Add ContravariantMonoidal

build.sbt

@@ -198,6 +198,39 @@ def mimaSettings(moduleName: String) = Seq(
       exclude[MissingTypesProblem]("cats.data.OneAndLowPriority3"),
       exclude[MissingTypesProblem]("cats.data.OneAndLowPriority2"),
       exclude[MissingTypesProblem]("cats.data.OneAndLowPriority1"),
+      exclude[DirectMissingMethodProblem]("cats.implicits.catsContravariantSemigroupalForEq"),
+      exclude[DirectMissingMethodProblem]("cats.implicits.catsContravariantSemigroupalForOrder"),
+      exclude[DirectMissingMethodProblem]("cats.implicits.catsContravariantSemigroupalForOrdering"),
+      exclude[DirectMissingMethodProblem]("cats.implicits.catsContravariantSemigroupalEquiv"),
+      exclude[ReversedMissingMethodProblem]("cats.ContravariantSemigroupal.contramap2"),
+      exclude[ReversedMissingMethodProblem]("cats.ContravariantSemigroupal#Ops.contramap2"),
+      exclude[InheritedNewAbstractMethodProblem]("cats.ContravariantMonoidal#ToContravariantMonoidalOps.toContravariantMonoidalOps"),
+      exclude[InheritedNewAbstractMethodProblem]("cats.ContravariantSemigroupal#ToContravariantSemigroupalOps.toContravariantSemigroupalOps"),
+      exclude[ReversedMissingMethodProblem]("cats.Applicative.composeContravariantMonoidal"),
+      exclude[UpdateForwarderBodyProblem]("cats.instances.Function1Instances.catsStdContravariantForFunction1"),
+      exclude[DirectMissingMethodProblem]("cats.instances.package=uiv.catsContravariantSemigroupalEquiv"),
+      exclude[DirectMissingMethodProblem]("cats.instances.OrderingInstances.catsContravariantSemigroupalForOrdering"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.OrderingInstances.cats$instances$OrderingInstances$_setter_$catsContravariantMonoidalForOrdering_="),
+      exclude[ReversedMissingMethodProblem]("cats.instances.OrderingInstances.catsContravariantMonoidalForOrdering"),
+      exclude[DirectMissingMethodProblem]("cats.instances.OrderInstances.catsContravariantSemigroupalForOrder"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.OrderInstances.catsContravariantMonoidalForOrder"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.OrderInstances.cats$instances$OrderInstances$_setter_$catsContravariantMonoidalForOrder_="),
+      exclude[DirectMissingMethodProblem]("cats.instances.package#ordering.catsContravariantSemigroupalForOrdering"),
+      exclude[DirectMissingMethodProblem]("cats.instances.package#all.catsContravariantSemigroupalForEq"),
+      exclude[DirectMissingMethodProblem]("cats.instances.package#all.catsContravariantSemigroupalForOrder"),
+      exclude[DirectMissingMethodProblem]("cats.instances.package#all.catsContravariantSemigroupalForOrdering"),
+      exclude[DirectMissingMethodProblem]("cats.instances.package#all.catsContravariantSemigroupalEquiv"),
+      exclude[DirectMissingMethodProblem]("cats.instances.EquivInstances.catsContravariantSemigroupalEquiv"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.EquivInstances.catsContravariantMonoidalForEquiv"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.EquivInstances.cats$instances$EquivInstances$_setter_$catsContravariantMonoidalForEquiv_="),
+      exclude[DirectMissingMethodProblem]("cats.instances.package=.catsContravariantSemigroupalForEq"),
+      exclude[DirectMissingMethodProblem]("cats.instances.package#order.catsContravariantSemigroupalForOrder"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.Function1Instances.catsStdContravariantMonoidalForFunction1"),
+      exclude[DirectMissingMethodProblem]("cats.instances.EqInstances.catsContravariantSemigroupalForEq"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.EqInstances.catsContravariantMonoidalForEq"),
+      exclude[ReversedMissingMethodProblem]("cats.instances.EqInstances.cats$instances$EqInstances$_setter_$catsContravariantMonoidalForEq_="),
+      exclude[InheritedNewAbstractMethodProblem]("cats.syntax.ContravariantSemigroupalSyntax.catsSyntaxContravariantSemigroupal"),
+      exclude[InheritedNewAbstractMethodProblem]("cats.syntax.ContravariantMonoidalSyntax.catsSyntaxContravariantMonoidal"),
       exclude[DirectMissingMethodProblem]("cats.data.OneAndLowPriority3.catsDataNonEmptyTraverseForOneAnd"),
       exclude[DirectMissingMethodProblem]("cats.data.OneAndLowPriority2.catsDataTraverseForOneAnd"),
       exclude[ReversedMissingMethodProblem]("cats.instances.ParallelInstances.catsStdNonEmptyParallelForZipVector"),

core/src/main/scala/cats/Applicative.scala

@@ -51,6 +51,12 @@ import simulacrum.typeclass
       val G = Applicative[G]
     }
 
