MR scala

mr.scala

//def
val l:List[A]
val m[K,V]:A=>List[(K, V)]
val r[V]:List[V]=>V
val s[K]:K=>K

//mr
l.flatMap(m)
  .groupBy(_._1)
  .mapValues(_.map(_._2))
  .groupBy{case (x, xs) => s(x) }
  .mapValues(map => map.map{ case (x, xs) => (x, r(xs))})
  .values
  .flatten