Scala 特征层次结构无法编译

Question

在我学习 Scala 的过程中，我创建了以下层次结构：

trait Animal {
  val name: String = "Animal"
}

trait HasLegs {
  this: Animal =>
  val numLegs: Int
}

abstract class Terrestrial extends Animal with HasLegs {
  val name: String = "Terrestrial"
}

class Dog(val name: String) extends Terrestrial {
  def numLegs = 4

  override def toString = {
    f"My name is: $name%s. I'm a Dog and I've $numLegs%d legs. I'm from the ${super.name}%s family."
  }
}

但是，编译器在 Dog.toString "super may be not be used on value name" 中抱怨。我做错了什么？

Answer 1

有几个问题，主要是由于在超类中使用 val，它将其锁定为一个值，而不是可以通过 super 覆盖和访问的东西。没有指定 override:

也有一些问题

trait Animal {
  // in order for super to be called on this it has to be a def
  def name: String = "Animal"
}

trait HasLegs {
  this: Animal =>
  val numLegs: Int
}

abstract class Terrestrial extends Animal with HasLegs {

  // this needs to be an override of Animal.name
  override def name: String = "Terrestrial"
}

class Dog(override val name: String) extends Terrestrial {

  // since the trait specified this as a val, the implementation
  // also needs to be a val
  val numLegs = 4

  override def toString = {
    f"My name is: $name%s. I'm a Dog and I've $numLegs%d legs. I'm from the ${super.name}%s family."
  }
}

我对内联更改发表了评论。

至于super不能用在val上的原因见SI-899

This is the intended behavior, and it was changed so that traits could override vals to be more uniform with classes.

Scala 特征层次结构无法编译

Scala traits hierarchy doesn't compile

scala

inheritance

traits