Experiment: Implementing Multiple Dispatch in D (need vararg call)

Bruno Medeiros brunodomedeirosATgmail at SPAM.com
Wed Aug 2 14:33:10 PDT 2006

Bruno Medeiros wrote:
> I watched some time ago a Google TechTalk about Common Lisp (
> http://video.google.com/videoplay?docid=448441135356213813&q=peter+seibel
> ) where, as expected from a Lispanatic, LISP is idolized and Java is 
> whined about.
> The second part of the talk, talks about the CLOS (Common Lisp Object
> System) multiple dispatch system (called multimethods on Lisp lingo),
> and how it can be useful, etc.., and how in Java it all had to be
> implemented manually and non-generically (i.e., for each particular usage).

Well, I think it is complete, although not fully working due to bug #274.

I've attached the code if anyone is interested. It is two files, the MDF 
dispacher MultiDisFunction.d, some user/test code MDF_usercode.d, and 
TomS's meta library (thx!). Basically the idea is this: suppose you have 
two types of objects, Circle and Rect, and you want to check for a 
collision between two objects of any of those types. The exact algorithm 
will vary depending on the combination of the objects involved (checking 
for collision between <Circle,Circle> is different from <Circle,Rect>), 
so we create a multiple dispatch function that will redirect to each 
algorithm depending on each parameter. (See the user code) This only 
works for class parameters of course.
The dispatcher works by maintaining a map of entries, where the value is 
a function pointer (a dispatch), and the key is an array of ClassInfo's, 
which are the parameters of that dispatch. A dispatch is only selected 
if the parameter's types match exactly, but this could be a more 
complicated algorithm, checking superclass relations, etc., but that was 
beyond my interest.

Bruno Medeiros - MSc in CS/E student
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