将使用访问者的 Traversable 迁移到 Scala 2.13 中的 Iterable

Question

migration guide to Scala 2.13 解释说 Traversable 已被删除，应该使用 Iterable 代替。这种变化对于一个项目来说尤其烦人，它使用访问者在树的Node 类中实现foreach 方法：

case class Node(val subnodes: Seq[Node]) extends Traversable[Node] {
  override def foreach[A](f: Node => A) = Visitor.visit(this, f)
}

object Visitor {
  def visit[A](n: Node, f: Node => A): Unit = {
    f(n)
    for (sub <- n.subnodes) {
      visit(sub, f)
    }
  }
}

object Main extends App {
  val a = Node(Seq())
  val b = Node(Seq())
  val c = Node(Seq(a, b))
  for (Node(subnodes) <- c) {
    Console.println("Visiting a node with " + subnodes.length + " subnodes")
  }
}

输出：

Visiting a node with 2 subnodes
Visiting a node with 0 subnodes
Visiting a node with 0 subnodes

迁移到 Scala 2.13 的一个简单解决方法是首先将访问的元素存储在 remaining 缓冲区中，然后用于返回迭代器：

import scala.collection.mutable
import scala.language.reflectiveCalls

case class Node(val subnodes: Seq[Node]) extends Iterable[Node] {
  override def iterator: Iterator[Node] = {
    val remaining = mutable.Queue.empty[Node]
    Visitor.visit(this, item => iterator.remaining.enqueue(item))
    remaining.iterator
  }
}

// Same Visitor object
// Same Main object

这个解决方案的缺点是它引入了新的分配给 GC 带来压力，因为访问的元素的数量通常非常大。

您对如何使用现有访问者但不引入新分配从Traversable 迁移到Iterable 有什么建议？

Answer 1

如您所见，您需要扩展 Iterable 而不是 Traversable。你可以这样做：

case class Node(name: String, subnodes: Seq[Node]) extends Iterable[Node] {
  override def iterator: Iterator[Node] = Iterator(this) ++ subnodes.flatMap(_.iterator)
}

val a = Node("a", Seq())
val b = Node("b", Seq())
val c = Node("c", Seq(a, b))
val d = Node("d", Seq(c))

for (node@Node(name, _) <- d) {
  Console.println("Visiting node " + name + " with " + node.subnodes.length + " subnodes")
}

输出：

Visiting node d with 1 subnodes
Visiting node c with 2 subnodes
Visiting node a with 0 subnodes
Visiting node b with 0 subnodes

然后你可以做更多的操作如：

d.count(_.subnodes.length > 1)

代码在Scastie 运行。

Answer 2

这是一个示例，您的代码可以使用LazyList 实现并且不需要访问者：

case class Node(val subnodes: Seq[Node]) {
  
  def recursiveMap[A](f: Node => A): LazyList[A] = {
    def expand(node: Node): LazyList[Node] = node #:: LazyList.from(node.subnodes).flatMap(expand)
    expand(this).map(f)
  }
}

val a = Node(Seq())
val b = Node(Seq())
val c = Node(Seq(a, b))

val lazyList = c.recursiveMap { node =>
  println("computing value")
  "Visiting a node with " + node.subnodes.length + " subnodes"
}

println("started computing values")

lazyList.iterator.foreach(println)

输出

started computing values
computing value
Visiting a node with 2 subnodes
computing value
Visiting a node with 0 subnodes
computing value
Visiting a node with 0 subnodes

如果您自己不存储 lazyList 引用并且只存储迭代器，那么 JVM 将能够随时 GC 值。

Answer 3

我们最终编写了一个最小的Traversable trait，只实现了我们代码库中使用的方法。这样就没有额外的开销，也不需要改变访问者的逻辑。

import scala.collection.mutable

/** A trait for traversable collections. */
trait Traversable[+A] {
  self =>

  /** Applies a function to all element of the collection. */
  def foreach[B](f: A => B): Unit

  /** Creates a filter of this traversable collection. */
  def withFilter(p: A => Boolean): Traversable[A] = new WithFilter(p)

  class WithFilter(p: A => Boolean) extends Traversable[A] {
    /** Applies a function to all filtered elements of the outer collection. */
    def foreach[U](f: A => U): Unit = {
      for (x <- self) {
        if (p(x)) f(x)
      }
    }

    /** Further refines the filter of this collection. */
    override def withFilter(q: A => Boolean): WithFilter = {
      new WithFilter(x => p(x) && q(x))
    }
  }

  /** Finds the first element of this collection for which the given partial
    * function is defined, and applies the partial function to it.
    */
  def collectFirst[B](pf: PartialFunction[A, B]): Option[B] = {
    for (x <- self) {
      if (pf.isDefinedAt(x)) {
        return Some(pf(x))
      }
    }
    None
  }

  /** Builds a new collection by applying a partial function to all elements
    * of this collection on which the function is defined.
    */
  def collect[B](pf: PartialFunction[A, B]): Iterable[B] = {
    val elements = mutable.Queue.empty[B]
    for (x <- self) {
      if (pf.isDefinedAt(x)) {
        elements.append(pf(x))
      }
    }
    elements
  }
}