rock_score.rb

class RockScore
  def initialize(value)
    @value = value
  end

  def available?
    !@value.nil?
  end

  def value
    # Alternatively, have a NoScore that just raises
    # when #value is called and keep this as a simple
    # reader.
    if @value.nil?
      raise "WAT!"
    else
      @value
    end
  end
end