如何编写一个返回与传递给它的集合类型相同的集合类型的 zipWith 方法? [英] How to write a zipWith method that returns the same type of collection as those passed to it?

问题描述

我已经走到这一步了:

implicit def collectionExtras[A](xs: Iterable[A]) = new {
  def zipWith[B, C, That](ys: Iterable[B])(f: (A, B) => C)(implicit cbf: CanBuildFrom[Iterable[A], C, That]) = {
    val builder = cbf(xs.repr)
    val (i, j) = (xs.iterator, ys.iterator)
    while(i.hasNext && j.hasNext) {
      builder += f(i.next, j.next)
    }
    builder.result
  }
}
// collectionExtras: [A](xs: Iterable[A])java.lang.Object{def zipWith[B,C,That](ys: Iterable[B])(f: (A, B) => C)(implicit cbf: scala.collection.generic.CanBuildFrom[Iterable[A],C,That]): That}

Vector(2, 2, 2).zipWith(Vector(4, 4, 4))(_ * _)
// res3: Iterable[Int] = Vector(8, 8, 8)

现在的问题是上面的方法总是返回一个Iterable.如何让它返回传递给它的类型集合?(在这种情况下，Vector)谢谢.

Now the problem is that above method always returns an Iterable. How do I make it return the collection of type as those passed to it? (in this case, Vector) Thanks.

推荐答案

你已经足够接近了.只是两行的小改动:

You got close enough. Just a minor change in two lines:

implicit def collectionExtras[A, CC[A] <: IterableLike[A, CC[A]]](xs: CC[A]) = new {
  def zipWith[B, C, That](ys: Iterable[B])(f: (A, B) => C)(implicit cbf: CanBuildFrom[CC[A], C, That]) = {
    val builder = cbf(xs.repr)
    val (i, j) = (xs.iterator, ys.iterator)
    while(i.hasNext && j.hasNext) {
      builder += f(i.next, j.next)
    }
    builder.result
  }
}

首先需要获取被传递的集合类型，所以我添加了CC[A]作为类型参数.此外，该集合需要能够复制"自身——这是由 IterableLike 的第二个类型参数保证的——所以 CC[A] <: IterableLike[A, CC[A]].请注意，IterableLike 的第二个参数是 Repr，正是 xs.repr 的类型.

First, you need to get the collection type being passed, so I added CC[A] as a type parameter. Also, that collection needs to be able to "reproduce" itself -- that is guaranteed by the second type parameter of IterableLike -- so CC[A] <: IterableLike[A, CC[A]]. Note that this second parameter of IterableLike is Repr, precisely the type of xs.repr.

自然，CanBuildFrom 需要接收CC[A] 而不是Iterable[A].仅此而已.

Naturally, CanBuildFrom needs to receive CC[A] instead of Iterable[A]. And that's all there is to it.

结果:

scala> Vector(2, 2, 2).zipWith(Vector(4, 4, 4))(_ * _)
res0: scala.collection.immutable.Vector[Int] = Vector(8, 8, 8)

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