Illustrates that Ruby modules *can* have state but only on the module level, not the instance level.

module-class-state.rb

#!/usr/bin/env ruby

module M
  class << self
    attr_accessor :foo
  end
end

class C1
  def call
    M.foo = 'C1 was here'
  end
end

class C2
  def call
    puts M.foo
  end
end

C1.new.call
C2.new.call # => "C1 was here"