代码之家 › 专栏 › 技术社区 › olle kullberg

“MyType”问题:是否必须在Scala中使用抽象类型(或泛型)才能返回实际的类?

scala generics

olle kullberg · 技术社区 · 14 年前

trait Animal {
  val name: String
  val weight: Int

  type SubAnimal <: Animal

  def updateName(n: String) = returnMe(n, this.weight)
  def updateWeight(w: Int) = returnMe(this.name, w)
  // Abstract protected method
  protected def returnMe(n: String, w: Int): SubAnimal
}

case class Dog(name: String, weight: Int) extends Animal {
  type SubAnimal = Dog
  override def returnMe(n: String, w: Int): Dog = Dog("Dog: " + name, w)
}
case class Cat(name: String, weight: Int) extends Animal {
  type SubAnimal = Cat
  override def returnMe(n: String, w: Int): Cat = Cat("Cat: " + name, w)
}

val fido = Dog("Fido", 11)
println( fido )
val fido2 = fido.updateWeight(12)
println( fido2 )

当我运行代码时,会得到以下输出:

$ scala animal.scala 
Dog(Fido,11)
Dog(Dog: Fido,12)

我想在 updateName 或 updateWeight Animal ). 我知道如果我超越和直接返回正确的类型,而不必使用抽象类型 SubAnimal .

对于抽象类型的值是 ?

(这就是所谓的“MyType”问题)。

3 回复 | 直到 14 年前

Alex Cruise 14 年前

一些 recent discussion 关于这个话题。。。您要寻找的通常称为“MyType”,它的典型Scala/Java编码使用递归类型参数:

public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable { 
    public final int compareTo(E o) 
    // ... 
}

Landei 14 年前

这应该有效:

trait Animal[T] {
  self:T =>

  val name: String
  val weight: Int

  def updateName(n: String): T = returnMe(n, this.weight)
  def updateWeight(w: Int): T = returnMe(this.name, w)
  // Abstract protected method
  protected def returnMe(n: String, w: Int): T
}

case class Dog(name: String, weight: Int) extends Animal[Dog] {
  override def returnMe(n: String, w: Int): Dog = Dog("Dog: " + name, w)
}

case class Cat(name: String, weight: Int) extends Animal[Cat] {
   override def returnMe(n: String, w: Int): Cat = Cat("Cat: " + name, w)
}

有点像 case class Cat(name: String, weight: Int) extends Animal[Dog] 被编译器拒绝。代码偷改编自 http://oldfashionedsoftware.com/2009/12/10/self-help/

one-zero-zero-one 14 年前

trait Animal[A <: Animal[A]] {
  val name: String
  val weight: Int

  def updateName(n: String) = returnMe(n, this.weight)
  def updateWeight(w: Int) = returnMe(this.name, w)
  // Abstract protected method
  protected def returnMe(n: String, w: Int): A
}

case class Dog(name: String, weight: Int) extends Animal[Dog] {
  override def returnMe(n: String, w: Int) = Dog("Dog: " + name, w)
}

case class Cat(name: String, weight: Int) extends Animal[Cat] {
  override def returnMe(n: String, w: Int) = Cat("Cat: " + name, w)
}