modules including other modules

[Tim Olsen]

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

 

[Jeremy Henty]

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

 

[Trans]

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.

 

[Adriano Mitre]

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

 

[Adriano Mitre]

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.