why do we need ClassMethods and InstanceMethods?

I read the API for ActiveSupport::Concern. There a开发者_如何转开发re ClassMethods and InstanceMethods, we can put class methods in ClassMethods.

But the M's host can use the methods defined in M, can't it? Why can't I just write:

module M
  def self.x
  end

  def y
  end
end

rather than:

module M
  module ClassMethods
    def x
    end
  end
  module InstanceMethods
    def y
    end
  end
end

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You might be interested in Yehuda's take on this pattern. I think the reason for some of it is historical, since they're not really needed unless you're doing manually what Ruby will do automatically through include and extend.

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Dependencies are taken care of. See the example provided here.