scala - 案例类继承弃用的更好解决方案？

Question

我正在研究（诚然有些老）Programming Scala [Subramaniam, 2009]，在完成第 9.7 节“使用案例类匹配”（见下文）时遇到了令人不安的编译器警告。

这是我根据对错误消息的解释设计的解决方案。我如何使用更接近本书示例初衷的解决方案来改进此代码？特别是如果我想使用sealed案例类功能？

/** 
 * [warn] case class `class Sell' has case ancestor `class Trade'.  
 * Case-to-case inheritance has potentially dangerous bugs which 
 * are unlikely to be fixed.  You are strongly encouraged to instead use 
 * extractors to pattern match on non-leaf nodes.
 */

// abstract case class Trade()
// case class Buy(symbol: String, qty: Int) extends Trade
// case class Sell(symbol: String, qty: Int) extends Trade
// case class Hedge(symbol: String, qty: Int) extends Trade

object Side extends Enumeration {
  val BUY = Value("Buy")
  val SELL = Value("Sell")
  val HEDGE = Value("Hedge")
}

case class Trade(side: Side.Value, symbol: String, qty: Int)

def process(trade: Trade) :String = trade match {
  case Trade(_, _, qty) if qty >= 10000 => "Large transaction! " + trade
  case Trade(_, _, qty) if qty % 100 != 0 => "Odd lot transaction! " + trade 
  case _ => "Standard transaction: " + trade
}

Answer 1

改为继承“密封特性贸易”。

sealed trait Trade
case class Buy(symbol: String, qty: Int) extends Trade
case class Sell(symbol: String, qty: Int) extends Trade
case class Hedge(symbol: String, qty: Int) extends Trade