+  def composeContravariantMonoidal[G[_]: ContravariantMonoidal]: ContravariantMonoidal[λ[α => F[G[α]]]] =
+    new ComposedApplicativeContravariantMonoidal[F, G] {
+      val F = self
+      val G = ContravariantMonoidal[G]
+    }
+
   /**
    * Returns the given argument if `cond` is `false`, otherwise, unit lifted into F.
    */

core/src/main/scala/cats/Composed.scala

@@ -113,6 +113,19 @@ private[cats] trait ComposedContravariantCovariant[F[_], G[_]] extends Contravar
     F.contramap(fga)(gb => G.map(gb)(f))
 }
 
+private[cats] trait ComposedApplicativeContravariantMonoidal[F[_], G[_]] extends ContravariantMonoidal[λ[α => F[G[α]]]] { outer =>
+  def F: Applicative[F]
+  def G: ContravariantMonoidal[G]
+
+  override def unit[A]: F[G[A]] = F.pure(G.unit)
+
+  override def contramap[A, B](fa: F[G[A]])(f: B => A): F[G[B]] =
+    F.map(fa)(G.contramap(_)(f))
+
+  override def product[A, B](fa: F[G[A]], fb: F[G[B]]): F[G[(A, B)]] =
+    F.map2(fa, fb)(G.product(_, _))
+}
+
 private[cats] trait ComposedSemigroupal[F[_], G[_]] extends ContravariantSemigroupal[λ[α => F[G[α]]]] with ComposedContravariantCovariant[F, G] { outer =>
   def F: ContravariantSemigroupal[F]
   def G: Functor[G]

core/src/main/scala/cats/ContravariantMonoidal.scala

@@ -0,0 +1,25 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * [[ContravariantMonoidal]] functors are functors that supply
+ * a unit along the diagonal map for the `contramap2` operation.
+ *
+ * Must obey the laws defined in cats.laws.ContravariantMonoidalLaws.
+ *
+ * Based on ekmett's contravariant library:
+ * https://hackage.haskell.org/package/contravariant-1.4/docs/Data-Functor-Contravariant-Divisible.html
+ */
+@typeclass trait ContravariantMonoidal[F[_]] extends ContravariantSemigroupal[F] { self =>
+  /**
+   * `unit` produces an instance of `F` for any type `A`
+   * that is trivial with respect to `contramap2` along
+   * the diagonal
+   */
+  def unit[A]: F[A]
+
+  def liftContravariant[A, B](f: A => B): F[B] => F[A] =
+    ContravariantMonoidal.contramap2(unit[B], _: F[B])(((b: B) => (b, b)) compose f)(self, self)
+}
+object ContravariantMonoidal extends SemigroupalArityFunctions

core/src/main/scala/cats/ContravariantSemigroupal.scala

@@ -13,3 +13,4 @@ import simulacrum.typeclass
       def G = Functor[G]
     }
 }
+object ContravariantSemigroupal extends SemigroupalArityFunctions

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

@@ -65,9 +65,12 @@ private[data] sealed abstract class ConstInstances extends ConstInstances0 {
     def show(f: Const[A, B]): String = f.show
   }
 
