import scala.util.Random

def tetrominos(amount: Int, bag: List[Char] = Nil): List[Char] = {
  val actualBag = if (bag.nonEmpty) bag else List('L', 'J', 'S', 'Z', 'T', 'O', 'I')
  val index = Random.nextInt(actualBag.size)
  actualBag(index) :: (amount match {
    case 0 => Nil
    case _ => tetrominos(amount - 1, actualBag.patch(index, Nil, 1))
  })
}

print(tetrominos(50).mkString)