clusterBy.scala

  import scala.collection.immutable.{TreeMap,SortedMap}
  def fromListWith[A,K<%Ordered[K]](seq:Iterable[(K,A)])(f: (A,A)=>A):SortedMap[K,A] =
    seq.foldLeft(TreeMap.empty[K,A]){
      (map:TreeMap[K,A],entry) =>       
      val (key,ele) = entry
      // update entry by either applying f to existing elements or 
      // just starting a new entry
      map.update(key,map.get(key).map(f(ele,_)).getOrElse(ele))              
    }
  
  def &&&[A,B,C](f1:A=>B,f2:A=>C) = (a:A) => (f1(a),f2(a))
  
  def clusterBy[A,B<%Ordered[B]](it:Iterable[A])(f:A=>B) =
     fromListWith(it.map(&&&(f,List(_))))(_++_)
  
  def words(str:String) = str.split(' ')

  def main(args:Array[String]){
    val ws = words("das ist das haus des nikolaus")
    clusterBy(ws)(_.length)
    clusterBy(ws)(_.first)
    clusterBy(ws)(_.last)
    ()
  }