http://www.nimrodstech.com/scala-map-merge/

Need to merge multiple maps by their keys and make manipulation on the values? this post is probably for you!
Why ?previous experience with map reduce, many times data collected was in a Map structure, found myself repeatedly summing/appending/listing/manipulating maps

Assuming we have 2 maps with names of people as the key and the amount of them as the value :

 //each map contains count of people names
 val names = Map("Sidney" -> 1, "Paul" -> 1, "Jacob" -> 7)
 val moreNames = Map("Sidney" -> 1, "Paul" -> 5, "Nick" -> 2)

For some reason we need to merge the 2 maps into 1 expecting to sum the values and get all keys.

 //need to merge them to 1 map that sums all people names
 //expecting : Map("Sidney" -> 2, "Paul" -> 6, "Nick" -> 2, "Jacob" -> 7)

Lets write our own implementation for this task :

 //note ++ on maps will get all keys with second maps values if exists, if not first map values.
 val mergedMap = names ++ moreNames.map {
    case (name,count) => name -> (count + names.getOrElse(name,0)) }

Next task: lets merge the maps but instead of summing the values lets list them

 //expecting : Map("Sidney" -> List(1,1), "Paul" -> List(5,1), "Nick" -> List(2), "Jacob" -> List(7))
 val mergedMap2 = (names.toSeq ++ moreNames.toSeq)
  .groupBy{case(name,amount) => name}
  .mapValues(person => person.map{ case(name,amount) => amount}.toList)

Ok that was nice, but can be achieved much easier using Scalaz semigroups

//in your projects dependencies
val scalazDep = Seq (org.scalaz %% scalaz-core % 7.0.6)

 //now lets use scalaz Semigroup concept
 //lets merge maps by key and sum the values !
 import scalaz.Scalaz._
 val mergedMap3 = names |+| moreNames
 //lets merge maps by key and list the values
 val mergedMap4 = names.map(p => p._1 -> List(p._2)) |+| moreNames.map(p => p._1 -> List(p._2))

Main idea with using scalaz |+| is to arrange the data structure to what you need to do. in case you need to sum values a int will do the job. case you need to list the values a list of Any will do the trick.
This example really comes in handy when used in reduce functions, resulting in clean and short code (if you aren’t a scalaz hater)

 //each map contains count of people names
 val names = Map("Sidney" -> 1, "Paul" -> 1, "Jacob" -> 7)
 val moreNames = Map("Sidney" -> 1, "Paul" -> 5, "Nick" -> 2)
 //Money time!
 val test = List(names , moreNames).reduce(_ |+| _)