How to merge a map of String to List functionally?

Question

I've built an abstraction such as this:

class Thing(val messages: Map[String, Seq[String]]) { 
  def and(that: Thing): Thing = {
    new Thing(this.messages ++ that.messages)
  }
}

this's map of String -> Seq[String] needs to be merged with that's map.

This is the best way I can think of to do this:

def and(that: Thing): Thing = {
  val keys = this.messages.keys ++ that.messages.keys
  val tuples = keys map {
    case key: String =>
      val theseMessages = this.messages.getOrElse(key, Seq[String]())
      val thoseMessages = that.messages.getOrElse(key, Seq[String]())
      (key, theseMessages ++ thoseMessages)
  }
  new Thing(tuples.toMap)
}

Possible duplicate of [this question](http://stackoverflow.com/questions/1262741/scala-how-to-merge-a-collection-of-maps). You can use the code provided in the accepted answerwith the combinator function being `{_ ++ _}` — Ratan Sebastian, Oct 21 '12 at 03:38

dhg · Answer 1 · 2012-10-21T03:43:40.463

3

Here's a version that uses groupBy:

def and(that: Thing): Thing = {
  new Thing(
    (this.messages.toIndexedSeq ++ that.messages)
      .groupBy(_._1)
      .map { case (k,vs) => k -> vs.flatMap(_._2) }
      .toMap)
}

edited Oct 21 '12 at 03:43

answered Oct 21 '12 at 03:37

dhg

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Not a bad answer, but maybe what I have is better? I think yours is harder to understand. – Scoobie Oct 21 '12 at 21:50

score 1 · Answer 2 · answered Oct 21 '12 at 09:59

1

The Scalaz library includes this function, the |+| (semigroup append) operator. Note that you will need to choose a more specific collection type than Seq, i.e. one that has a semigroup instance, so I've used List.

import scalaz._, Scalaz._

class Thing(val messages: Map[String, List[String]]) { 
  def and(that: Thing): Thing =
    new Thing(this.messages |+| that.messages)
}

You could also define a semigroup instance for Thing, and use the |+| operator to combine Things.

case class Thing(val messages: Map[String, List[String]])

implicit val ThingSemigroup = new Semigroup[Thing] {
  def append(t1: Thing, t2: => Thing) = new Thing(t1.messages |+| t2.messages)
}

Thing(Map("foo" -> List("bar"))) |+| Thing(Map("foo" -> List("baz")))
// Thing(Map(foo -> List(bar, baz)))

answered Oct 21 '12 at 09:59

Ben James

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`implicit val ThingSemigroup = semigroup[Thing]((x, y) => Thing(x.messages |+| y.messages))` is more concise – Luigi Plinge Oct 21 '12 at 10:30
I don't think that works in Scalaz 7, but `Semigroup.instance` looks like it does the same thing, thanks – Ben James Oct 21 '12 at 11:03
Crafty, but I can't pull in Scalaz – Scoobie Oct 21 '12 at 21:51

score 0 · Answer 3 · answered Oct 21 '12 at 03:45

A more specific, slightly modified version of the answer I linked to in the comment which might make the approach a little clearer:

def and(that: Thing): Thing = {
  new Thing(this.messages.foldLeft(that.messages) { case (out, (keyToAdd, valuesToAdd)) =>
    out ++ Map(keyToAdd -> out.get(keyToAdd).map(_ ++ valuesToAdd).getOrElse(valuesToAdd))
  })
}

Also has the advantage of not looping over one of the maps.

How to merge a map of String to List functionally?

3 Answers3