How to merge 2 concurrent maps, given an associative function

Question

What is the best way to merge two maps, given an associative function when keys are present in both maps.

Here is my current implementation

private def mergeMap[A, B](map1: concurrent.Map[A, B], map2: concurrent.Map[A, B], f: (B, B) => B) : concurrent.Map[A, B] = {
  val keys = map1.keys ++ map2.keys
  val newMap = new TrieMap[A, B]()
  keys.foreach { k =>
    (map1.get(k), map2.get(k)) match {
      case (Some(v1), Some(v2)) => newMap.put(k, f(v1, v2))
      case (Some(v1), None) => newMap.put(k, v1)
      case (None, Some(v2)) => newMap.put(k, v2)
      case _ => // could not happen
    }
  }
  newMap
}

Answer 1

Is 'concurrent map' a requirement for some reason? If not you can easily use scalaz

Associative functions for keys is the same as Semigroup: http://eed3si9n.com/learning-scalaz/Functor+Laws.html#Semigroup+Laws

And when you have semigroup available Map becomes Monoid and you can merge them like this:

  import scala.collection.concurrent

  import scalaz._
  import Scalaz._

  val map1 = concurrent.TrieMap("a" -> 1, "b" -> 1)
  val map2 = concurrent.TrieMap("b" -> 1, "c" -> 1)

  val merge = map1.toMap |+| map2.toMap

  println(merge)

And result is:

  Map(c -> 1, b -> 2, a -> 1)

You can pretty easy write you own Monoid for concurrent map if you don't want convert them to immutable maps, but conversion is almost free, so I don't see why not to use it