斯卡拉类型差异

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A协变，它会出错，因为那样我可以说

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

这非常令人困惑，所以我可以接受。

那么在这一点上，如何删除最后一个错误，让编译器明白Warrior是Person呢？

Answer 1

如果你的目的是让某种类型的A能够委托给A和所有类型B，它们是A子类型，那么你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