snoclist.scala

import scala.annotation.tailrec

sealed abstract class SnocList[+A]:
    inline def :<[B >: A](a: B): SnocList[B] = new :<(this, a)
    final def toList: List[A] =
        @tailrec def go(xs: SnocList[A], acc: List[A]): List[A] = xs match
            case Lin     => acc
            case xs :< x => go(xs, x :: acc)
        go(this, Nil)
    override def toString: String = toList.mkString("SnocList(", ", ", ")")

final case class :<[+A](init: SnocList[A], last: A) extends SnocList[A]
case object Lin extends SnocList[Nothing]

object SnocList:
    def apply[A](as: A*): SnocList[A] =
        @tailrec def go(as: List[A], acc: SnocList[A]): SnocList[A] = as match
            case Nil     => acc
            case x :: xs => go(xs, acc :< x)
        go(as.toList, Lin)

extension [A](xs: IterableOnce[A])
    def toSnocList: SnocList[A] = SnocList(xs.iterator.toSeq*)

val xs = Lin :< 1 :< 2 :< 3
val ys = xs match
    case Lin     => println("Empty")
    case xs :< x => println(s"Snoc($xs, $x)")