Scala-抽象基類中的參數類型，可以擴展

Question

如何為抽象類中的方法提供參數，以便以后可以擴展該參數？ 我將在下面說明一個簡單的場景。

abstract class Car {
  def drive(x: Driver)
}

abstract class Driver
case class DriverA(name: String) extends Driver
case class DriverB(name: String, age: Int) extends Driver

class Audi extends Car {
  // each child class should have a more specific type for param "x"
  def drive(x: DriverA) = { ... }
}

class BMW extends Car {
  // each child class should have a more specific type for param "x"
  def drive(x: DriverB) = { ... }
}

但這在Scala中不起作用：“錯誤：[..]方法參數類型必須完全匹配”

我也嘗試指定像def drive [T <：Driver]（x：T）：Int這樣的上限類型，但是仍然沒有運氣。 似乎我缺少一些簡單而明顯的東西。

這樣的設計有什么問題嗎？

Answer 1

您可以在scala中使用抽象類型 ：

scala> :paste
// Entering paste mode (ctrl-D to finish)

abstract class Car {
  type D <: Driver
  def drive(x: D): Unit
}
abstract class Driver
case class DriverA(name: String) extends Driver
case class DriverB(name: String) extends Driver
class Audi extends Car {
  type D = DriverA
  def drive(x: DriverA): Unit = println(x.name)
}

// Exiting paste mode, now interpreting.

defined class Car
defined class Driver
defined class DriverA
defined class DriverB
defined class Audi

scala> new Audi().drive(DriverA("test"))
test

或使用通過類型參數化的類：

scala> :paste
// Entering paste mode (ctrl-D to finish)

abstract class Driver
case class DriverA(name: String) extends Driver
abstract class Car[D <: Driver] {
  def drive(x: D): Unit
}
class Audi extends Car[DriverA] {
  def drive(x: DriverA) = println(x.name)
}

// Exiting paste mode, now interpreting.

scala> new Audi().drive(DriverA("zzz"))
zzz