compact.scala

case class StringList(xs: List[String]) {
  def prepend(element: String) = StringList(element :: xs)
} 

val xs = List(1, 2, 3, "aa", "bb", "cc", 4, 5.5, 7, "dd", "ee", "ff", "gg", 8, 9.2, 10)

val r = xs.foldRight(List.empty[Any]) {
  case (current: String, compactedList @ ((strList: StringList) :: xs)) => strList.prepend(current) :: xs
  case (current: String, compactedList @ (_ :: xs)) => new StringList(current :: Nil) :: compactedList
  case (current: Int, compactedList) => current :: compactedList
  case (current, compactedList) => compactedList
} 

println(r)