Recursion examples do not handle negative number input

[MariaLysik]

For example, calling chessboard(-1) will lead to endless recursion and eventually stack overflow.

def chessboard(count: Int): Image = {
  val blackSquare = Image.rectangle(30, 30) fillColor Color.black
  val redSquare   = Image.rectangle(30, 30) fillColor Color.red
  val base = (redSquare beside blackSquare) above (blackSquare beside redSquare)
  count match {
    case 0 => base
    case n =>
      val unit = chessboard(n-1)
      (unit beside unit) above (unit beside unit)
  }
}

The solution could be just adding additional case on top:

case n if n<0 => Image.Empty

However, Dave suggested to turn it into a learning opportunity by adding:

• Exercise: What would happen if we called chessboard() with a negative number? Why? How could we prevent this happening?
• Solution: Explain that we fail to terminate. Explain underflow. Show your code using a guard clause to protect against this.

I like his approach better.