斯卡拉隐式排序

abstract class Common
case class A extends Common
case class B extends Common

implicit val KeyOrdering = new Ordering[Common] {
    override def compare(x: Common,y: Common): Int = {
      x.toString.compareTo(y.toString)
   }
}

正如@ntn所指出的,列表的推断类型 – 它的两个元素的最小上限 – 是带有Commonizable的Serializable.由于scala.Ordering在其类型参数上没有逆变,因此隐式解析失败,因为它不符合Ordering [Common]&lt ;: Ordering [Product with Serializable with Common].

您可以通过编写隐式排序来解决此问题,以便始终具有所考虑的隐式参数的确切类型：

abstract class Common
case class A() extends Common
case class B() extends Common

object Common {
  implicit def ordering[A <: Common]: Ordering[A] = new Ordering[A] {
    override def compare(x: A,y: A): Int = {
      x.toString.compareTo(y.toString)
    }
  }
}

或者简洁：

object Common {
  implicit def ordering[A <: Common]: Ordering[A] = Ordering.by(_.toString)
}