How to share methods between ruby modules

Question

Here is what I tried:

module A
  def self.method1; "method1"; end
  def method2; "method2"; end
end

module B; include A; end

B.method1  # => error
B.method2  # => error
B::method1 # => error
B::method2 # => error

I want to avoid copying and pasting equivalent code between two modules. The reason I'm using modules instead of classes here is because I don't need more than one instance of each module, as they simply hold constants (other modules, at this point).

What is the best way to solve this problem?

Answer 1

Plain include only gives you instance methods (method2 in your particular piece of code). If you want to share module-level methods - extract them to separate module and extend other modules with it:

module A
  extend self     # to be able to use A.method1

  def method1
    "method1"
  end
end

module B
  extend A
end

B.method1       # => "method1"

It is also possible get module-level methods by include, but with a little twist, using hook method:

module A
  def self.included(other)
    other.extend ModuleMethods    # this is where the magic happens
  end

  def instance_method
    'instance method'
  end

  module ModuleMethods
    def module_method
      'module method'
    end
  end

  extend ModuleMethods     # to be able to use A.module_method
end


module B
  include A
end

B.module_method        #=> "module method"
B.instance_methods     #=> [:instance_method]

Answer 2

First of all, please note that A.method2 won't work either. You can create objects including A (or B) that will have method2:

class C
  include B    # (or A)
end
c = C.new
c.method2

So, for method2 it just works as you intended.

Regarding method1, it is a singleton method of the object A and there is no way to inherit it.