Forum: Ruby modules including other modules

Hello,

It appears that including module A inside another module B can cause
problems for classes that have already included A.  What seems to
happen is that a class can only see the methods from module B if that
class include module A before module A includes module B.

For example,

module A

  def a
    puts "a"
  end

end

module B

  def b
    puts "b"
  end

end

class C

  include B

  def c
    puts "c"
  end

end

module B

  include A

  def b2
    puts "b2"
  end
end


C.new.c
C.new.b
C.new.b2
C.new.a

produces

tolsen@neurofunk:~/svn/limeade$ /var/tmp/tst.rb
c
b
b2
/var/tmp/tst.rb:42: undefined method `a' for #<C:0x2ac204fe7180> 
(NoMethodError)

Notice I can define b2() after C has already included B but including
A into B does nothing for C.

Any idea what's going on here?

Thanks,
Tim

On 2007-05-29, Tim Olsen <tolsen718@gmail.com> wrote:

> It appears that including module A inside another module B can cause
> problems for classes that have already included A.  What seems to
> happen is that a class can only see the methods from module B if
> that class include module A before module A includes module B.

It's a known problem that is apparently almost impossible to fix.  See
http://eigenclass.org/hiki/The+double+inclusion+problem .

Unless there's been some development that I've failed to Google up,
your only option is "Don't Do That!".  :-(  Unless Ruby 1.9 handles
things better.

Regards,

Jeremy Henty

On May 29, 12:25 pm, Jeremy Henty <onepo...@starurchin.org> wrote:
> On 2007-05-29, Tim Olsen <tolsen...@gmail.com> wrote:
>
> > It appears that including module A inside another module B can cause
> > problems for classes that have already included A.  What seems to
> > happen is that a class can only see the methods from module B if
> > that class include module A before module A includes module B.
>
> It's a known problem that is apparently almost impossible to fix.  Seehttp://eigenclass.org/hiki/The+double+inclusion+problem.

Also know as the "Dynamic Inclusion Problem" or just the "Inclusion
Problem".

> Unless there's been some development that I've failed to Google up,
> your only option is "Don't Do That!".  :-(  Unless Ruby 1.9 handles
> things better.

While, it would be great if someone did find a way, from what I
understand its too difficult to fix given how Ruby works, so don't
hold your breath.

T.

Well, this limitation can be circumvented by runnning the following
after each change in MyModule:

ObjectSpace.each_object(Module) do |m|
  if m <= Enumerable
    m.module_eval("include MyModule")
  end
end


In order to understand the drawbacks of this solution, I suggest reading
http://ola-bini.blogspot.com/2007/07/objectspace-to-have-or-not-to-have.html

Update: the aforementioned solution was encapsulated into a gem which 
makes it feels like it were part of the Ruby language.

Synopsis:
Enumerable.module_eval { retroactively_include MyModule }

At http://rubygems.org/gems/retroactive_module_inclusion one may find 
the link to the GitHub repository, documentation, bug tracker, etc.