Add updateWith method to NonEmptyMap

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

@@ -93,6 +93,15 @@ sealed class NonEmptyMapOps[K, A](val value: NonEmptyMap[K, A]) {
    */
   def lookup(k: K): Option[A] = toSortedMap.get(k)
 
+  /**
+   * Applies f to the value stored at k. If lookup misses, does nothing.
+   */
+  def updateWith(k: K)(f: A => A): NonEmptyMap[K, A] =
+    lookup(k) match {
+      case Some(v) => add((k, f(v)))
+      case None    => value
+    }
+
   /**
    * Returns a `SortedSet` containing all the keys of this map.
    */

tests/src/test/scala/cats/tests/NonEmptyMapSuite.scala

@@ -209,4 +209,22 @@ class NonEmptyMapSuite extends CatsSuite {
       nem.toNel should ===(NonEmptyList.fromListUnsafe(nem.toSortedMap.toList))
     }
   }
+
+  test("NonEmptyMap#updateWith identity should be a no-op") {
+    forAll { (nem: NonEmptyMap[String, Int], i: (String, Int)) =>
+      nem.add(i) should ===(nem.add(i).updateWith(i._1)(identity))
+    }
+  }
+
+  test("NonEmptyMap#updateWith on existing value should behave like Option#map on the same value") {
+    forAll { (nem: NonEmptyMap[String, Int], i: (String, Int)) =>
+      nem.add(i).lookup(i._1).map(_ + 1) should ===(nem.add(i).updateWith(i._1)(_ + 1).lookup(i._1))
+    }
+  }
+
+  test("NonEmptyMap#updateWith should not act when key is missing") {
+    val single = NonEmptyMap[String, Int](("here", 1), SortedMap())
+    single.lookup("notHere") should ===(single.updateWith("notHere")(_ => 1).lookup("notHere"))
+  }
+
 }