MonoidK instances for Map and SortedMap

core/src/main/scala/cats/implicits.scala

@@ -12,3 +12,4 @@ object implicits
     with instances.AllInstancesBinCompat1
     with instances.AllInstancesBinCompat2
     with instances.AllInstancesBinCompat3
+    with instances.AllInstancesBinCompat4

core/src/main/scala/cats/instances/all.scala

@@ -46,3 +46,5 @@ trait AllInstancesBinCompat1
 trait AllInstancesBinCompat2 extends DurationInstances with FiniteDurationInstances
 
 trait AllInstancesBinCompat3 extends AllCoreDurationInstances
+
+trait AllInstancesBinCompat4 extends SortedMapInstancesBinCompat1 with MapInstancesBinCompat1

core/src/main/scala/cats/instances/map.scala

@@ -134,3 +134,11 @@ trait MapInstancesBinCompat0 {
     }
 
 }
+
+trait MapInstancesBinCompat1 {
+  implicit def catsStdMonoidKForMap[K]: MonoidK[Map[K, ?]] = new MonoidK[Map[K, ?]] {
+    override def empty[A]: Map[K, A] = Map.empty
+
+    override def combineK[A](x: Map[K, A], y: Map[K, A]): Map[K, A] = x ++ y
+  }
+}

core/src/main/scala/cats/instances/package.scala

@@ -22,7 +22,7 @@ package object instances {
   object list extends ListInstances with ListInstancesBinCompat0
   object long extends LongInstances
   object option extends OptionInstances with OptionInstancesBinCompat0
-  object map extends MapInstances with MapInstancesBinCompat0
+  object map extends MapInstances with MapInstancesBinCompat0 with MapInstancesBinCompat1
   object order extends OrderInstances
   object ordering extends OrderingInstances
   object parallel extends ParallelInstances

core/src/main/scala/cats/instances/sortedMap.scala

@@ -1,6 +1,6 @@
 package cats.instances
 
-import cats.{Always, Applicative, Eval, FlatMap, Foldable, Monoid, Order, Show, Traverse, TraverseFilter}
+import cats.{Always, Applicative, Eval, FlatMap, Foldable, Monoid, MonoidK, Order, Show, Traverse, TraverseFilter}
 import cats.kernel._
 import cats.kernel.instances.StaticMethods
 
@@ -223,3 +223,11 @@ trait SortedMapInstancesBinCompat0 {
         traverseFilter(fa)(a => G.map(f(a))(if (_) Some(a) else None))
     }
 }
+
+trait SortedMapInstancesBinCompat1 {
+  implicit def catsStdMonoidKForSortedMap[K: Order]: MonoidK[SortedMap[K, ?]] = new MonoidK[SortedMap[K, ?]] {
+    override def empty[A]: SortedMap[K, A] = SortedMap.empty[K, A](Order[K].toOrdering)
+
+    override def combineK[A](x: SortedMap[K, A], y: SortedMap[K, A]): SortedMap[K, A] = x ++ y
+  }
+}

testkit/src/main/scala/cats/tests/CatsSuite.scala

@@ -2,23 +2,8 @@ package cats
 package tests
 
 import catalysts.Platform
-
-import cats.instances.{
-  AllInstances,
-  AllInstancesBinCompat0,
-  AllInstancesBinCompat1,
-  AllInstancesBinCompat2,
-  AllInstancesBinCompat3
-}
-import cats.syntax.{
-  AllSyntax,
-  AllSyntaxBinCompat0,
-  AllSyntaxBinCompat1,
-  AllSyntaxBinCompat2,
-  AllSyntaxBinCompat3,
-  AllSyntaxBinCompat4,
-  EqOps
-}
+import cats.instances._
+import cats.syntax._
 import org.scalactic.anyvals.{PosInt, PosZDouble, PosZInt}
 import org.scalatest.{FunSuite, FunSuiteLike, Matchers}
 import org.scalatest.prop.{Configuration, GeneratorDrivenPropertyChecks}
@@ -55,6 +40,7 @@ trait CatsSuite
     with AllInstancesBinCompat1
     with AllInstancesBinCompat2
     with AllInstancesBinCompat3
+    with AllInstancesBinCompat4
     with AllSyntax
     with AllSyntaxBinCompat0
     with AllSyntaxBinCompat1

tests/src/test/scala/cats/tests/MapSuite.scala

@@ -5,6 +5,7 @@ import cats.laws.discipline.{
   ComposeTests,
   FlatMapTests,
   FunctorFilterTests,
+  MonoidKTests,
   SemigroupalTests,
   SerializableTests,
   UnorderedTraverseTests
@@ -13,7 +14,6 @@ import cats.laws.discipline.arbitrary._
 import cats.arrow.Compose
 
 class MapSuite extends CatsSuite {
-  implicit val iso = SemigroupalTests.Isomorphisms.invariant[Map[Int, ?]]
 
   checkAll("Map[Int, Int]", SemigroupalTests[Map[Int, ?]].semigroupal[Int, Int, Int])
   checkAll("Semigroupal[Map[Int, ?]]", SerializableTests.serializable(Semigroupal[Map[Int, ?]]))
@@ -31,6 +31,9 @@ class MapSuite extends CatsSuite {
   checkAll("Map[Int, Long]", ComposeTests[Map].compose[Int, Long, String, Double])
   checkAll("Compose[Map]", SerializableTests.serializable(Compose[Map]))
 
+  checkAll("Map[Int, Int]", MonoidKTests[Map[Int, ?]].monoidK[Int])
+  checkAll("MonoidK[Map[Int, ?]]", SerializableTests.serializable(MonoidK[Map[Int, ?]]))
+
   test("show isn't empty and is formatted as expected") {
     forAll { (map: Map[Int, String]) =>
       map.show.nonEmpty should ===(true)

tests/src/test/scala/cats/tests/SortedMapSuite.scala

@@ -3,7 +3,14 @@ package tests
 
 import cats.kernel.CommutativeMonoid
 import cats.kernel.laws.discipline.{CommutativeMonoidTests, HashTests, MonoidTests}
-import cats.laws.discipline.{FlatMapTests, SemigroupalTests, SerializableTests, TraverseFilterTests, TraverseTests}
+import cats.laws.discipline.{
+  FlatMapTests,
+  MonoidKTests,
+  SemigroupalTests,
+  SerializableTests,
+  TraverseFilterTests,
+  TraverseTests
+}
 import cats.laws.discipline.arbitrary._
 
 import scala.collection.immutable.SortedMap
@@ -39,4 +46,7 @@ class SortedMapSuite extends CatsSuite {
            SerializableTests.serializable(CommutativeMonoid[SortedMap[String, Int]]))
   checkAll("Monoid[SortedMap[String, String]]", MonoidTests[SortedMap[String, String]].monoid)
   checkAll("Monoid[SortedMap[String, String]]", SerializableTests.serializable(Monoid[SortedMap[String, String]]))
+
+  checkAll("SortedMap[String, String]", MonoidKTests[SortedMap[String, ?]].monoidK[String])
+  checkAll("MonoidK[SortedMap[String, ?]]", SerializableTests.serializable(MonoidK[SortedMap[String, ?]]))
 }