Scala Map.mapValues stackoverflower

object StackTest extends App{
    @tailrec def incrementValues(acc: Map[String,Int],inc: Int): Map[String,Int] = {
        if(inc == 0) acc
        else incrementValues(acc.mapValues(_ + 1),inc - 1)
    }

    val myMap = incrementValues(Map("key" -> 0),10000)
    myMap.foreach(println)
}

在Scala 2.11.2中：

Exception in thread "main" java.lang.StackOverflowError
at scala.collection.MapLike$MappedValues.foreach(MapLike.scala:245)
at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:777)
at scala.collection.MapLike$MappedValues.foreach(MapLike.scala:245)
at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:777)
at scala.collection.MapLike$MappedValues.foreach(MapLike.scala:245)
...

看看MapLike的源代码,我可以看到它使用的是MappedValues对象,它看起来像一个视图：

protected class MappedValues[C](f: B => C) extends AbstractMap[A,C] with DefaultMap[A,C] {
    override def foreach[D](g: ((A,C)) => D): Unit = for ((k,v) <- self) g((k,f(v)))
    def iterator = for ((k,v) <- self.iterator) yield (k,f(v))
    override def size = self.size
    override def contains(key: A) = self.contains(key)
    def get(key: A) = self.get(key).map(f)
  }

  /** Transforms this map by applying a function to every retrieved value.
   *  @param  f   the function used to transform values of this map.
   *  @return a map view which maps every key of this map
   *          to `f(this(key))`. The resulting map wraps the original map without copying any elements.
   */
  def mapValues[C](f: B => C): Map[A,C] = new MappedValues(f)

让它实际映射值的最佳方法是什么？