Ruby: extend self

It is a convenient way to make instance methods into class methods. But you can also use it as a more efficient singleton.

For me it always helps to think of extend as include inside the singleton class (also known as meta or eigen class).

You probably know that methods defined inside the singleton class are basically class methods:

module A
  class << self
    def x
      puts 'x'
    end
  end
end

A.x #=> 'x'

Now that we know that, extend will include the methods in the module inside the singleton class and thus expose them as class methods:

module A
  class << self
    include A

    def x
      puts 'x'
    end
  end

  def y
    puts 'y'
  end
end

A.x #=> 'x'
A.y #=> 'y'

In a module, self is the module class itself. So for example

puts self

will return Rake so,

extend self

basically makes the instance methods defined in Rake available to it, so you can do

Rake.run_tests