From: Joel VanderWerf <vjoel@...>
Date: 2002-09-12T05:37:28+09:00
Subject: Re: Multimethods (was Re: Larry Wall's comments on Ruby)

Christoph wrote:
> "Justin Johnson" wrote
> ....
> 
>>I'd be interested to see if anyone could come up with some satisfactory
>>syntax with regards to implementing multi-dispatch in Ruby.
> 
> 
> Well the following is commonly used  (Matz does not like it
> if I remember correctly)
> 
>   <(a,b)>.atan
> 
> or  if you declare new key words like pair,multi,sym or alt
> you could write for example
> 
>    multi(a.b).atan(), pair(a,b).atan  or   multi(a,b,c).mth(d,e,*f)
> 
> If you prefer a more conventional look how about
> 
>   a.atan(b;)  or    a.mth(b,c ; d , e, *f)  ?

Without changing Ruby internals, what about something like:


module Physics

   Collide = GenericFunction.new {|actor, object, *args|}

   #
   # The generic function will dispatch on actor and object, but
   # simply pass args to the selected method.
   #
   # The distinction in the block can be detected by checking whether #arity
   # returns 3 or -3.
   #

end

class Ship < Actor

   # tell Physics how collisions involving ships work

   Physics::Collide.add_method Ship, Object do |ship, object, *args|
     ...
   end

   # a more specific method:

   Physics::Collide.add_method Ship, Ship do |ship, other_ship, *args|
     ...
   end

end

class Rock

   Physics::Collide.add_method Actor, Rock {|actor, rock, *args|
     ...
   }

end