Map a single entry of a Map

Question

I want to achieve something like the following:

(_ : Map[K,Int]).mapKey(k, _ + 1)

And the mapKey function applies its second argument (Int => Int) only to the value stored under k. Is there something inside the standard lib? If not I bet there's something in Scalaz.

Of course I can write this function myself (m.updated(k,f(m(k))) and its simple to do so. But I've come over this problem several times, so maybe its already done?

For Scalaz I imagine something along the following code:

(m: Map[A,B]).project(k: A).map(f: B => B): Map[A,B]

Answer 1

You could of course add

def changeForKey[A,B](a: A, fun: B => B): Tuple2[A, B] => Tuple2[A, B] = { kv =>
  kv match {
    case (`a`, b) => (a, fun(b))
    case x => x
  }
}

val theMap = Map('a -> 1, 'b -> 2)
theMap map changeForKey('a, (_: Int) + 1)
res0: scala.collection.immutable.Map[Symbol,Int] = Map('a -> 2, 'b -> 2)

But this would circumvent any optimisation regarding memory re-use and access.

I came also up with a rather verbose and inefficient scalaz solution using a zipper for your proposed project method:

theMap.toStream.toZipper.flatMap(_.findZ(_._1 == 'a).flatMap(elem => elem.delete.map(_.insert((elem.focus._1, fun(elem.focus._2)))))).map(_.toStream.toMap)

or

(for {
  z <- theMap.toStream.toZipper
  elem <- z.findZ(_._1 == 'a)
  z2 <- elem.delete
} yield z2.insert((elem.focus._1, fun(elem.focus._2)))).map(_.toStream.toMap)

Probably of little use. I’m just posting for reference.

Answer 2

Here is one way:

scala> val m = Map(2 -> 3, 5 -> 11)
m: scala.collection.immutable.Map[Int,Int] = Map(2 -> 3, 5 -> 11)

scala> m ++ (2, m.get(2).map(1 +)).sequence
res53: scala.collection.immutable.Map[Int,Int] = Map(2 -> 4, 5 -> 11)

scala> m ++ (9, m.get(9).map(1 +)).sequence
res54: scala.collection.immutable.Map[Int,Int] = Map(2 -> 3, 5 -> 11)

This works because (A, Option[B]).sequence gives Option[(A, B)]. (sequence in general turns types inside out. i.e. F[G[A]] => [G[F[A]], given F : Traverse and G : Applicative.)

Answer 3

You can pimp it with this so that it creates a new map based on the old one:

class MapUtils[A, B](map: Map[A, B]) {
  def mapValueAt(a: A)(f: (B) => B) = map.get(a) match {
    case Some(b) => map + (a -> f(b))
    case None => map
  }
}

implicit def toMapUtils[A, B](map: Map[A, B]) = new MapUtils(map)

val m = Map(1 -> 1)
m.mapValueAt(1)(_ + 1)
// Map(1 -> 2)
m.mapValueAt(2)(_ + 1)
// Map(1 -> 1)