循环类型参数定义在Scala中 [英] Circular type parameters definition in scala

问题描述

我试图定义一个通用容器，其元素可以返回封闭的容器。例如：

I am trying to define a generic container whose elements can return the enclosing container. Something like:

abstract class Container[E <: Element] { // compile error
  def contains( e: E ): Boolean
  def addNewElement(): Unit
}

abstract class Element[C <: Container] { // compile error
  def enclosingContainer(): C
}

class MyContainer extends Container[MyElement] {
  private var elements = List[MyElement]()
  override def contains( elem: MyElement ) = elements.contains( elem )
  override def addNewElement() { elements ::= new MyElement(this) }
}

class MyElement( container: MyContainer ) extends Element[MyContainer] {
  override val enclosingContainer = container
}

然而，没有编译，因为我应该在抽象类Container [E<：Element] 定义中为元素提供一个类型参数以及抽象类元素中的 Container 类型元素[C<：Container] 定义。

However that snippet does not compile because I should give a type parameter to Element in the abstract class Container[E <: Element] definition and a type to Container in the abstract class Element[C <: Container] definition.

我有一种方法可以实现我所寻找的行为吗？对于 Container 和 Element 是否有适当的声明？我应该定义第三方对象吗？

I there a way to achieve the behavior I am looking for ? Is there an appropriate declaration for Container and Element? Should I define a third-party object ?

推荐答案

其他解决方案未能强制实施类型匹配：给定类型 ContainerImpl extends Container ，您应该确保 ContainerImpl.EC 应该是 ContainerImpl 而不是其他容器。这是一个强制执行此操作（改编自 http：//programming-scala.labs.oreilly .com / ch13.html ）：

The other solutions already given fail to enforce that the types match: that is, given a type ContainerImpl extends Container, you should be sure that ContainerImpl.E.C should be ContainerImpl and not some other container. Here is one which does enforce this (adapted from http://programming-scala.labs.oreilly.com/ch13.html):

abstract class ContainerWithElement {
  type C <: Container
  type E <: Element

  trait Container {
    self: C =>
    def contains( e: E ): Boolean
    def addNewElement(): Unit
  }

  trait Element {
    self: E =>
    def enclosingContainer(): C
  }
}

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