Scala Map.mapValues stackoverflowerror

Question

The following code results in a StackOverflowError on the last line.

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)
}

In Scala 2.11.2:

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)
...

Looking at the source for MapLike I can see it's using a MappedValues object, which seems like a view:

  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)

What's the best way to make it actually map the values?

Answer 1

mapValues is known to be a trap, as it indeed creates only a wrapping view function instead of eagerly producing a new collection. Therefore, in your example you create a data structure of nesting level 10,000.

You can use the regular map method:

acc.map(tup => (tup._1, tup._2 + 1))

or

acc.map { case (key, value) => (key, value + 1) }