idea taken from http://xprogramming.com/articles/scala-bowling-i-think

Game.functional.scala

class Game(rolls: List[Int]) {
  def score:Int = score(rolls, 10)
  private def score(rolls:List[Int], index:Int):Int = {
    if (index == 0) return 0

    rolls match {
      case 10 :: second :: third :: rest 
        => 10 + second + third + score(second :: third :: rest, index-1)
      case first :: second :: third :: rest if first + second == 10 
        => 10 + third + score(third :: rest, index-1)
      case first :: second :: rest 
        => first + second + score(rest, index-1)
      case List()
        => error("too few frames")
    }  
  }
}

Game.scala

object Game {
    abstract class Frame { def score : Int }
    case object LastFrame extends Frame { def score = 0 }
    case class FrameList(total:Int, next:Frame) extends Frame {
        def score = total + next.score
    }

    def fromRolls(remainingrolls: List[Int]): Frame = fromRolls(remainingrolls, 10)

    private def fromRolls(remainingrolls: List[Int], index:Int): Frame = {
        if (index == 0) return LastFrame 
        remainingrolls match {
            case 10 :: b :: c :: rest => FrameList(10+b+c, fromRolls(b::c::rest, index-1))
            case a :: b :: c :: rest if a+b == 10 => FrameList(10+c, fromRolls(c :: rest, index-1))
            case a :: b :: rest => FrameList(a + b, fromRolls(rest, index-1))
            case List() => LastFrame
        }	
    }    
}

class Game(val frames:Game.Frame) {
    def this(rolls: List[Int]) = this(Game.fromRolls(rolls))
    def score = frames.score
}

Here is a simple functional version

class Game(rolls: List[Int]) {
def score:Int = score(rolls, 10)
private def score(rolls:List[Int], index:Int):Int = {
if (index == 0) return 0
rolls match {
case 10 :: second :: third :: rest
=> 10 + second + third + score(second :: third :: rest, index-1)
case first :: second :: third :: rest if first + second == 10
=> 10 + third + score(third :: rest, index-1)
case first :: second :: rest
=> first + second + score(rest, index-1)
case List()
=> error("too few frames")
}
}
}