scala - 评估惰性字段

Question

这是斯卡拉代码：

  class FullDescription(val description: String) 
  class Logger {
      val description = //?????

      def onAttach(description: String) = 
       description = new FullDescription(description)

      // val lazy description = onAttach(???) -- 

      // ops! we don't know its actual argument, 
      // besides we should be able to make 
      // val description be evaluated from onAttach(...) and not vise versa
    }

假设对于任何类Logger，都保证该方法onAttach只会被调用一次。但不知道何时onAttach调用该方法。

是否有可能保持val description 不变，我该怎么做？评价lazy还是别的？