safe head/tail on lists using covariance

list.scala

// Initial idea courtesy of @dwijnand
// Improved by @fthomas

sealed trait List[+T] {
  def head: Option[T]
}

final case class Cons[T](h: T, t: List[T]) extends List[T] {
  val head: Some[T] = Some(h)
}

final case object Nil extends List[Nothing] {
  val head = None
}

// scala> Cons(1, Nil).head.x
// res0: Int = 1
// scala> Nil.head
// res1: None.type = None

Some#x: scala/scala#5275