S99 Problem 6

Palindrome.scala

import scala.annotation.tailrec

object Palindrome extends App {

  //don't want to use name "reverse" because of conflict with built-in
  def rev[A](lst: List[A]): List[A] = {
    @tailrec
    def loop(rlst: List[A], accumulator: List[A]): List[A] = rlst match {
      case Nil          => accumulator
      case head :: tail => loop(tail, head :: accumulator)
    }
    loop(lst, Nil)
  }

  //using custom reverse
  def custPalindrome[A](l: List[A]): Boolean = rev(l) == l

  //using built-ins
  def palindroeme[A](l: List[A]): Boolean = l.reverse == l


  val palList = List(1,2,3,2,1)
  
  assert(custPalindrome(palList) == true)
  assert(palindroeme(palList) == true)
  
  //using pimp-my-library pattern
  class CustPal(rl: List[Any]){
    def palindrome = palindroeme(rl)
  }
  implicit def listToCustRev(l: List[Any]) = new CustPal(l)

  assert(palList.palindrome)

}