7

假设我想向mapScala添加类似的功能List,类似于list mapmap f,它将函数应用于两次f的每个元素。list(一个更严重的例子可能是实现一个并行或分布式地图,但我不想被那个方向的细节分散注意力。)

我的第一种方法是

object MapMap {
    implicit def createFancyList[A](list: List[A]) = new Object {
        def mapmap(f: A => A): List[A] = { list map { a: A => f(f(a)) } }
    }
}

这现在很好用

scala> import MapMap._
import MapMap._

scala> List(1,2,3) mapmap { _ + 1 }
res1: List[Int] = List(3, 4, 5)

当然,它只适用于Lists,我们没有理由不希望它适用于任何东西Traverseable,具有map函数,例如Sets 或Streams。所以第二次尝试看起来像

object MapMap2 {
    implicit def createFancyTraversable[A](t: Traversable[A]) = new Object {
        def mapmap(f: A => A): Traversable[A] = { t map { a: A => f(f(a)) } }
    }
}

但是现在,当然,结果不能分配给 a List[A]

scala> import MapMap2._
import MapMap2._

scala> val r: List[Int] = List(1,2,3) mapmap { _ + 1 }
<console>:9: error: type mismatch;
 found   : Traversable[Int]
 required: List[Int]

有没有中间立场?我可以编写一个隐式转换,将方法添加到 Traversable 的所有子类,并成功返回该类型的对象吗?

(我猜这涉及理解可怕的CanBuildFrom特征,甚至可能breakout!)

4

2 回答 2

11

您不能对所有 Traversable 执行此操作,因为它们不保证 map 返回比 Traversable 更具体的任何内容。 请参阅下面的更新 2。

import collection.generic.CanBuildFrom
import collection.TraversableLike

class TraversableW[CC[X] <: TraversableLike[X, CC[X]], A](value: CC[A]) {
  def mapmap(f: A => A)(implicit cbf: CanBuildFrom[CC[A], A, CC[A]]): CC[A] 
      = value.map(f andThen f)
  def mapToString(implicit cbf: CanBuildFrom[CC[A], String, CC[String]]): CC[String]
      = value.map(_.toString)
}

object TraversableW {
  implicit def TraversableWTo[CC[X] <: TraversableLike[X, CC[X]], A](t: CC[A]): TraversableW[CC, A] 
      = new TraversableW[CC, A](t)
}

locally {
  import TraversableW._

  List(1).mapmap(1+)
  List(1).mapToString
  // The static type of Seq is preserved, *and* the dynamic type of List is also
  // preserved.
  assert((List(1): Seq[Int]).mapmap(1+) == List(3))
}

更新 我添加了另一个 pimped 方法,mapToString来演示为什么TraversableW接受两个类型参数,而不是像 Alexey 的解决方案中那样接受一个参数。参数CC是更高种类的类型,它代表原始集合的容器类型。第二个参数A表示原始集合的元素类型。因此,该方法mapToString能够返回具有不同元素类型的原始容器类型:CC[String.

更新 2 感谢@oxbow_lakes 评论,我重新考虑了这一点。确实可以直接 pimp CC[X] <: Traversable[X]TraversableLike不是严格需要的。内联评论:

import collection.generic.CanBuildFrom
import collection.TraversableLike

class TraversableW[CC[X] <: Traversable[X], A](value: CC[A]) {
  /**
   * A CanBuildFromInstance based purely the target element type `Elem`
   * and the target container type `CC`. This can be converted to a
   * `CanBuildFrom[Source, Elem, CC[Elem]` for any type `Source` by
   * `collection.breakOut`.
   */
  type CanBuildTo[Elem, CC[X]] = CanBuildFrom[Nothing, Elem, CC[Elem]]

  /**
   * `value` is _only_ known to be a `Traversable[A]`. This in turn
   * turn extends `TraversableLike[A, Traversable[A]]`. The signature
   * of `TraversableLike#map` requires an implicit `CanBuildFrom[Traversable[A], B, That]`,
   * specifically in the call below `CanBuildFrom[Traversable[A], A CC[A]`.
   *
   * Essentially, the specific type of the source collection is not known in the signature
   * of `map`.
   *
   * This cannot be directly found instead we look up a `CanBuildTo[A, CC[A]]` and
   * convert it with `collection.breakOut`
   *
   * In the first example that referenced `TraversableLike[A, CC[A]]`, `map` required a
   * `CanBuildFrom[CC[A], A, CC[A]]` which could be found.
   */
  def mapmap(f: A => A)(implicit cbf: CanBuildTo[A, CC]): CC[A]
      = value.map[A, CC[A]](f andThen f)(collection.breakOut)
  def mapToString(implicit cbf: CanBuildTo[String, CC]): CC[String]
      = value.map[String, CC[String]](_.toString)(collection.breakOut)
}

object TraversableW {
  implicit def TraversableWTo[CC[X] <: Traversable[X], A](t: CC[A]): TraversableW[CC, A]
      = new TraversableW[CC, A](t)
}

locally {
  import TraversableW._

  assert((List(1)).mapmap(1+) == List(3))

  // The static type of `Seq` has been preserved, but the dynamic type of `List` was lost.
  // This is a penalty for using `collection.breakOut`. 
  assert((List(1): Seq[Int]).mapmap(1+) == Seq(3))   
}

有什么不同?我们不得不使用collection.breakOut,因为我们无法从 . 中恢复特定的集合子类型Traversable[A]

def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
  val b = bf(repr)
  b.sizeHint(this) 
  for (x <- this) b += f(x)
  b.result
}

使用Builder b原始集合进行初始化,这是通过 . 保存动态类型的机制map。但是,我们通过类型参数否认了对FromCanBuildFrom的所有知识。你所能做的就是忽略它,这正是它的作用:NothingNothingbreakOut

def breakOut[From, T, To](implicit b : CanBuildFrom[Nothing, T, To]) =
  new CanBuildFrom[From, T, To] {
    def apply(from: From) = b.apply();
    def apply() = b.apply()
  }

我们不能打电话b.apply(from),就像你不能打电话一样def foo(a: Nothing) = 0

于 2010-07-12T05:43:26.197 回答
5

作为一般规则,当您想要返回具有相同类型的对象时,您需要TraversableLike( IterableLike,SeqLike等) 而不是Traversable. 这是我能想到的最通用的版本(单独的FancyTraversable类是为了避免推断结构类型和反射命中):

class FancyTraversable[A, S <: TraversableLike[A, S]](t: S) {
  def mapmap(f: A => A)(implicit bf: CanBuildFrom[S,A,S]): S = { t map { a: A => f(f(a)) } }
}

implicit def createFancyTraversable[A, S <: TraversableLike[A, S]](t: S): FancyTraversable[A, S] = new FancyTraversable(t)
于 2010-07-12T05:30:32.473 回答