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

This can be made even simpler:

scala> :paste
// Entering paste mode (ctrl-D to finish)

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
}

// Exiting paste mode, now interpreting.

defined trait List
defined class Cons
defined object Nil

scala> Cons(1, Nil).head.x
res0: Int = 1

scala> Nil.head
res1: None.type = None

Some#x: scala/scala#5275