Scala自我类型和collection.issue中的this.type [英] Scala self type and this.type in collections issue

问题描述

我正在尝试把自己的头放在scala中的抽象和显式自身类型上. 让我们考虑这个例子: 我想像这样简单地为可扩展树创建基础:

I'm trying to wrap my head around abstract and explicit self types in scala. Lets consider this example: I want to create a base for extensible tree as simple as this:

trait Tree {
  def children: Iterable[Tree]
  def descendants: Iterable[Tree] = { val dv = children.view; dv ++ (dv.flatMap { _.children }) }
}

但是，我希望能够使用某些方法扩展树节点并使用以下方法，例如:tree.children foreach { _.newMethod() }

However, I want to be able to extend tree nodes with some methods and use these methods like: tree.children foreach { _.newMethod() }

为此，我已经尝试过:

A.此类型:失败

trait Tree {
    def children: Iterable[this.type] 
    def descendants: Iterable[this.type] = {
      val dv = children.view
      // FAIL: type mismatch;  found   :  scala.collection.IterableView[com.abovobo.data.Tree,Iterable[_]]  required: Iterable[Tree.this.type] 
      // dv ++ (dv.flatMap { _.children })
      // OK: 
      dv.++[this.type, Iterable[this.type]](dv.flatMap[this.type, Iterable[this.type]]{ _.children })
    }
}

工作变体非常笨拙.

B.抽象类型:失败

trait Tree {
    type Node <: Tree

    def children: Iterable[Node]  
    def descendants: Iterable[Node] = {
        val dv = children.view
        // FAIL: type mismatch;  found   : scala.collection.IterableView[com.abovobo.data.Tree#Node,Iterable[_]]  required: Iterable[Tree.this.Node] 
        dv ++ (dv.flatMap { _.children })
    }
}

由于我所理解的路径特定类型不匹配，因此根本无法工作.

Doesn't work at all due to path specific type mismatch as I understood.

C.键入params(泛型):确定

C. Type params (generics): OK

trait Tree[+Node <: Tree[Node]] {

    def children: Iterable[Node]

    def descendants: Iterable[Node] = {
       val dv = children.view
       dv ++ (dv.flatMap { _.children })
    }
}

可以，但是在派生类中维护不太好.

Works OK, but not so good to maintain in derived classes.

有什么想法可以使前两个变体在没有大量代码的情况下工作?

Any ideas how to make first two variants working without a tons of code?

此外，使用this.type我在实现方面遇到了问题.

Also, with this.type I've run into problems with implementation.

trait BiDTree extends Tree {
    def parent: Option[this.type]
}

// how to accept this param? Option[TreeImpl] doesn't work. 
class TreeImpl(val parent: Option[???]) extends BiDTree {
  // ...
}

谢谢！

推荐答案

最后，我已经解决了本次讨论中提出的建议

At the end I've settled with what was proposed in this discussion http://www.scala-lang.org/node/6649:

trait Tree[+Node <: Tree[Node]] {
    this: Node =>

    def children: Iterable[Node]

    def descendants: Iterable[Node] = {
       val dv = children.view
       dv ++ (dv.flatMap { _.children })
    }
}  

即变体(C)，但具有明确的自我类型.这提供了在其他方法(例如方法find(path: String): Option[Node])中使用this的机会.

I.e. variant (C) but with explicit self type. This gives a chance to use this in other methods (say, method find(path: String): Option[Node]).

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