CraftingCode.scala

case class Person(name: String, flag: Boolean)

def sortedKeyList(xs: List[List[Person]]) = {
  xs.flatten
    .map(_.name)
    .distinct
    .sortBy(identity)
    .mkString(", ") match {
      case "" => "<none>"
      case s => s
    }
}

class Tests extends FunSuite with Matchers {
  test("sorted key list") {
    def people(xs: List[String]) = xs.map(Person(_, flag = true))

    val data = List(
      (Nil, Nil, "<none>"),
      (List("fred"), List("barney", "wilma"), "barney, fred, wilma"),
      (List("fred"), Nil, "fred"),
      (List("fred"), List("fred", "barney"), "barney, fred"))

    for ((a, b, res) <- data)
      sortedKeyList(List(people(a), people(b))) should equal(res)
  }
}