S99 P07 http://aperiodic.net/phil/scala/s-99/

S9907.scala

package scalaninetynine
/*
 * S99 P07 http://aperiodic.net/phil/scala/s-99/
 * 
 * Flatten a nested list structure.
 * 
 * Example:
 * scala> flatten(List(List(1, 1), 2, List(3, List(5, 8))))
 * res0: List[Any] = List(1, 1, 2, 3, 5, 8)
 * 
 * D. Holbrook 2012-01-28
 */

object S9907 extends App {

  //having lists with more than one type in them
  //isn't very idiomatic for Scala
  //here typesafe approach using Either
  def flatten[A](l: List[Either[List[A], A]]): List[A] = {
    l.foldLeft(List[A]()) {
      case (acc, Left(sublst)) => sublst.reverse ::: acc
      case (acc, Right(i))     => i :: acc
    }.reverse
  }

  def flattenAny(l: List[Any]): List[Any] = {
    val nl = l.map {
      case ils: List[_] => Left(ils)
      case i            => Right(i)
    }
    flatten(nl)
  }

  //typesafe
  val v = List(Left(List("a", "b", "c")), Right("d"), Right("e"), Right("f"), Left(List("g", "h", "i")))
  assert(flatten(v) == List("a", "b", "c", "d", "e", "f", "g", "h", "i"))

  //not typesafe
  val w = List(List("a", "b", "c"), "d", "e", "f", List("g", "h", "i"))
  assert(flattenAny(w) == List("a", "b", "c", "d", "e", "f", "g", "h", "i"))

}