switch.rb

class Switch
  def initialize(obj, &block)
    @obj, @procs = obj, []
    instance_eval(&block)
    @proc = @procs.detect{|p| p.call }
  end
  
  def call
    @proc.call if @proc
  end
  
  def on(val, &block)
    deffer(block) { val === @obj }
  end
  
  def responds_to(val,&block)
    deffer(block) { @obj.respond_to?(val) }
  end
  
private
  def deffer(block, &condition)
    @procs << lambda { return block.call if condition.call }
  end
end

def switch(obj,&block)
  Switch.new(obj,&block).call
end

out = switch "Dog farts" do
  responds_to :<< do
    "hey!"
  end
  
  responds_to :<< do
    "I respond <<!"
  end
  
  on String do
    "I'm a string!"
  end
end

puts out