gistfile1.scala

def transformKeys[A,B,C](m: Map[A, B], transformFunc: (A => C)): Map[C, B] = {
  m.foldLeft(Map.empty[C, B]) { (acc, entry) =>
    acc + (transformFunc(entry._1) -> entry._2)
  }
}

val numMap: Map[Int, String] = Map(1 -> "Two", 2 -> "Four", 3 -> "Six")
val transformed = transformKeys(numMap, { (key:Int) => key * 2 })

// without (key:Int) = error: missing parameter type