Require an Order instance for NonEmptyList's groupBy function

core/src/main/scala/cats/data/NonEmptyList.scala

@@ -5,9 +5,9 @@ import cats.instances.list._
 import cats.syntax.order._
 
 import scala.annotation.tailrec
-import scala.collection.immutable.TreeSet
+import scala.collection.immutable.{ TreeMap, TreeSet }
+import scala.collection.mutable
 import scala.collection.mutable.ListBuffer
-import scala.collection.{immutable, mutable}
 
 /**
  * A data type which represents a non empty list of A, with
@@ -325,24 +325,26 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) {
     *
     * {{{
     * scala> import cats.data.NonEmptyList
+    * scala> import cats.instances.boolean._
     * scala> val nel = NonEmptyList.of(12, -2, 3, -5)
     * scala> nel.groupBy(_ >= 0)
     * res0: Map[Boolean, cats.data.NonEmptyList[Int]] = Map(false -> NonEmptyList(-2, -5), true -> NonEmptyList(12, 3))
     * }}}
     */
-  def groupBy[B](f: A => B): Map[B, NonEmptyList[A]] = {
-    val m = mutable.Map.empty[B, mutable.Builder[A, List[A]]]
+  def groupBy[B](f: A => B)(implicit B: Order[B]): Map[B, NonEmptyList[A]] = {
+    var m = TreeMap.empty[B, mutable.Builder[A, List[A]]](B.toOrdering)
+
     for { elem <- toList } {
-      m.getOrElseUpdate(f(elem), List.newBuilder[A]) += elem
-    }
-    val b = immutable.Map.newBuilder[B, NonEmptyList[A]]
-    for { (k, v) <- m } {
-      val head :: tail = v.result // we only create non empty list inside of the map `m`
-      b += ((k, NonEmptyList(head, tail)))
+      val k = f(elem)
+
+      m.get(k) match {
+        case None => m += ((k, List.newBuilder[A] += elem))
+        case Some(builder) => builder += elem
+      }
     }
-    b.result
-  }
 
+    m.map { case (k, v) => (k, NonEmptyList.fromListUnsafe(v.result)) }
+  }
 }
 
 object NonEmptyList extends NonEmptyListInstances {

core/src/main/scala/cats/syntax/list.scala

@@ -27,6 +27,6 @@ final class ListOps[A](val la: List[A]) extends AnyVal {
     * }}}
     */
   def toNel: Option[NonEmptyList[A]] = NonEmptyList.fromList(la)
-  def groupByNel[B](f: A => B): Map[B, NonEmptyList[A]] =
+  def groupByNel[B : Order](f: A => B): Map[B, NonEmptyList[A]] =
     toNel.fold(Map.empty[B, NonEmptyList[A]])(_.groupBy(f))
 }