List.scala

package week4

import scala.annotation.tailrec

abstract class List[T] {
  def isEmpty: Boolean
  def head: T
  def tail: List[T]

  override def toString =
    toStringAcc(new StringBuilder("["), true)

  @tailrec
  final def toStringAcc(builder: StringBuilder, first: Boolean): String =
    if (isEmpty)
      builder.append("]").toString()
    else {
      if (!first) builder.append(", ")
      tail.toStringAcc(builder.append(head), false)
    }
}

class Cons[T](val head: T, val tail: List[T]) extends List[T] {
  def isEmpty = false
}

class Nil[T]() extends List[T] {
  def isEmpty = true
  def head: Nothing = throw new NoSuchElementException("Nil.head")
  def tail: Nothing = throw new NoSuchElementException("Nil.tail")
}

object List {
  def apply[T](x1: T, x2: T) = new Cons(x1, new Cons(x2, new Nil))
}