不可变的Scala Map实现，保留插入顺序 [英] Immutable Scala Map implementation that preserves insertion order

问题描述

LinkedHashMap 用于在地图中保留插入顺序，但这仅适用于可变地图。

LinkedHashMap is used to preserve insertion order in the map, but this only works for mutable maps. Which is the immutable Map implementation that preserves insertion order?

推荐答案

a href =http://www.scala-lang.org/api/current/scala/collection/immutable/ListMap.html =noreferrer> ListMap 使用基于列表的方式实现不可变的地图数据结构，从而保留插入顺序。

ListMap implements an immutable map using a list-based data structure, and thus preserves insertion order.

scala> import collection.immutable.ListMap
import collection.immutable.ListMap

scala> ListMap(1 -> 2) + (3 -> 4)
res31: scala.collection.immutable.ListMap[Int,Int] = Map(1 -> 2, 3 -> 4)

scala> res31 + (6 -> 9)
res32: scala.collection.immutable.ListMap[Int,Int] = Map(1 -> 2, 3 -> 4, 6 -> 9)

以下扩展方法 - Seq＃toListMap 可以是在使用 ListMap s时非常有用。

The following extension method - Seq#toListMap can be quite useful when working with ListMaps.

scala> import scalaz._, Scalaz._, Liskov._
import scalaz._
import Scalaz._
import Liskov._

scala> :paste
// Entering paste mode (ctrl-D to finish)

implicit def seqW[A](xs: Seq[A]) = new SeqW(xs)
class SeqW[A](xs: Seq[A]) {
  def toListMap[B, C](implicit ev: A <~< (B, C)): ListMap[B, C] = {
    ListMap(co[Seq, A, (B, C)](ev)(xs) : _*)  
  }
}


// Exiting paste mode, now interpreting.

seqW: [A](xs: Seq[A])SeqW[A]
defined class SeqW

scala> Seq((2, 4), (11, 89)).toListMap
res33: scala.collection.immutable.ListMap[Int,Int] = Map(2 -> 4, 11 -> 89)

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