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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 <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.  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.

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