Require and Order instance for NonEmptyList's groupBy function

This addresses: typelevel#1959
igstan · Oct 11, 2017 · 2516f36 · 2516f36
1 parent 523ac99
commit 2516f36
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Showing 2 changed files with 4 additions and 3 deletions.
diff --git a/core/src/main/scala/cats/data/NonEmptyList.scala b/core/src/main/scala/cats/data/NonEmptyList.scala
@@ -325,13 +325,14 @@ 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]] = {
+    val m = mutable.TreeMap.empty[B, mutable.Builder[A, List[A]]](B.toOrdering)
     for { elem <- toList } {
       m.getOrElseUpdate(f(elem), List.newBuilder[A]) += elem
     }

diff --git a/core/src/main/scala/cats/syntax/list.scala b/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))
 }