-  implicit def catsDataContravariantForConst[C]: Contravariant[Const[C, ?]] = new Contravariant[Const[C, ?]] {
-    override def contramap[A, B](fa: Const[C, A])(f: (B) => A): Const[C, B] =
+  implicit def catsDataContravariantMonoidalForConst[D: Monoid]: ContravariantMonoidal[Const[D, ?]] = new ContravariantMonoidal[Const[D, ?]] {
+    override def unit[A] = Const.empty[D, A]
+    override def contramap[A, B](fa: Const[D, A])(f: B => A): Const[D, B] =
       fa.retag[B]
+    override def product[A, B](fa: Const[D, A], fb: Const[D, B]): Const[D, (A, B)] =
+      fa.retag[(A, B)] combine fb.retag[(A, B)]
   }
 
   implicit def catsDataTraverseForConst[C]: Traverse[Const[C, ?]] = new Traverse[Const[C, ?]] {
@@ -102,11 +105,15 @@ private[data] sealed abstract class ConstInstances extends ConstInstances0 {
 }
 
 private[data] sealed abstract class ConstInstances0 extends ConstInstances1 {
-
   implicit def catsDataSemigroupForConst[A: Semigroup, B]: Semigroup[Const[A, B]] = new Semigroup[Const[A, B]] {
     def combine(x: Const[A, B], y: Const[A, B]): Const[A, B] = x combine y
   }
 
+  implicit def catsDataContravariantForConst[C]: Contravariant[Const[C, ?]] = new Contravariant[Const[C, ?]] {
+    override def contramap[A, B](fa: Const[C, A])(f: (B) => A): Const[C, B] =
+      fa.retag[B]
+  }
+
   implicit def catsDataPartialOrderForConst[A: PartialOrder, B]: PartialOrder[Const[A, B]] = new PartialOrder[Const[A, B]]{
     def partialCompare(x: Const[A, B], y: Const[A, B]): Double =
       x partialCompare y

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

@@ -73,6 +73,18 @@ private[data] sealed trait IdTApplicative[F[_]] extends Applicative[IdT[F, ?]] w
   def pure[A](a: A): IdT[F, A] = IdT.pure(a)
 }
 
+private[data] sealed trait IdTContravariantMonoidal[F[_]] extends ContravariantMonoidal[IdT[F, ?]] {
+  implicit val F0: ContravariantMonoidal[F]
+
+  override def unit[A]: IdT[F, A] = IdT(F0.unit[A])
+
+  override def contramap[A, B](fa: IdT[F, A])(f: B => A): IdT[F, B] =
+    IdT(F0.contramap(fa.value)(f))
+
+  override def product[A, B](fa: IdT[F, A], fb: IdT[F, B]): IdT[F, (A, B)] =
+    IdT(F0.product(fa.value, fb.value))
+}
+
 private[data] sealed trait IdTFlatMap[F[_]] extends FlatMap[IdT[F, ?]] with IdTApply[F] {
   implicit val F0: FlatMap[F]
 
@@ -123,7 +135,12 @@ private[data] sealed trait IdTNonEmptyTraverse[F[_]] extends IdTTraverse[F] with
     fa.reduceRightTo(f)(g)
 }
 
-private[data] sealed abstract class IdTInstances5 {
+private[data] sealed abstract class IdTInstances6 {
+  implicit def catsDataContravariantMonoidalForIdT[F[_]](implicit F: ContravariantMonoidal[F]): ContravariantMonoidal[IdT[F, ?]] =
+    new IdTContravariantMonoidal[F] { implicit val F0: ContravariantMonoidal[F] = F }
+}
+
+private[data] sealed abstract class IdTInstances5 extends IdTInstances6 {
   implicit def catsDataFunctorForIdT[F[_]](implicit F: Functor[F]): Functor[IdT[F, ?]] =
     new IdTFunctor[F] { implicit val F0: Functor[F] = F }
 }

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

@@ -230,6 +230,10 @@ private[data] sealed abstract class IndexedStateTInstances extends IndexedStateT
   implicit def catsDataAlternativeForIndexedStateT[F[_], S](implicit FM: Monad[F],
     FA: Alternative[F]): Alternative[IndexedStateT[F, S, S, ?]] with Monad[IndexedStateT[F, S, S, ?]] =
     new IndexedStateTAlternative[F, S] { implicit def F = FM; implicit def G = FA }
+
+  implicit def catsDataContravariantMonoidalForIndexedStateT[F[_], S](implicit FD: ContravariantMonoidal[F],
+    FA: Applicative[F]): ContravariantMonoidal[IndexedStateT[F, S, S, ?]] =
+    new IndexedStateTContravariantMonoidal[F, S] { implicit def F = FD; implicit def G = FA }
 }
 
 private[data] sealed abstract class IndexedStateTInstances1 extends IndexedStateTInstances2 {
@@ -362,6 +366,28 @@ private[data] sealed abstract class IndexedStateTSemigroupK[F[_], SA, SB] extend
     IndexedStateT(s => G.combineK(x.run(s), y.run(s)))
 }
 
