mixin-class-methods.rb

## Abstraction for common pattern of using Module#included to
## add class methods to the host class.

class Module
  def extends_host_with(class_methods)
    @_extension_module = class_methods
    def included(base)
      base.extend(const_get(@_extension_module))
    end
  end
end

## Example
module MyMod
  extends_host_with :ClassMethods

  def an_instance_method; end

  module ClassMethods
    def a_class_method; end
  end
end