Ruby, how to reference Root namespace?

Question

When I have a module like this:

module MyModule
  class MyClass
  end
end

I can access/modify MyModule referencing it:

MyModule.const_set("MY_CONSTANT", "value")

But what about the Root namespace, the :: one?, I'm looking for something like:

::.const_set("MY_CONSTANT", "value")

The const_set thing is just an example, please don't try to solve this concrete situation but the way of actually making reference to the Root namespace

Answer 1

What is the root object? If you mean main object, you can't set constant at this level:

TOPLEVEL_BINDING.eval('self').const_set("MY_CONSTANT", "value")
# NoMethodError: undefined method `const_set' for main:Object
#   from (irb):71
#   from /home/malo/.rvm/rubies/ruby-2.1.0/bin/irb:11:in `<main>'

If you mean Object object, do as follows:

Object.const_set("MY_CONSTANT", "value")
# => "value"

then you can use at the main, or at any other level:

::MY_CONSTANT
# => "value"

Adding another confirmation

We can set a constant using Kernel or using Object and in both cases the constant will be accessible from the root namespace:

Kernel.const_set("KERNEL_CONSTANT", "value")
Object.const_set("OBJECT_CONSTANT", "value")

puts !!(defined? ::KERNEL_CONSTANT) # => true
puts !!(defined? ::OBJECT_CONSTANT) # => true

But if we set a constant in the root namespace this constant is actually set in Object and not in Kernel:

::ROOT_CONSTANT = "value"

puts !!(defined? Object::ROOT_CONSTANT) # => true
puts !!(defined? Kernel::ROOT_CONSTANT) # => false

Answer 2

There is no such a thing like a Root module, however you can get some quite similar thing operating on Object class:

def bar
  'bar'
end

class A
  def bar
    'not bar'
  end
  def test
    bar
  end
  def test2
    Object.bar
  end
end

A.new.test  #=> 'not bar'
A.new.test2 #=> 'bar'

You can read about the main object here: http://banisterfiend.wordpress.com/2010/11/23/what-is-the-ruby-top-level/

Answer 3

There is no such thing as a root module in Ruby.

You can define a module in the top-level namespace like this:

::MY_CONSTANT = "value"

If for some reason you have to use const_set you can do:

module Hi
  Kernel.const_set("X", "3")
end

puts X # => 3

Kernel is mixed-in into the top-level namespace so you're effectively defining a constant in the global namespace this way.