Quick example of how to override an instance method in an existing Ruby library. We use this approach when producing Redmine plugins.

lion_patch.rb

#!/usr/bin/env ruby -rubygems

module Zoo
  module Animal
    class Lion
      def make_noise
        "rawr"
      end
    end
  end
end

lion = Zoo::Animal::Lion.new
puts 'Noise before patch: ' + lion.make_noise

# Patch the lion to make him purr
module LionPatch
  def self.included(base)
    base.send(:include, InstanceMethods)

    base.class_eval do
      alias_method :make_noise, :make_noise_with_feeling
    end    
  end 
  
  module InstanceMethods
    def make_noise_with_feeling
      "purrrrr"
    end    
  end
end

unless Zoo::Animal::Lion.included_modules.include? LionPatch
  Zoo::Animal::Lion.send(:include, LionPatch)
end

lion_after_patch = Zoo::Animal::Lion.new
puts 'Noise after patch: ' + lion_after_patch.make_noise

# Expected output:
#   Noise before patch: rawr
#   Noise after patch: purrrrr

@bluefuton can it be done with passing arguments to make_noise_with_feeling method? Tnx!