Preconditioning (require) an uninitialized value's property in a Scala trait

Question

Given the below trait, instance a throws NullPointerException:

trait T {
  val l: List[Int]

  require(l.size > 1)
}

case class A(list: List[Int]) extends T {
  override val l: List[Int] = list
}

val a = A(List(1,2))

Based on this StackOverflow post, I tried the below variations for T:

trait T {
  def l(): List[Int]

  require(l().size > 1)
}

trait T {
  val l: List[Int]
  lazy val s: Int = l.size

  require(s > 1)
}

trait T {
  val l: List[Int]
  def s: Int = l.size

  require(s > 1)
}

but all of them gives NullPointerException.

Is there a way to use precondition (require clause) for an uninitialized value's property in a trait or do I have to copy the precondition(s) to all classes implementing the trait?

Answer 1

You can easily do this as follow:

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

trait T {
  def l: List[Int]
  require(l.size > 1)
}

case class A(l: List[Int]) extends T

val a = A(List(1,2))


// Exiting paste mode, now interpreting.

defined trait T
defined class A
a: A = A(List(1, 2))