singleton_vs_module_instance_variable.rb

require 'singleton'

class MyLogger
  include Singleton

  def initialize
    @logger = $stdout
  end

  def write(msg)
    @logger.puts msg
  end
end

MyLogger.instance.write('hoge')

module MyLogger2
  class << self
    def write(msg)
      logger.puts msg
    end

    def logger
      @logger ||= $stdout
    end
  end
end

MyLogger2.write 'foo'