Capabilities of foldLeft of Scala

FoldLeft.scala

object FoldLeft extends App {

  def reverse(list: List[Int]): List[Int] =
    list.foldLeft[List[Int]](Nil)((acc, element) => element :: acc)

  def dedupe(list: List[Int]): List[Int] = {
    list.foldLeft[List[Int]](Nil)((acc, element) => if (acc.contains(element)) acc else acc :+ element)
  }

  def span(list: List[Int], p1: Int => Boolean, p2: Int => Boolean, p3: Int => Boolean, p4: Int => Boolean): (List[Int], List[Int], List[Int], List[Int]) =
    list.foldLeft[(List[Int], List[Int], List[Int], List[Int])]((Nil, Nil, Nil, Nil)) { case ((p1List, p2List, p3List, p4List), element) =>
      (
        if (p1(element)) p1List :+ element else p1List,
        if (p2(element)) p2List :+ element else p2List,
        if (p3(element)) p3List :+ element else p3List,
        if (p4(element)) p4List :+ element else p4List
      )
    }

  def combine(list1: List[Int], list2: List[Int]) =
    list2.foldLeft[List[Int]](list1)((acc, element) => acc :+ element)

  def zip(list1: List[Int], list2: List[String]): List[(Int, String)] =
    if (list1.length > list2.length) {
      list2.foldLeft[(List[Int], List[(Int, String)])]((list1, Nil)) {
        case ((list, zip), element) => (list.tail, zip :+ (list.head, element))
      }._2
    } else {
      list1.foldLeft[(List[String], List[(Int, String)])]((list2, Nil)) {
        case ((list, zip), element) => (list.tail, zip :+ (element, list.head))
      }._2
    }

  def unzip(list: List[(Int, String)]): (List[Int], List[String]) =
    list.foldLeft[(List[Int], List[String])]((Nil, Nil)) {
      case ((list1, list2), (number, str)) => (list1 :+ number, list2 :+ str)
    }


  def sort(list: List[Int]): List[Int] =
    list.foldLeft[List[Int]](Nil) { (acc, element) => insert(element, acc) }

  def insert(elem: Int, list: List[Int]): List[Int] =
    list match {
      case head :: tail => if (head <= elem) head :: insert(elem, tail) else elem :: list
      case Nil => elem :: Nil
    }

  def sum(list: List[Int]): Int = list.foldLeft(0)(_ + _)


  def map(list: List[Int], f: Int => Int): List[Int] =
    list.foldLeft[List[Int]](Nil)((acc, element) => acc :+ f(element))


  def flatMap(list: List[Int], f: Int => List[Int]): List[Int] =
    list.foldLeft[List[Int]](Nil)((acc, element) => acc ++ f(element))

  println(" ----" + flatMap(List(2, 3, 4, 4, 1), a => List(a * a)))
}