scala - 使用方法丰富 Scala 集合

Question

如何在 Scala 集合上添加 foreachWithIndex 方法？

到目前为止，这是我能想到的:

implicit def iforeach[A, CC <: TraversableLike[A, CC]](coll: CC) = new {
  def foreachWithIndex[B](f: (A, Int) => B): Unit = {
    var i = 0
    for (c <- coll) {
      f(c, i)
      i += 1
    }
  }
}

这不起作用:

Vector(9, 11, 34).foreachWithIndex { (el, i) =>
  println(el, i)
}

引发以下错误:

error: value foreachWithIndex is not a member of scala.collection.immutable.Vector[Int]
Vector(9, 11, 34).foreachWithIndex { (el, i) =>

但是，当我明确应用转换方法时，代码有效:

iforeach[Int, Vector[Int]](Vector(9, 11, 34)).foreachWithIndex { (el, i) =>
  println(el, i)
}

输出:

(9,0)
(11,1)
(34,2)

如何在不显式应用转换方法的情况下使其工作？谢谢。

Answer 1

您需要扩展 Iterable:

class RichIter[A, C](coll: C)(implicit i2ri: C => Iterable[A]) {
    def foreachWithIndex[B](f: (A, Int) => B): Unit = {
    var i = 0
    for (c <- coll) {
      f(c, i)
      i += 1
    }
  }
}

implicit def iter2RichIter[A, C[A]](ca: C[A])(
    implicit i2ri: C[A] => Iterable[A]
): RichIter[A, C[A]] = new RichIter[A, C[A]](ca)(i2ri)

Vector(9, 11, 34) foreachWithIndex {
  (el, i) => println(el, i)
}

输出:

(9,0)
(11,1)
(34,2)

有关更多信息，请参阅 this post by Rex Kerr。