minGroupBy.scala

import scala.language.higherKinds
import scala.collection.TraversableLike

def minGroupBy[CC[A], A, B](c: CC[A])(f: A => B)
    (implicit ev1: CC[A] => TraversableLike[A, CC[A]],
              ev2: Ordering[B]): CC[A] = {
  val grouped = c.groupBy(f)
  if (grouped.nonEmpty) grouped.minBy(_._1)._2 else c
}                                                 //> minGroupBy: [CC[A], A, B](c: CC[A])(f: A => B)(implicit ev1: CC[A] => scala.
                                                  //| collection.TraversableLike[A,CC[A]], implicit ev2: Ordering[B])CC[A]

minGroupBy(Set("a", "b", "ab"))(_.length)         //> res0: scala.collection.immutable.Set[String] = Set(a, b)
minGroupBy(List("a", "b", "ab"))(_.length)        //> res1: List[String] = List(a, b)
minGroupBy(List.empty[String])(_.length)          //> res2: List[String] = List()