EmptyArray: an array variant of the null object pattern

empty_array_minitest.rb

require 'minitest/autorun'

class EmptyArray
  def self.new
    @instance ||= super
  end

  private :initialize

  def method_missing(*args, &block)
    return empty_array.send(*args, &block) if empty_array.respond_to?(args.first)
    self
  end

  def respond_to?(method_name)
    true
  end

  private

  def empty_array
    @empty_array
  end
end

describe EmptyArray do
  it "returns itself until it gets a message a regular array responds to" do
    empty_array = EmptyArray.new
    empty_array.foo.bar.baz.qux.first.must_equal nil
  end

  it "returns [] when passed to Kernel#Array" do
    Array(EmptyArray.new).must_equal []
  end

  it "returns [] when passed to Kernel#Array even after several chained methods" do
    Array(EmptyArray.new.foo.bar.baz.qux).must_equal []
  end

  describe "cheesy singleton hack" do
    it "is an instance of EmptyArray" do
      EmptyArray.new.must_be_kind_of EmptyArray
    end

    it "one instance is the same as another" do
      one = EmptyArray.new
      two = EmptyArray.new
      one.object_id.must_equal two.object_id
    end
  end
end

empty_array_test_unit.rb

require 'test/unit/autorunner'

class EmptyArray
  def self.new
    @instance ||= super
  end

  private :initialize

  def method_missing(*args, &block)
    return empty_array.send(*args, &block) if empty_array.respond_to?(args.first)
    self
  end

  def respond_to?(method_name)
    true
  end

  private

  def empty_array
    @empty_array
  end
end

class EmptyArrayTest < Test::Unit::TestCase
  def test_returns_itself_until_it_gets_a_message_a_regular_array_responds_to
    empty_array = EmptyArray.new
    assert_nil empty_array.foo.bar.baz.qux.first
  end

  def test_returns_empty_array_when_passed_to_Kernel_Array
    assert_equal [], Array(EmptyArray.new)
  end

  def test_returns_empty_array_when_passed_to_Kernel_Array_even_after_several_chained_methods
    assert_equal [], Array(EmptyArray.new.foo.bar.baz.qux)
  end

  def test_is_an_instance_of_EmptyArray
    assert_kind_of EmptyArray, EmptyArray.new
  end

  def test_one_instance_is_the_same_as_another
    one = EmptyArray.new
    two = EmptyArray.new
    assert_same one, two
  end
end