gistfile1.rb

module A
  # prefer when there are less than three class methods
  # and there are instance methods
  def self.boo
  end

  def self.soo
  end

  def fuuu
  end
end

module B
  # prefer when there are three or more class methods
  # and there are instance methods
  class << self
    def boo
    end

    def soo
    end

    def moo
    end
  end

  def fuuu
  end
end

module C
  # prefer when there are no private/protected
  # and there are no instance methods
  module_function

  def boo
  end

  def moo
  end
end

module D
  # prefer when there are private/protected scopes
  # and there are no instance methods
  extend self

  def boo
  end

  private

  def moo
  end
end