Ruby 2.0 Refined #only? Method on Array

refined-array.rb

module Only
  refine Array do
    def only?
      case self.size
      when 1
        self.first
      when 0
        nil
      else
        false
      end
    end
  end
end

require 'minitest/autorun'
require 'minitest/pride'

describe Array do
  using Only
  
  describe 'a single element' do
    it 'returns the element' do
      assert_equal 0, [0].only?
      assert_equal 'Just me!', ['Just me!'].only?
    end
  end
  
  describe 'multiple elements' do
    it 'returns false' do
      assert_equal false, [1.0/0, 2.71828].only?
      assert_equal false, %w[O M G W T F B B Q].only?
    end
  end
  
  describe 'no elements' do
    it 'returns nil' do
      assert_nil [].only?
    end
  end
end