Talk Like A Duck

A Subtle Change to Mixin Semantics in Ruby 1.9

Posted by Rick DeNatale Mon, 09 Oct 2006 20:07:00 GMT

I’ve been working on a little tool to peek behind the curtain and see a bit of what’s going on behind the scenes in the standard Ruby implementation (i.e ‘ruby’ as opposed to ‘Ruby’)

While doing this I was looking at the code which ruby runs when you include a module in a class or another module. I noticed that ruby 1.8 was going to some pains not to move the proxy for an included module in the inheritance chain.

To verify what my eyes seemed to be telling me, I wrote a silly little test program which created a module with one method, and several classes.

This test verified my reading of the 1.8 code. I then tried the same test using the latest ruby 1.9 and found that module mixin semantics have changed.

Read on

module M1
  def foo
    "foo in M1"
  end
end

module M2
  include M1
end

class C1
  def foo
    "foo in C1"
  end
end

class C2 < C1
  include M1
end

class C3 < C2
  def foo
    "foo in C3"
  end
end

class C4 < C3
  include M1
end

class C5 < C3
  include M2
end

puts "C1 #{C1.new.foo}"
puts "C2 #{C2.new.foo}"
puts "C3 #{C3.new.foo}"
puts "C4 #{C4.new.foo}"
puts "C5 #{C5.new.foo}"

Module mix-in semantics in ruby 1.8

rick@frodo:/public/rubyscripts$ ruby include_test.rb
C1 foo in C1

C2 foo in M1

C3 foo in C3

C4 foo in C3

C5 foo in C3

I find the 1.8 implementation counter-intuitive. As I said it goes to lengths to find an existing inclusion of a module, and re-use it, even if that inclusion was in a superclass.

Change in ruby 1.9

$ ruby1.9 include_test.rb
C1 foo in C1
C2 foo in M1
C3 foo in C3
C4 foo in M1
C5 foo in M1

$ ruby include_test.rb
C1 foo in C1
C2 foo in M1
C3 foo in C3
C4 foo in C3
C5 foo in C3

I don’t know about you, but this seems more natural to me.

It’s a subtle change. I don’t know how much ruby code re-mixes in modules. I only wrote my test code to verify a quirk I saw in reading the ruby sources.

Since the ruby 1.8 implementation made doing so ineffective, I doubt that there is much code which does, but if there is such code, it might break under 1.9, but I think that the change is a good one.