about_constants.rb

require File.expand_path(File.dirname(__FILE__) + '/neo')

C = "top level"

class AboutConstants < Neo::Koan

  C = "nested"

  def test_nested_constants_may_also_be_referenced_with_relative_paths
    assert_equal "nested", C
  end

  def test_top_level_constants_are_referenced_by_double_colons
    assert_equal "top level", ::C
  end

  def test_nested_constants_are_referenced_by_their_complete_path
    assert_equal "nested", AboutConstants::C
    assert_equal "nested", ::AboutConstants::C
  end

  # ------------------------------------------------------------------

  class Animal
    LEGS = 4
    def legs_in_animal
      LEGS
    end

    class NestedAnimal
      def legs_in_nested_animal
        LEGS
      end
    end
  end

  def test_nested_classes_inherit_constants_from_enclosing_classes
    assert_equal 4, Animal::NestedAnimal.new.legs_in_nested_animal
  end

  # ------------------------------------------------------------------

  class Reptile < Animal
    def legs_in_reptile
      LEGS
    end
  end

  def test_subclasses_inherit_constants_from_parent_classes
    assert_equal 4, Reptile.new.legs_in_reptile
  end

  # ------------------------------------------------------------------

  class MyAnimals
    LEGS = 2

    class Bird < Animal
      def legs_in_bird
        LEGS
      end
    end
  end

  def test_who_wins_with_both_nested_and_inherited_constants
    assert_equal 2, MyAnimals::Bird.new.legs_in_bird
  end

  # QUESTION: Which has precedence: The constant in the lexical scope,
  # or the constant from the inheritance hierarchy?

  # The lexical scope.

  # ------------------------------------------------------------------

  class MyAnimals::Oyster < Animal
    def legs_in_oyster
      LEGS
    end
  end

  def test_who_wins_with_explicit_scoping_on_class_definition
    assert_equal 4, MyAnimals::Oyster.new.legs_in_oyster
  end

  # QUESTION: Now which has precedence: The constant in the lexical
  # scope, or the constant from the inheritance hierarchy?  Why is it
  # different than the previous answer?

=begin 

When you define MyAnimals::Oyster you are still in the global scope, 
so ruby has no knowledge of the LEGS value set to 2 in MyAnimals 
because you never actually are in the scope of MyAnimals 
(a little counterintuitive).

However, things would be different if you were to define Oyster this way:

class MyAnimals
  class Oyster < Animal
    def legs_in_oyster
      LEGS # => 2
    end
  end
end
The difference is that in the code above, by the time you define Oyster, 
you have dropped into the scope of MyAnimals, 
so ruby knows that LEGS refers to MyAnimals::LEGS (2) and not Animal::LEGS (4).

https://stackoverflow.com/questions/5464811/ruby-koans-explicit-scoping-on-a-class-definition-part-2

=end
end