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How to generate transitive closure of set of tuples?
//one transitive step
def addTransitive[A, B](s: Set[(A, B)]) = {
  s ++ (for ((x1, y1) <- s; (x2, y2) <- s if y1 == x2) yield (x1, y2))
}

//repeat until we don't get a bigger set
def transitiveClosure[A,B](s:Set[(A,B)]):Set[(A,B)] = {
  val t = addTransitive(s)
  if (t.size == s.size) s else transitiveClosure(t)
}

println(transitiveClosure(Set((1,2), (2,3), (3,4))))

def unfoldRight[A, B](seed: B)(f: B => Option[(A, B)]): List[A] = f(seed) match {
  case Some((a, b)) => a :: unfoldRight(b)(f)
  case None => Nil
}

def unfoldLeft[A, B](seed: B)(f: B => Option[(B, A)]) = {
  def loop(seed: B)(ls: List[A]): List[A] = f(seed) match {
    case Some((b, a)) => loop(b)(a :: ls)
    case None => ls
  }

  loop(seed)(Nil)
}

def transitiveClosure(input: Set[(Int, Int)]) = {
    val map = input.toMap
    def closure(seed: Int) = unfoldLeft(map get seed) {
        case Some(`seed`) => None
        case Some(n)      => Some(seed -> n -> (map get n))
        case _            => None
    }
    map.keySet flatMap closure
}

def closure(seed: Int) = unfoldRight(seed) {
    case n if map.get(n) != seed => map get n map (x => seed -> x -> x)
    case _ => None
}

def closure(seed: Int) = unfoldRight(map get seed) {
        case Some(`seed`) => Some(seed -> seed -> None)
        case Some(n)      => Some(seed -> n -> (map get n))
        case _            => None
    }

input.groupBy(_._1) mapValues ( _ map (_._2) )

def transitiveClosure[T]( dag: Map[ T, List[T] ] ) = {
  var tc = Map.empty[ T, List[T] ]
  def getItemTC( item:T ): List[T] = tc.get(item) match {
    case None =>
      val itemTC = dag(item) flatMap ( x => x::getItemTC(x) )
      tc += ( item -> itemTC )
      itemTC
    case Some(itemTC) => itemTC
  }
  dag.keys foreach getItemTC
  tc
}

scala> val a = List((1,2), (2,3), (3,4))
a: List[(Int, Int)] = List((1,2), (2,3), (3,4))

scala> val (xx, yy) = a unzip
xx: List[Int] = List(1, 2, 3)
yy: List[Int] = List(2, 3, 4)

scala> for(x <- xx; y <- yy if (x != y && x < y)) yield (x,y)
res32: List[(Int, Int)] = List((1,2), (1,3), (1,4), (2,3), (2,4), (3,4))