singleton.rb

require 'forwardable'

# A singleton class inspired by Primer class in library

class Class
  # We need that method like this become a bultin
  def metaclass; class << self; self end end
end

class Foo

  class << self
    extend Forwardable

    _instance = Foo.new
    define_method(:instance) { _instance }

    # Primer class's cool technic with forwardable
    def method_added(meth)
      self.metaclass.def_delegator :instance, meth
    end

    private :new
  end

  def say
    puts 'hello'
  end
end

Foo.say #=> hello