Static typed Safe Dictionary

Task: Map [String] -> [String]

I need to map String keys to some other String keys [String] -> [String]. I have some defined keys and I want to map them to different keys. This could be a StoryboardIds, some actions from server etc.

As example let's say we have ["Play", "Stop"] keys and we need to map them to ["1", "2"].
The fist idea is to use Dictionary.

var keys = [
  "Play" : "1",
  "Stop" : "2"
]

Ok this works, But. There is problem with this solution. The keys in the dictionary are not static typed. You can make a Typo or use whatever string value you want for keys.

var keys = [
  "Plau" : "1",
  "Puppies" : "Puppies_HACK_THE_APP" // I want a Compiler ERROR! No Puppies here
]

keys["Puppies"]

That's not what we wanted. Can we do it better? Yes we can. We can use enum

enum Actions : String {
  case Play = "Play"
  case Stop = "Stop"
}
// In Swift 2.0 it's even better, no need to define string raw values. 😍 Swift 2.0

let items: [Actions: String] = [
  .Play : "1",
  .Stop : "2",
//  .Puppies : "Puppies_HACK_THE_APP" // - Error, yes :)

]

Ok now we have a static typed safe dictionary of Actions and its related keys [Actions: String], but we wanted [String : String] mapping. What we need to do is map every Action key to its raw String value.

let safeKeys =  items.map { key in key.rawValue }

[Actions: String] doest not have a member named 'map'

Ok, we see an error that dictionary doesn't have a map method. We can fix it by adding it.

extension Dictionary {

  init(_ elements: [Element]){
    self.init()
    for (k, v) in elements {
      self[k] = v
    }
  }

  func map<U>(transform: Key -> U) -> [U : Value] {
    return Dictionary<U, Value>(Swift.map(self, { (key, value) in
      (transform(key), value) }))
  }
}

This is it. Now it works. Here is a Gist with code.
This solution should work not only for [String] -> [String] mapping but for every value you want.