Question

I read the API for ActiveSupport::Concern. There are 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

Answer 1

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.