Card deck with Shuffle

Deck.scala

import scala.util.Random

trait Shuffle {
    var cards: List[Card]
    lazy val random = new Random
    def shuffle {
        def shuffle(cards: List[Card], size: Int, pile: List[Card] = Nil): List[Card] = {
            if (size > 0) {
                val card = cards(random.nextInt(size))
                val (bef, aft) = cards.span(_ != card)
                shuffle(bef ::: aft.tail, size - 1, card :: pile)
            } else pile
        }
        cards = shuffle(cards, cards.size)
    }
}

class Deck extends Shuffle {
    var cards = for (r ← Rank.values.toList; s ← Suit.values) yield r of s
    def next = {
        if (!cards.isEmpty) {
            val card = cards.head
            cards = cards.tail
            Some(card)
        } else None
    }
}