extend self是在单例类父类链中加入当前module。如果改变以后删除或重定义，会影响类方法（其实一般都用这个，很少见module_function）

module E
  extend self
  def e
    1
  end
end

o = String.new
o.extend E

puts E.singleton_class.ancestors.join ', '
puts o.singleton_class.ancestors.join ', '

=> #, E, Module, Object, Kernel, BasicObject
=> #<class:#>, E, String, Comparable, Object, Kernel, BasicObject</class:#</class:e></code></pre>


而Module functions are copies of the original and so may be changed independently. The instance-method versions are made private. If used with no arguments, subsequently defined methods become module functions

其实现大概如下：

class Module
  def my_module_function *methods
    if methods.empty?
      class << self
        def method_added name
          private name
          singleton_class.send :define_method, name, instance_method(name)
        end
      end
    else
      methods.each do |name|
        private name
        singleton_class.send :define_method, name, instance_method(name)
      end
    end
  end
end


若想做到changed independently，需带参数（方法名）使用，这样，当重定义该方法时，因已用define_method指向旧方法体，所以不影响类方法

其private特性可带来以下好处：

The biggest advantage of module_function methods is that when their encompassing module gets included into a class the methods do not become public method of that class – which is a good thing, as they don’t extend the class’s public API (and don’t need to be kept backward-compatible)

此外，module一旦调用extend self，所有方法都会成为类方法，因为每次调用类方法时都重新查找继承链。而module_function只影响其后定义的方法，或带参数时其前定义的方法