simple scala unionWith

Maps.scala

object Maps {
  import scalaz._, syntax.semigroup._

  def unionWithKey[A, K](f: (K, A, A) => A): (Map[K, A], Map[K, A]) => Map[K, A] =
    (m1, m2) =>
      (m1 -- m2.keySet) ++ m2.map {
        case (k, v) => k -> (m1 get k map { f(k, v, _) } getOrElse v)
      }

  def unionWith[K, A: Semigroup]: (Map[K, A], Map[K, A]) => Map[K, A] =
    unionWithKey { (_, a, b) => a |+| b }
}