require 'erb'

def madlib(str, &block)
  madlibber = Class.new do

    attr_reader :vars, :story

    def initialize(story)
      @vars  = {}
      @story = story
    end

    %w{name animal verb adjective thing name2 name3}.each do |var|
      class_eval <<-EOR
        def #{var}(val)
          vars[:#{var}] = val
        end
      EOR
    end

    def to_s
      vars.each_pair do |name, value|
        instance_variable_set("@#{name}".to_sym, value)
      end

      ERB.new(story).result(binding)
    end
  end

  m = madlibber.new(str)
  m.instance_eval(&block)
  m.to_s
end

story = < said: "Now you just fought one hell of a <%= @animal %>
And I know you hate me, and you got the right
To <%= @verb %> me now, and I wouldn't blame you if you do. 
But ya ought to thank me, before I die, 
For the gravel in ya guts and the spit in ya eye 
Cause I'm the <%= @adjective %> that named you "Sue.'" 

I got all choked up and I threw down my <%= @thing %> 
And I called him my pa, and he called me his son, 
And I came away with a different point of view. 
And I think about him, now and then, 
Every time I try and every time I win, 
And if I ever have a son, I think I'm gonna name him 
<%= @name2 %> or <%= @name3 %>! Anything but Sue! I still hate that name!
EOS

str = madlib(story) do
  name 'Alistair'
  animal 'duck'
  verb 'tickle'
  adjective 'fellow'
  thing 'spatula'
  name2 'Horace'
  name3 'Cynthia'
end

puts str