Scala: Merge map

Using the standard lib, you can do it as follows:

// convert maps to seq, to keep duplicate keys and concat
val merged = Map(1 -> 2).toSeq ++ Map(1 -> 4).toSeq
// merged: Seq[(Int, Int)] = ArrayBuffer((1,2), (1,4))

// group by key
val grouped = merged.groupBy(_._1)
// grouped: scala.collection.immutable.Map[Int,Seq[(Int, Int)]] = Map(1 -> ArrayBuffer((1,2), (1,4)))


// remove key from value set and convert to list
val cleaned = grouped.mapValues(_.map(_._2).toList)
// cleaned: scala.collection.immutable.Map[Int,List[Int]] = Map(1 -> List(2, 4))