From: Dan Sugalski <dan@...>
Date: 2002-09-11T23:56:39+09:00
Subject: Multimethods (was Re: Larry Wall's comments on Ruby)

At 10:48 PM +0900 9/11/02, Gavin Sinclair wrote:
>Despite the discussion and links, I haven't picked up on what multimethods
>actually are, so I can't comment on those.

Multimethod dispatch is reasonably simple. It's when you have 
multiple methods with the same name, which are only differentiated by 
their method signature. At runtime, the types of the arguments are 
checked and you dispatch to the matching method. If no method 
matches, or more than one method matches equally well, an exception 
is thrown.

This is a useful thing to do, as it means you can avoid casting or 
converting. For example, if you have:

    a = b + c

b and c are both integers, you want to return an integer. If one is 
an integer and the other a float, you want to return a float but do 
integer math (as its faster to add an int and a float than it is to 
add two floats), and if they're both floats you want to do a float 
add. (And if they're extended precision numbers you might want to go 
to even more lengths, as it's usually much cheaper to add a platform 
int to an extended precision number than coerce an int to an extended 
precision number and do the add)

That's pretty much multimethods. They're terribly useful, as they cut 
out a lot of checks for the second type in methods. (Checks which 
tend to be slow, error prone, and missing stuff)
-- 
                                         Dan

--------------------------------------"it's like this"-------------------
Dan Sugalski                          even samurai
dan@sidhe.org                         have teddy bears and even
                                       teddy bears get drunk