TotalPriceComparison

equals.rb

class GreaterThan
  def display
    "more than"
  end

  def evaluate one, two
    one > two
  end
end

total_price_comparison.rb

class TotalPriceComparison
  def comparison= comparison
    @comparison
  end

  def display
    "Customer spent #{comparison.display()} #{threshold}"
  end

  def evaluate
    comparison.evaluate purchase.total_price, threshold
  end
end