The Seasoned Schemer, chapter 19, page 176

ch19.scala

// page 176

// recursive non-consing solution with var

def hasTwoInARow[T](lists: List[Any]) = {
  var previous: Option[Any] = None
  def recurse(rest: Any): Boolean =
    rest match {
      case xs: List[_] =>
        xs.exists(recurse)
      case _ =>
        previous.exists(_ == rest) ||
          { previous = Some(rest); false }
    }
  recurse(lists)
}

// recursive pure-functional lazy consing solution

def flatten[T](xs: Stream[Any]): Stream[Any] =
  xs flatMap {
    case x: Stream[_] => flatten(x)
    case x => Stream(x)
  }

def hasTwoInARow[T](xs: Stream[Any]) =
  flatten(xs).sliding(2).exists(pair => pair(0) == pair(1))