Override module method from another module

Question

I want to override a method from a module A from another module B that will monkey-patch A.
http://codepad.org/LPMCuszt

module A
  def foo; puts 'A' end
end

module B
  def foo; puts 'B'; super; end
end

A.module_eval { include B } # why no override ???

class C
  include A
end

# must print 'A B', but only prints 'A' :(
C.new.foo

Answer 1

Including a module places it above the module/class that is including it in the class hierarchy. In other words, A#foo is not super of B#foo but rather the other way round.

If you think of including a module as a way of doing multiple inheritance this makes sense, include SomeModule is a way of saying, "Treat SomeModule like it is a parent class for me".

To get the output you wanted you need to reverse the inclusion so that B includes A:

module A
  def foo; puts 'A' end
end

module B
  def foo; puts 'B'; super; end
end

B.module_eval { include A } # Reversing the inclusion

class C
  include B # not include A
end

puts C.new.foo

---

Edit in response to comment:

Then either include both A and B in C with B included after A:

# A and B as before without including B in A.

class C
  include A
  include B
end

or patch A in C itself and don't bother with B.

# A as before, no B.

class C
  include A

  def foo; puts 'B'; super; end
end

The only way for this to work is if the method lookup on C is C -> B -> A and there is no way to do this without including B into C.