Implement Haskell's insertWith for Scala Map types.

insertWith.scala

  implicit class RichMap[K, +V](private val m: Map[K, V]) extends AnyVal {
    /**
     * Based on Haskell's insertWith for Map types.
     * If the provided key already exists in the map, the value inserted will be f(newValue, oldValue).
     */
    def insertWith[V1 >: V](k: K, v: V1, f: (V1, V1) => V1): Map[K, V1] = {
      m.updated(k, m.get(k).map(f(v, _)).getOrElse(v))
    }
  }