Including a Module with append_features

module MyMod

  def MyMod.append_features(someClass)
    def someClass.modmeth
      puts "Module (class) method"
    end
    super   # This call is necessary!
  end

  def meth1
    puts "Method 1"
  end

end


class MyClass

  include MyMod

  def MyClass.classmeth
    puts "Class method"
  end

  def meth2
    puts "Method 2"
  end

end


x = MyClass.new

                      # Output:
MyClass.classmeth     #   Class method
x.meth1               #   Method 1
MyClass.modmeth       #   Module (class) method
x.meth2               #   Method 2

This example is worth examining in detail. First, we should understand that append_features isn't just a hook that is called when an include happens; it actually does the work of the include operation. That's why the call to super is needed; without it, the rest of the module (in this case, meth1) wouldn't be included at all.

Also note that within the append_features call, there is a method definition. This looks unusual, but it works because the inner method definition is a singleton method (class-level or module-level). An attempt to define an instance method in the same way would result in a Nested method error.

Conceivably a module might want to determine the initiator of a mixin. The append_features method can also be used for this because the class is passed in as a parameter.

It is also possible to mix in the instance methods of a module as class methods. 

module MyMod

  def meth3
    puts "Module instance method meth3"
    puts "can become a class method."
  end

end


class MyClass

  class << self    # Here, self is MyClass
    include MyMod
  end

end


MyClass.meth3

# Output:
#   Module instance method meth3
#   can become a class method.

The extend method is useful here. This example simply becomes:

class MyClass
    extend MyMod
end