Bowling Scoring Kata (in Scala)

// Original formulation of the problem: http://butunclebob.com/ArticleS.UncleBob.TheBowlingGameKata

import org.scalatest._
import org.scalatest.matchers._

class Game {
  private var rolls:List[Int] = Nil
  
  def roll(pins:Int) {
    rolls += pins
  }
  
  def roll(pins:List[Any]) {
    pins.foreach(_ match {
      case p:Int => roll(p)
      case (p1:Int, p2:Int) => roll(p1); roll(p2)
    })
  }

  def strikeBonus(rolls:List[Int]) = {
    rolls.head + rolls.tail.head
  }

  def spareBonus(rolls:List[Int]) = {
    rolls.head
  }

  def computeScore(rolls:List[Int]):Int = {
    if (rolls != Nil) {
      var frameScore:Int = rolls.head
      var remainingRolls:List[Int] = rolls.tail

      if (frameScore == 10) {
        frameScore += strikeBonus(remainingRolls)
        if (remainingRolls.length == 2) {
          remainingRolls = Nil
        }
      } else {
        frameScore += remainingRolls.head
        remainingRolls = remainingRolls.tail
      
        if (frameScore == 10) {
          frameScore += spareBonus(remainingRolls)
          if (remainingRolls.length == 1) {
            remainingRolls = Nil
          }
        }
      }

      frameScore + computeScore(remainingRolls)
    } else {
      0
    }
  }

  def score:Int = {
    this.computeScore(rolls)
  }
}


class GameSpecs extends Spec with ShouldMatchers {

  describe("The bowling game scoring rules") {

    it("should be zero when no pin is dropped in any roll") {
      val game = new Game
      game.roll(List((0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0)))
      game.score should equal (0)
    }

    it("should be the simple sum (20) if just one pin is dropped in every roll") {
      val game = new Game
      game.roll(List((1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1), (1, 1)))
      game.score should equal (20)
    }

    it("should add the extra point of the next score when doing a spare") {
      val game = new Game
      game.roll(List((5, 5), (3, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0)))
      game.score should equal (16)
    }

    it("should add the extra point of the next two scores when doing a strike") {
      val game = new Game
      game.roll(List((10), (3, 4), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0), (0, 0)))
      game.score should equal (24)
    }

    it("should get 300 points for a perfect game") {
      val game = new Game
      game.roll(List(10, 10, 10, 10, 10, 10, 10, 10, 10, 10, /* extra rolls */ 10, 10))
      game.score should equal (300)
    }
  }
}