defmulti ?

#!/usr/bin/env ruby

class GenericFunction
  def initialize
    @dispatchers = {} 
  end
  def add_dispatcher match, action
    @dispatchers[match] = action
  end
  def dispatch *args
    @dispatchers.each do |match,action|
      action.call *args if match.call *args
    end
  end
end

gf = GenericFunction.new

gf.add_dispatcher(
  Proc.new {|a,b| a.respond_to? :quack and b.respond_to? :listen },
  Proc.new {|a,b| b.listen a.quack }
)


class Animal; def listen message; puts "I heard #{message}"; end; end
class Duck < Animal; def quack; "quack"; end; end

gf.dispatch Duck.new, Animal.new