具有路径依赖类型和协变参数的Scala复制类
具有路径依赖类型和协变参数的Scala复制类
提问于 2018-06-08 03:58:15
我在路径依赖类型和协变参数方面遇到了一些问题。我需要创建一个新的SomeFancyCollection实例，但由于依赖于路径的类型，它无法编译。当我从类中取出像Node和Branch这样的东西时，我必须将root参数声明为privatethis，而且我也不能获得类的新实例。我的代码看起来像这样：
class SomeFancyCollection[+A] {
private var root: Node = Branch(0x0, Vector[Node]())
trait Node {
val id: Byte
def put[A1 >: A](ids: Seq[Byte], item: A1): Node
def remove[A1 >: A](ids: Seq[Byte], item: A1): Node
}
case class Branch(id: Byte, subs: Vector[Node]) extends Node {
...
}
case class Leaf[A1 >: A](id: Byte, subs: Vector[A1]) extends Node {
...
}
def add[A1 >: A](item: A1): SomeFancyCollection[A1] = {
val ids: Seq[Byte] = getIds() // doesn't matter
val newRoot = root.put(ids, item)
val newInstance = new SomeFancyCollection[A1]()
newInstance.root = newRoot
newInstance
}
}回答 1
Stack Overflow用户
回答已采纳
发布于 2018-06-08 06:35:34
在您的代码中，没有明显的理由使Node和Leaf嵌套路径依赖类。只需使它们成为协变的独立类，那么路径依赖的所有问题就会消失：
trait Node[+A] {
val id: Byte
def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
}
case class Branch[+A](id: Byte, subs: Vector[Node[A]]) extends Node[A] {
def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
}
case class Leaf[+A](id: Byte, subs: Vector[A]) extends Node[A] {
def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
}
class SomeFancyCollection[+A](val root: Node[A] = Branch(0x0, Vector[Node[A]]())) {
def add[A1 >: A](item: A1): SomeFancyCollection[A1] = {
val ids: Seq[Byte] = ???// getIds() // doesn't matter
val newRoot = root.put(ids, item)
new SomeFancyCollection[A1](newRoot)
}
}如果不想污染名称空间，只需将Node类声明为package-private，或者甚至将所有这些辅助实现细节类隐藏在SomeFancyCollection的配套对象中
class SomeFancyCollection[+A] private[SomeFancyCollection](
val root: SomeFancyCollection.AnnoyingDetails.Node[A]
) {
def add[A1 >: A](item: A1): SomeFancyCollection[A1] = {
val ids: Seq[Byte] = ???// getIds() // doesn't matter
val newRoot = root.put(ids, item)
new SomeFancyCollection[A1](newRoot)
}
}
object SomeFancyCollection {
def empty[A]: SomeFancyCollection[A] = new SomeFancyCollection[A](
AnnoyingDetails.Branch(0x0, Vector[AnnoyingDetails.Node[A]]())
)
private[SomeFancyCollection] object AnnoyingDetails {
trait Node[+A] {
val id: Byte
def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1]
}
case class Branch[+A](id: Byte, subs: Vector[Node[A]]) extends Node[A] {
def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
}
case class Leaf[+A](id: Byte, subs: Vector[A]) extends Node[A] {
def put[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
def remove[A1 >: A](ids: Seq[Byte], item: A1): Node[A1] = ???
}
}
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/50749309
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