Scala: map operation. Didactic.

map_using.scala

// An operation for each of the elements in a list
def scaleList(xs: List[Double], factor: Double) = xs match {
  case Nil     => xs
  case y :: ys => y * factor :: scaleList(ys, factor)
}

// A simple way to define map
// In reality it's more complicated because:
// * It works for arbitrary collections, not only lists
// * Its tail recursive
abstract class List[T] { ...
  def map[U](f: T => U): List[U] = this match {
    case Nil     => this
    case x :: xs => f(x) :: xs.map(f)
  }
}

// Back to scaleList
def scaleList(xs: List[Double], factor: Double) = xs.map(x => x * factor)