最近学习ruby过程中，extend self 跟 module_function 傻傻分不清楚，查资料后明白之间的差别，虽记录之，原文地址 github

module A

  extend self

  def a

    puts 'In a'

  end

  private

  def b

    puts 'In b'

  end

end

module B

  def c

    puts 'In c'

  end

  private

  def d

    puts 'In d'

  end

  module_function :c, :d

end

A.a => In a

A.b => NoMethodError: private method b called for A:Module

B.c => In c

B.d => In d

class O

  include A

  include B

end

o = O.new => #<O:0x007f90ed2cdb18>

o.a => In a

o.b => NoMethodError: private method b called for #<O:0x007f90ed2cdb18>

o.c => NoMethodError: private method c called for #<O:0x007f90ed2cdb18>

o.d => NoMethodError: private method d called for #<O:0x007f90ed2cdb18>

总结：module_function 会将module自身的方法变为public(即使方法本身声明为private), mixin之后所有的方法都变为实例的私有方法.