Take a look at the implementation of the following

Take a look at the implementation of the following function, select the correct result:

def someFunction[A](l: List[A], h: A): List[A] = l match {
  case Nil => sys.error("empty list")
  case Cons(_, t) => Cons(h, t)
}

someFunction(List(1, 2, 3), 3)

Assuming the following code is available for your reference

sealed trait List[+A]
case object Nil extends List[Nothing]
case class Cons[+A](head: A, tail: List[A]) extends List[A]

Explanation

This is possible implementation of setHead function.
List(1,2,3) can be represented as Cons(1, Cons(2, Cons(3, Nil))). It doesn’t match Nil. When matching against Cons(_, t) the following values are substituted: _ => 1, t => List(2, 3) == Cons(2, Cons(3, Nil)). h==3, therefore Cons(h, t) == Cons(3, Cons(2, Cons(3, Nil))) == List(3, 2, 3).

Source: Scala Exercises: functional data structures