Error Calling Generic Function In Scala - scala

Question

So I'm following fpinscala and I have this Object:
sealed trait List[+A]
case object Nil extends List[Nothing]
case class Cons[+A](head: A, tail: List[A]) extends List[A]
object List {
def sum(ints: List[Int]): Int = ints match {
case Nil => 0
case Cons(x, xs) => x + sum(xs)
}
def product(ds: List[Double]): Double = ds match {
case Nil => 1
case Cons(x, xs) => x * product(xs)
}
def apply[A](as: A*): List[A] =
if (as.isEmpty) Nil
else Cons(as.head, apply(as.tail: _*))
def tail[A](as: List[A]): List[A] = as match {
case Nil => as
case Cons(_, xs) => xs
}
def setHead[A](as: List[A], a: A): List[A] = as match {
case Nil => Cons(a, Nil)
case Cons(_, xs) => Cons(a, xs)
}
def main(args: Array[String]): Unit =
println(tail(List(1,2,3)))
}
When I try to run main I get:
error: type mismatch;
found : List[Int] (in scala.collection.immutable)
required: List[?] (in <empty>)
println(tail(List[Int](1,2,3)))
I have followed the book pretty exactly so I don't know what I'm missing here. It says you should now be able to call List(1, 2, 3, 4) or List("hello", "goodbye") with no issues. But I can't pass that to the function? Some insight on what I'm missing would be appreciated.

Answer 1

The code compiles correctly, the only problem I see is that you are declaring the main method in the List companion object and that is incorrect, you should move it to other object:
sealed trait List[+A]
case object Nil extends List[Nothing]
case class Cons[+A](head: A, tail: List[A]) extends List[A]
object List {
def sum(ints: List[Int]): Int = ints match {
case Nil => 0
case Cons(x, xs) => x + sum(xs)
}
def product(ds: List[Double]): Double = ds match {
case Nil => 1
case Cons(x, xs) => x * product(xs)
}
def apply[A](as: A*): List[A] =
if (as.isEmpty) Nil
else Cons(as.head, apply(as.tail: _*))
def tail[A](as: List[A]): List[A] = as match {
case Nil => as
case Cons(_, xs) => xs
}
def setHead[A](as: List[A], a: A): List[A] = as match {
case Nil => Cons(a, Nil)
case Cons(_, xs) => Cons(a, xs)
}
}
object Main {
def main(args: Array[String]): Unit =
println(List.tail(List(1,2,3)))
}