module_function.rb

module Foo
  module_function

  def function
    :ok
  end
end

class Bar
  include Foo

  def it_works_privately
    function
  end
end

Foo.function
#=> :ok

bar = Bar.new

bar.it_works_privately
#=> :ok

bar.function
#!> NoMethodError: private method `function' called for #<Bar:...>