Using module_function instead of extend self

module_function.rb

# a lot of people in ruby do this
module Foo
  extend self
  
  def bar
    'bar'
  end
end

Foo.bar
# => 'bar'
class Baz; include Foo; end
Baz.new.bar
# => 'bar'

# but what they really should be doing is this:
module Foo
  def bar
    'bar'
  end
  
  module_function :bar
end

Foo.bar
# => 'bar'
class Baz; include Foo; end
Baz.new.bar
# => 'bar'