Scoping in Ruby

Around a month back, a colleague of mine and I were playing around with nesting and variable scopes in ruby. Here is a small piece of code I wrote for better understanding what is going on.

A = 'I am A'
module Foo
  B = 'I am B'
  puts Module.nesting.inspect
  class Bar
    C = 'I am C'
    puts Module.nesting.inspect
    module World
      D = 'I am D'
      puts Module.nesting.inspect
      puts ::A
      puts ::Foo::Bar::C
    end
  end
end
    
p A
p Foo::B
p Foo::Bar::C
p Foo::Bar::World::D

While not in the scope a particular variable, the :: operator can be used to specify which scope this variable belongs to. So If we're currently outside module Foo, Foo::Bar::World::D can be used to access constant D.

The statements with a :: right at the very start are used to set the current scope to root level. This can be observed in module world where ::A is being used to access the constant defined outside of module Foo.

The code above generates the following output.

[Foo]
[Foo::Bar, Foo]
[Foo::Bar::World, Foo::Bar, Foo]
I am A
I am C
"I am A"
"I am B"
"I am C"
"I am D"

Oh and the nesting method is used to see the current level of nesting.