Scala 类型差异

Question

我有以下代码

class Person
class Warrior extends Person
trait Commander[A] {
  def giveOrder(to: A)
  def delegate(to: Commander[A])
}

val warCommander: Commander[Warrior] = new Commander[Warrior] {
  override def giveOrder(to: Warrior): Unit = ???
  override def delegate(to: Commander[Warrior]): Unit = ???
}

val president: Commander[Person] = new Commander[Person] {
  override def giveOrder(to: Person): Unit = ???
  override def delegate(to: Commander[Person]): Unit = ???
}

warCommander.giveOrder(new Person) // GOOD ERROR: Person is not a Warrior
president.giveOrder(new Person)
warCommander.delegate(president) // GOOD ERROR: Commander[Person] is not a Commander[Warrior]
president.delegate(warCommander) // UNWANTED ERROR: Commander[Warrior] is not a Commander[Person]

所以最后一个错误不是预期的，但是如果我让 A 是协变的，它就会出错，因为那时我可以说

class Civilian extends Person
val a: Commander[Warrior] = new Commander[Person] { def giveOrder(to: Civilian) ...

这很混乱，所以我可以接受。

那么此时，如何去掉最后一个错误，让编译器明白一个Warrior是一个Person？

Answer 1

如果您的意图是让某些类型 A 能够委托给 A 以及属于 A 子类型的所有类型 B，那么您可以在没有协方差的情况下实现这一点（注意 def delegate[B <: A](to: Commander[B])）：

class Person
class Warrior extends Person
trait Commander[A] {
  def giveOrder(to: A)
  def delegate[B <: A](to: Commander[B])
}

val warCommander: Commander[Warrior] = new Commander[Warrior] {
  override def giveOrder(to: Warrior): Unit = ???
  override def delegate[B <: Warrior](to: Commander[B]): Unit = ???
}

val president: Commander[Person] = new Commander[Person] {
  override def giveOrder(to: Person): Unit = ???
  override def delegate[B <: Person](to: Commander[B]): Unit = ???
}

warCommander.giveOrder(new Person) // ERROR: Person is not a Warrior
president.giveOrder(new Person)    // OK
warCommander.delegate(president)   // ERROR: Commander[Person] is not a Commander[Warrior]
president.delegate(warCommander)   // OK

Scala 类型差异

Scala type variance

scala

covariance

contravariance

variance

invariance