Ruby class variables in dynamic anonymous module

Question

I'm creating an anonymous module because I need it to be dynamic, but using class variables like in the example below doesn't work. I have this code:

class Filter

  def self.mod(var)
    # Creating anonymous module
    m = Module.new do
      def hello
        puts @var
      end

      def self.var=(var)
        @@var = var
      end

      def self.var
        @@var
      end

    end
    # Setting the class variable
    m.var = var
    m
  end
end

f1 = Filter.mod("hello")

puts f1.var # => hello

f2 = Filter.mod("goodbye")

puts f2.var # => goodbye
puts f1.var # => goodbye

Why does f1 change when I assign to f2? I need each module to maintain its own variable values. Is there a way to circumvent this with anonymous / dynamic modules?

If I include the Filter in a class:

class Main
  include Filter.mod("hello")
end

m = Main.new

puts m.hello # => nil

How do I access the @var variable?

This is different from what I understood from your comments in that other question. Just use @var and f1.var won't change. — Sergio Tulentsev
– Sergio Tulentsev, Commented Jul 13, 2015 at 13:34
I didn't really know how to ask it, what am I looking for? A dynamic module? I updated the example. I'm trying to avoid values being changed on all instances. — Vidar
– Vidar, Commented Jul 13, 2015 at 14:07
Perhaps, show your exact use-case and not a made-up one? Why do [you think] you need this module thing at all? — Sergio Tulentsev
– Sergio Tulentsev, Commented Jul 13, 2015 at 14:13

Kristján · Accepted Answer · 2015-07-13 13:40:09Z

1

The @@ defines a class variable on Filter, not on your new Module like you expect. You can see it with:

puts Filter.class_variables.inspect
# [:@@var]

This only exists once, and gets written to by every reference to @@var that happens in the scope of Filter.

What you really want is class instance variables on each of your Module.news, which you do with just a single @. This defines @var on your new instance of the Module class.

  def self.var=(var)
    @var = var
  end

  def self.var
    @var
  end

# ...

puts f1.class
# Module
puts f1.instance_variables.inspect
# [:@var]

puts f2.var
# goodbye
puts f1.var
# hello

answered Jul 13, 2015 at 13:40

Kristján

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Piotr Kruczek · Accepted Answer · 2015-07-13 13:35:14Z

0

Because you assign to @@var which is a class variable. Try changing it to @var.

answered Jul 13, 2015 at 13:35

Piotr Kruczek

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