Tail Recursion Optimization

foldLeft.scala

  private def foldLeftBad[T, TResult](list: List[T], initial: TResult)(folder: (TResult, T) => TResult): TResult = list match {
    case Nil => initial
    case head :: tail => folder(foldLeftBad(tail, initial)(folder), head)
  }

  private def foldLeft[T, TResult](list: List[T], initial: TResult)(folder: (TResult, T) => TResult): TResult = {
    
    @tailrec
    def foldLeftSoFar(list: List[T], computer: () => TResult): TResult = list match {
      case Nil => computer()
      case head :: tail => {
        val result = computer()
        foldLeftSoFar(tail, () => folder(result, head))
      }
    }
    
    foldLeftSoFar(list, () => initial)
  }
  
  // ...
  
  def sum(list: List[Int]): Int = foldLeft(list, 0)(_ + _)
  def multiply(list: List[Int]): Int = foldLeft(list, 1)(_ * _)