From: Dan Sugalski <dan@...>
Date: 2002-09-12T21:14:46+09:00
Subject: Re: Multimethods (was Re: Larry Wall's comments on Ruby)

At 6:59 PM +0900 9/12/02, Friedrich Dominicus wrote:
>Dan Sugalski <dan@sidhe.org> writes:
>
>>  At 12:57 AM +0900 9/12/02, Christoph wrote:
>>  >"Dan Sugalski"  wrote in
>>  >....
>>  >>  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)
>>  >
>>  >Thanks Dan, for making such an eloquent case for multimethods.
>>  >Personal I cannot image (surely an effect of being single and
>>  >simple minded;-) how their use could be less complex then the
>>  >alternatives like Rubys coerce framework  or double-dispatch ..
>>
>>  Their big problem is that they introduce methods that don't live in
>>  any class. If you have the methods:
>>
>>
>>      add(int, num)
>>      add(num, int)
>>      add(bignum, int)
>>
>>  those three, while methods, don't live in any class, so if you did:
>>
>>      foo.add(bar)
>sorry any from the above has nothing to do with this one.
>you could have
>a foo.add(int,num)
>  foo.add(num,int)
>..
>and yo can have a foo.add(bar).

Sorry, I wasn't being clear--foo would have been an int, a num, or a 
bigint. The first parameter's generally taken to be the type of the 
object the method ultimately gets invoked in. Since the various 
incarnations of a multimethod live outside any class, you can't count 
on the type of the class as implicit in the signature.

Also, since multimethods live outside any class, you can have a class 
that provides a version of a method where the left hand argument is 
not the class where the method happens to be defined. For example, if 
you had a method like:

     blah(int, some_blah_thing);
     blah(string, some_blah_thing);
     blah(file, some_blah_thing);

they'd be defined (probably) in the some_blah_thing class, since 
that's the class that has knowledge of them, but they'd still work if 
you did:

     1.blah(some_blah_object)

(You'll pardon the syntax--take it as pseudocode, as I don't really do syntax)
-- 
                                         Dan

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