gistfile1.rb

#!/usr/bin/env ruby

# http://clojure.org/runtime_polymorphism

=begin
(defmulti encounter (fn [x y] [(:Species x) (:Species y)]))
(defmethod encounter [:Bunny :Lion] [b l] :run-away)
(defmethod encounter [:Lion :Bunny] [l b] :eat)
(defmethod encounter [:Lion :Lion] [l1 l2] :fight)
(defmethod encounter [:Bunny :Bunny] [b1 b2] :mate)
(def b1 {:Species :Bunny :other :stuff})
(def b2 {:Species :Bunny :other :stuff})
(def l1 {:Species :Lion :other :stuff})
(def l2 {:Species :Lion :other :stuff})
(encounter b1 b2)
-> :mate
(encounter b1 l1)
-> :run-away
(encounter l1 b1)
-> :eat
(encounter l1 l2)
-> :fight
=end

class DefMulti
  def initialize &block
    @dispatcher = block
    @methods = []
  end
  def defmethod pattern, &block
    @methods << [pattern,block]
  end
  def dispatch *args
    _pattern = @dispatcher.call *args
    @methods.each do |pattern,method|
      return method.call *args if _pattern == pattern
    end
  end
end

encounter = DefMulti.new { |x,y| [x[:Species],y[:Species]] }
encounter.defmethod([:Bunny,:Lion]){|b,l| :run_away }
encounter.defmethod([:Lion,:Bunny]){|l,b| :eat }
encounter.defmethod([:Lion,:Lion]){|l1,l2| :fight }
encounter.defmethod([:Bunny,:Bunny]){|b1,b2| :mate }

b1={:Species => :Bunny, :other => :stuff}
b2={:Species => :Bunny, :other => :stuff}
l1={:Species => :Lion, :other => :stuff}
l2={:Species => :Lion, :other => :stuff}

puts encounter.dispatch b1, b2
puts encounter.dispatch b1, l1
puts encounter.dispatch l1, b1
puts encounter.dispatch l1, l2

=begin
# Output
mate
run_away
eat
fight
=end