sized list.scala

import scala.compiletime.ops.int._

sealed trait Lst[+A, N <: Int]
case object Nil extends Lst[Nothing, 0]
case class Cons[+A, N <: Int](head: A, tail: Lst[A, N]) extends Lst[A, N + 1]

def map2[A, B, C, N <: Int](list1: Lst[A, N], list2: Lst[B, N])(f: (A, B) => C): Lst[C, N] = 
  (list1, list2) match
    case (Nil, Nil) => Nil
    case (Cons(h1, t1), Cons(h2, t2)) =>
      Cons(f(h1, h2), map2(t1, t2)(f))

extension [A] (head: A)
  def :: [N <: Int](tail: Lst[A, N]) = Cons(head, tail)

extension [A : Numeric, N <: Int](list1: Lst[A, N])
  def + (list2: Lst[A, N]) = map2(list1, list2)(summon[Numeric[A]].plus)

@main def main = 
  val list1 = 1 :: 2 :: Nil
  val list2 = 3 :: 4 :: Nil
  println(list1 + list2)