A class that wraps 2 maps in it and makes it look like a single map

LazyMap.scala

// this is unfinished, but shows the idea
// It's an answer to:
// http://stackoverflow.com/questions/25028214/merging-scala-maps#comment38926797_25028214

package com.izmailoff

class LazyMap[A, B, C, D](x: Map[A, B], y: Map[A, C], f: (Option[B], Option[C]) => Option[D])
  extends Map[A, D] {

  def get(key: A): Option[D] =
    f(x get key, None) orElse f(None, y get key)

  def iterator: Iterator[(A, D)] = // wrong implementation - should produce unique keys
    x.iterator.map { case (a, b) => a -> f(Some(b), None).get } ++
      y.iterator.map { case (a, b) => a -> f(None, Some(b)).get }

  def +[B1 >: D](kv: (A, B1)): Map[A, B1] = ???
    //new LazyMap(x + kv, y, f)

  def -(key: A): Map[A, D] =
    new LazyMap(x - key, y - key, f)
}


object LazyMapMerge extends App {

  val m1 = Map(1 -> "one", 2 -> "two")
  val m2 = Map(1 -> 1L, 2 -> 2L, 3 -> 3L)
  val m = new LazyMap(m1, m2, (first: Option[String], second: Option[Long]) => (first orElse second) map (_.toString))

  println("GET 1: " + m(1))
  println("ALL KEYS: " + m.keys)
  println("ALL VALUES: " + m.values)
}