Why no mapKeys in Scala?

Question

The Scala Collection library has mapValues and filterKeys. The reason it doesn't have mapKeys is likely the performance aspect (with regard to HashMap implementation), as discussed here for Haskell: Why there's not mapKeys in Data.Hashmap?

However.

Performance implications aside, I find myself needing mapKeys at least as much as mapValues, simply for massaging data (i.e. I'm using a map for data abstraction, not for its fetch speed).

Am I wrong, and which data model would you use for this? Tuples?

Answer 1

No idea why it's not in standard library, but you can easily pimp your library with implicit class

  implicit class MapFunctions[A, B](val map: Map[A, B]) extends AnyVal {
    def mapKeys[A1](f: A => A1): Map[A1, B] = map.map({ case (a, b) => (f(a), b) })
  }

  val m = Map(1 -> "aaa", 2 -> "bbb")

  println(m.mapKeys(_ + 1))