foo.rb

module Foo
  def self.bar
  end

  def self.baz
  end
end

module Foo
  class << self
    def bar
    end

    def baz
    end
  end
end

module Foo
  module_function

  def bar
  end

  def baz
  end
end

module Foo
  def bar
  end

  def baz
  end

  module_function :bar, :baz
end