Class variables in modules, annoying

ruby-talk
1

Hi, let me show the following code:

···

--------------------------
module M
  @@var="M"

  def var
    @@var
  end

  def self.var
    @@var
  end
end

class A
  include M

  def var= v
     @@var=v
  end
end

a=A.new

a.var
=> "M"

a.var="A"
=> "A"

M.var
=> "A"
--------------------------

WHY is @@var replaced within M module? I do know that this behaviour
exists when inheriting classes, so a children class shares the class
variables of its parent class (which is a documented but undesirable
feature of Ruby). But I didn't expect it to occur using a module as
I'm not inheriting from a class.

In short, don't use class variables within a module, am I right?

Thanks a lot.

--
Iñaki Baz Castillo
<ibc@aliax.net>

2

Including a module inserts that module into the inheritance hierarchy.

class A
end
class B < A
end
module M
end

p B.ancestors
#=> [B, A, Object, Kernel, BasicObject]
class B
  include M
end

p B.ancestors
#=> [B, M, A, Object, Kernel, BasicObject]

More details than you'd probably like about this mechanism can be
found in a blog post of mine:

http://carboni.ca/blog/p/Modules-How-Do-They-Work

Michael Edgar
adgar@carboni.ca
http://carboni.ca/

···

On Jul 29, 2011, at 5:27 AM, Iñaki Baz Castillo wrote:

But I didn't expect it to occur using a module as
I'm not inheriting from a class.

3

"Iñaki Baz Castillo" <ibc@aliax.net> wrote in post #1013702:

In short, don't use class variables within a module, am I right?

A class is a module. So the statement should be: don't use class
variables period.

···

--
Posted via http://www.ruby-forum.com/\.

4

Great. Thanks a lot for the information.

···

2011/7/29 Michael Edgar <adgar@carboni.ca>:

Including a module inserts that module into the inheritance hierarchy.

More details than you'd probably like about this mechanism can be
found in a blog post of mine:

http://carboni.ca/blog/p/Modules-How-Do-They-Work

--
Iñaki Baz Castillo
<ibc@aliax.net>

5

Right. Is it not planned to change the behaviour of class variables in Ruby?

···

2011/7/29 7stud -- <bbxx789_05ss@yahoo.com>:

A class is a module. So the statement should be: don't use class
variables period.

--
Iñaki Baz Castillo
<ibc@aliax.net>

6

A class is a module. So the statement should be: don't use class
variables period.

Right. Is it not planned to change the behaviour of class variables in Ruby?

I have no idea, you might ask at ruby-core. But there is quite a
consensus not to use them. What is your use case?
As much as I can guess from your example code, class instance
variables should do the trick for you.

e.g.

module A
   class << self
     attr_accessor :a
   end
end

HTH
Robert

···

On Fri, Jul 29, 2011 at 12:32 PM, Iñaki Baz Castillo <ibc@aliax.net> wrote:

2011/7/29 7stud -- <bbxx789_05ss@yahoo.com>:

--
Iñaki Baz Castillo
<ibc@aliax.net>

--
I'm not against types, but I don't know of any type systems that
aren't a complete pain, so I still like dynamic typing.
--
Alain Kay

7

Yes, I try to use instance variables (but let's say "class's instance
variables" rather than "class instance's instance variable" XDD).
I was just wondering.

Thanks a lot.

···

2011/7/29 Robert Dober <robert.dober@gmail.com>:

I have no idea, you might ask at ruby-core. But there is quite a
consensus not to use them. What is your use case?
As much as I can guess from your example code, class instance
variables should do the trick for you.

--
Iñaki Baz Castillo
<ibc@aliax.net>