[EN] include vs extend in Ruby

module Commentable
  def submit
    "Commented"
  end
end

class Post
  include Commentable # `submit` method is an instance method
end

class Post
  include Commentable # `submit` method is an instance method
  extend Commentable  # `submit` method is a class method
end

class Post
  extend Commentable  # `submit` method is a class method
end

# `submit` method can be an instance method with extend keyword when doing:
post = Post.new
post.extend Commentable

module A
  def a
    p "method a"
  end

  def self.c
    p "method c"
  end
end

class B
  include A
end

B.new.a    # print 'method a'
B.c        # undefined method c
A.c        # print 'method c'