Forum: Ruby include Module in another Module

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Brian B. (Guest)
on 2007-02-19 18:48
(Received via mailing list)
What is the proper syntax/code to include a Module into another
Module?  My attempt below is not working.

--Brian

module Foo
  def xxx; "this is xxx" end
end

module Enumerable
  include Foo
end

puts [].xxx  #does not  work, xxx not defined for Array.  Array
includes Enumerable, right?
Pit C. (Guest)
on 2007-02-19 20:10
(Received via mailing list)
Brian B. schrieb:
>
> puts [].xxx  #does not  work, xxx not defined for Array.  Array
> includes Enumerable, right?

Brian, this isn't a problem of your syntax/code, but a (well known)
problem of the Ruby interpreter. There's no known solution yet. You have
to change the Enumerable module directly.

Regards,
Pit
Brian B. (Guest)
on 2007-02-20 04:00
(Received via mailing list)
Thanks, Pit.

Rounding this out, I notice now that classes that "include Enumerable"
after Enumerable is enhanced by "include Foo" have the extra method,
but classes that "include Enumerable" before Enumerable is enhanced
don't (core class Array is in this category).

Interesting to learn this behavior is considered a bug, and may one
day be fixed.

Interestingly, re-including Enumerable to Array, i.e., the line

class Array; include Enumerable end

corrects the bug for Array --- this is not necessarily a useful thing
to know since as you suggest putting code directly into Enumerable
rather than by using an include appears to be the way to go here.

--Brian
Marcello B. (Guest)
on 2007-02-20 07:33
(Received via mailing list)
Hi,

(i'm no ruby internals expert, just having some fun poking at rhg[2])

On Tuesday 20 February 2007 02:59, Brian B. wrote:
> Thanks, Pit.

> Interesting to learn this behavior is considered a bug, and may one
> day be fixed.

It is due the way method inheritance is implemented right now.

In MRI, every object has a linked list of classes from which inherits
methods.
This list contains either "plain" ruby classes, and other "hidden"
metaclasses, that serve to purposes such as singleton methods, class
methods
and module inclusion[1].
The latter is implemented by inserting in the linked list a new "include
class" whose mehtod table points to the included module's method table.
This means that when you do

class A
  include M
end

the ruby interpreter will build such an inheritance list:

       M
       |
       |
A -> (iM) -> Object

so, instances of A will have access to M's methods via the iM include
class.

When you do

module M1
end
module M2
  include M1
end

class A
  include M2
end

two include classes are created for A, pointing to every module in the
chain:

       +-------> M2
       |         |
       |       (iM2)
       |         |
       |         M1
       |         |
A -> (iM2') -> (iM1) -> Object

... that's why:

> Interestingly, re-including Enumerable to Array, i.e., the line
>
> class Array; include Enumerable end

... fixes the problem, because this include causes the include class of
your
module to be added to Array's inheritance chain ...

> corrects the bug for Array --- this is not necessarily a useful thing
> to know since as you suggest putting code directly into Enumerable
> rather than by using an include appears to be the way to go here.

... and also why adding code to enumerable works, because you are
changing the
method table already pointed by the include class in Array's inheritance
linked list.

Ruby internals are really really beautiful :)

[1]: http://www.ruby-doc.org/core/classes/Class.html
[2]: http://rhg.rubyforge.org/chapter04.html
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