From: mlucius@... (Mike Lucius)
Date: 2002-09-12T14:59:22+09:00
Subject: Re: Multimethods (was Re: Larry Wall's comments on Ruby)

I think two concepts are getting confused here.  Multimethod dispatch
and
generic programming.

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.  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.

The executive summary for function overloading:
   For a particular function name different combinations of
   argument types are mapped to different implementations.

Generic programming is when an algorithm (function) is defined, but
types are
not specified (completely).  Types are bound at compile time (C++
Haskell OCAML) or, in the case of dynamic typing, types are checked by
the programmer.

Ruby does generic programming naturally because types aren't checked. 

One problem that crops up here is syntax may be confused with
semantics
(e.g. the + operator doesn't commute with strings). I've noticed that
Mix-ins
are used to guarantee the semantics of operators in some cases.

The executive summary for generic programming:
    For a particular function name different combinations of
    argument types are mapped to the same implementation.
   

In C++ generic functions are implemented by the compiler generating
overloaded functions as required.  IMO it is very ugly

Some functional languages, like Haskell, combine the two ideas by
allowing
functions to be implemented in serveral different ways, but the types
of the
arguments are specified by patterns so that many different type
"signatures"
get mapped to one implementation.

Mike Lucius