left outer join

gistfile1.scala

package experiments

object LeftOuterJoinMap {
	
	def main(args: Array[String]) {
		
		val map1 = Map(1 -> "a", 2 -> "b", 3 -> "c")
		val map2 = Map(1 -> "1", 3 -> "3")

		val joined = (new LeftOuterJoinMap(map1, map2)).join
		
		println(joined)
	}

}

class LeftOuterJoinMap[K, V1, V2](map1: Map[K, V1], map2: Map[K, V2]) {
	
	type JoinValue = Tuple2[Option[V1], Option[V2]]
	
	def join: Map[K, JoinValue] = {

		map1.map {case (k1, v1) =>
			map2.get(k1) match {
				case v2: Some[V2] => (k1 -> (Some(v1), v2))
				case None => (k1 -> (Some(v1), None))
			}
		}
	}
	
}

Posted on scala-user. I got a more direct and shorter code.

map1.map {case (k, v) => (k -> (Some(v), map2.get(k)))}