Is it possible to mix-in a module method?

Question

Let's say I have a module that declares a module method (not an instance method):

module M
  def self.foo
    puts 'foo'
  end
end

Now let's say I want to mix-in M.foo into another class C such that C.foo is defined.

Finally, I want to do this without changing the way M.foo is defined and without just creating a method in C that calls M.foo. (i.e. rewriting foo as an instance method doesn't count. Neither does using module_function.)

Is this impossible in Ruby?

Answer 1

I want to do this without changing the way M.foo is defined

Unfortunately, that's not possible. Ruby only allows to include modules, not classes. foo however is defined on M's singleton class which is a class. Therefore, you can't include it. The same restriction applies to extend. Trying to do so results in a TypeError:

module M
  def self.foo
    puts 'foo'
  end
end

class C
  extend M.singleton_class # TypeError: wrong argument type Class (expected Module)
end

You can however achieve what you want by defining foo as an instance method in a separate module which can then be mixed into both, M and C via extend: (that module doesn't have to be nested under M)

module M
  module SingletonMethods
    def foo
      puts 'foo'
    end
  end

  extend SingletonMethods     # <- this makes foo available as M.foo
end

class C
  extend M::SingletonMethods  # <- this makes foo available as C.foo
end

Or with some metaprogramming magic using Ruby's included callback:

module M
  module SingletonMethods
    def foo
      puts 'foo'
    end
  end

  extend SingletonMethods

  def self.included(mod)
    mod.extend(SingletonMethods)
  end
end

class C
  include M
end

That's a simplified version of how ActiveSupport::Concern works in Rails.