A `groupBy` implementation in which the result's values are guaranteed not to be empty on the type level.

GroupToNel.scala

import scalaz.{Foldable, NonEmptyList}
import scalaz.syntax.foldable._

implicit class GroupToNel[F[_] : Foldable, T](foldable: F[T]){
  def groupToNel[K](fun: T => K): Map[K, NonEmptyList[T]] = {
    f.foldLeft(Map.empty[K, NonEmptyList[T]]) { (map, elem) =>
      val funOfElem = fun(elem)

      val newValue = map.get(funOfElem).fold(NonEmptyList(elem))(list => elem <:: list)

      map + (funOfElem -> newValue)
    }
  }
}

//scala> List(1,2,3,4).groupToNel(_ % 2)
//res0: Map[Int,scalaz.NonEmptyList[Int]] = Map(1 -> NonEmptyList(3, 1), 0 -> NonEmptyList(4, 2))