Manipulating maps with Monoids

int_additions.scala

object MindBlown extends App {
  trait Monoid[A] {
    def op(a1: A, a2: A): A
    def zero: A
  }

  val intAddition = new Monoid[Int] {
    override def op(a1: Int, a2: Int) = a1 + a2
    override def zero = 0
  }

  def mapMergeMonoid[K, V](V: Monoid[V]): Monoid[Map[K, V]] = new Monoid[Map[K, V]] {
    def zero = Map[K, V]()
    def op(a: Map[K, V], b: Map[K, V]) =
      (a.keySet ++ b.keySet).foldLeft(zero) { (acc, k) =>
        acc.updated(k, V.op(a.getOrElse(k, V.zero),
          b.getOrElse(k, V.zero)))
      }
  }
  
  // Add numbers for the same keys on a map of maps
  val merging: Monoid[Map[String, Map[String, Int]]] = mapMergeMonoid(mapMergeMonoid(intAddition))
  val m1 = Map("o1" -> Map("i1" -> 1, "i2" -> 2))
  val m2 = Map("o1" -> Map("i2" -> 3))
  val m3 = Map("o1" -> Map("i2" -> 2))
  println(merging.op(merging.op(m1, m2), m3)) // Map(o1 -> Map(i1 -> 1, i2 -> 7))
}