Why 'self' method of module cannot become a singleton method of class?

Question

module Test
  def self.model_method
    puts \"this is a module method\"
  end
end

class A
  include Test
end

A.model_method

this will be error w

Answer 1

Including a module is analogous to copying its instance methods over.

In your example, there are no instance methods to copy to A. model_method is actually an instance method of Test's singleton class.

---

Given:

module A
  def method
  end
end

This:

module B
  include A
end

Is analogous to this:

module B
  def method
  end
end

When you think of it this way, this makes perfect sense:

module B
  class << self
    include A
  end
end

B.method

Here, the methods are being copied to the B module's singleton class, which makes them the "class methods" of B.

Note that this is exactly the same thing as:

module B
  extend A
end

---

In reality, the methods are not being copied; there is no duplication. The module is simply included in the method lookup list.