In ruby all 'functions' belong to some class although this is not
apparent to the
casual observer.
In C++ a 'virtual member function' can be overridden by a derived
class.
In Ruby (and other languages) all methods can be overridden so there
is no
such distinction made.

>
> > 
> > In C++ generic functions are implemented by the compiler generating
> > overloaded functions as required.  IMO it is very ugly
> 
> These overloaded functions are generated by templates.  Having a
> different function generated at compile-time for every combination of
> parameters may be ugly, but does offer speed.  You can get a similar
> effect as Ruby if everything inherits from a common base class and you
> add a little bit of extra type information to your classes (XTTI
> perhaps?).

Sorry, I meant the syntax is ugly.  You're right the resulting code is
fast. However, I can hardly read the stuff and I have no trouble
understanding Perl :)
I think extending C++ is useless, its overextended now. I've been
looking for
alternatives to C++, by which I mean a type-safe compiled(fast)
language that
has al or most of the goodies I like about Ruby.  The best I've found
so far
is OCaml ( http://www.ocaml.org ), the biggest drawback to OCaml is
that it is not very popular.

Here is another place to read about this stuff: 
http://c2.com/cgi/wiki?GenericFunction

Mike Lucius