NonEmptyList

NonEmptyList.scala

object Main {
  // NonEmpty because the constructor forces you to provide at least one
  // instance of A.
  case class NonEmptyList[A](a: A, as: List[A]) {
    def map[B](f: A => B): NonEmptyList[B] = {
      NonEmptyList[B](f(a), as.map(f))
  }

    def flatMap[B](f: A => NonEmptyList[B]): NonEmptyList[B] = flatten(map(f))
  }

  def flatten[A](nel: NonEmptyList[NonEmptyList[A]]): NonEmptyList[A] = {
    val NonEmptyList(NonEmptyList(h, hs), as) = nel
    val bs = as.flatMap{case NonEmptyList(x, xs) => x :: xs}
    NonEmptyList(h, hs ++  bs)
  }

  def run(): Unit = {
    val original = NonEmptyList(1, List(2,3,4))
    // Multiply all elements by 2.
    val doubled = original.map(x => x * 2)
    // Use flatMap to duplicate each element.
    val duplicated = original.flatMap(x => NonEmptyList(x, List(x)))

    println(s"Original: $original")
    println(s"Doubled: $doubled")
    println(s"Duplicated: $duplicated")
  }
}

Output:

Original: NonEmptyList(1,List(2, 3, 4))
Doubled: NonEmptyList(2,List(4, 6, 8))
Duplicated: NonEmptyList(1,List(1, 2, 2, 3, 3, 4, 4))