require 'singleton'

class SingleInstanceOfState
  include Singleton

  attr_accessor :state

  def initialize
    @state = nil
  end

  def foo
    @state.reverse!
  end
end

singleton_instance = SingleInstanceOfState.instance
singleton_instance.state = 'wombat'
singleton_instance.foo
#=> "tabmow"
singleton_instance.foo
#=> "wombat"