+private[data] sealed abstract class IndexedStateTContravariantMonoidal[F[_], S] extends ContravariantMonoidal[IndexedStateT[F, S, S, ?]]{
+  implicit def F: ContravariantMonoidal[F]
+  implicit def G: Applicative[F]
+
+  override def unit[A]: IndexedStateT[F, S, S, A]  =
+    IndexedStateT.applyF(G.pure((s: S) => F.unit[(S, A)]))
+
+  override def contramap[A, B](fa: IndexedStateT[F, S, S, A])(f: B => A): IndexedStateT[F, S, S, B] =
+    contramap2(fa, unit)(((a: A) => (a, a)) compose f)
+
+  override def product[A, B](fa: IndexedStateT[F, S, S, A], fb: IndexedStateT[F, S, S, B]): IndexedStateT[F, S, S, (A, B)] =
+    contramap2(fa, fb)(identity)
+
+  def contramap2[A, B, C](fb: IndexedStateT[F, S, S, B], fc: IndexedStateT[F, S, S, C])(f: A => (B, C)): IndexedStateT[F, S, S, A] =
+    IndexedStateT.applyF(
+      G.pure((s: S) =>
+        ContravariantMonoidal.contramap2(G.map(fb.runF)(_.apply(s)), G.map(fc.runF)(_.apply(s)))(
+          (tup: (S, A)) => f(tup._2) match {
+            case (b, c) => (G.pure((tup._1, b)), G.pure((tup._1, c)))
+          })(G, F)))
+}
+
 private[data] sealed abstract class IndexedStateTAlternative[F[_], S] extends IndexedStateTMonad[F, S] with Alternative[IndexedStateT[F, S, S, ?]] {
   def G: Alternative[F]
 

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

@@ -136,11 +136,8 @@ private[data] sealed abstract class KleisliInstances0 extends KleisliInstances1
   implicit val catsDataCommutativeArrowForKleisliId: CommutativeArrow[Kleisli[Id, ?, ?]] =
     catsDataCommutativeArrowForKleisli[Id]
 
-  implicit def catsDataContravariantForKleisli[F[_], C]: Contravariant[Kleisli[F, ?, C]] =
-    new Contravariant[Kleisli[F, ?, C]] {
-      override def contramap[A, B](fa: Kleisli[F, A, C])(f: B => A): Kleisli[F, B, C] =
-        fa.local(f)
-    }
+  implicit def catsDataContravariantMonoidalForKleisli[F[_], A](implicit F0: ContravariantMonoidal[F]): ContravariantMonoidal[Kleisli[F, A, ?]] =
+    new KleisliContravariantMonoidal[F, A] {  def F: ContravariantMonoidal[F] = F0 }
 }
 
 private[data] sealed abstract class KleisliInstances1 extends KleisliInstances2 {
@@ -160,6 +157,12 @@ private[data] sealed abstract class KleisliInstances1 extends KleisliInstances2
     def parallel: Kleisli[M, A, ?] ~> Kleisli[F, A, ?] =
       λ[Kleisli[M, A, ?] ~> Kleisli[F, A, ?]](_.mapK(P.parallel))
   }
+
+  implicit def catsDataContravariantForKleisli[F[_], C]: Contravariant[Kleisli[F, ?, C]] =
+    new Contravariant[Kleisli[F, ?, C]] {
+      override def contramap[A, B](fa: Kleisli[F, A, C])(f: B => A): Kleisli[F, B, C] =
+        fa.local(f)
+    }
 }
 
 private[data] sealed abstract class KleisliInstances2 extends KleisliInstances3 {
@@ -294,6 +297,18 @@ private[data] trait KleisliAlternative[F[_], A] extends Alternative[Kleisli[F, A
   implicit def F: Alternative[F]
 }
 
+private[data] sealed trait KleisliContravariantMonoidal[F[_], D] extends ContravariantMonoidal[Kleisli[F, D, ?]] {
+  implicit def F: ContravariantMonoidal[F]
+
+  override def unit[A]: Kleisli[F, D, A] = Kleisli(Function.const(F.unit[A]))
+
+  override def contramap[A, B](fa: Kleisli[F, D, A])(f: B => A): Kleisli[F, D, B] =
+    Kleisli(d => F.contramap(fa.run(d))(f))
+
+  override def product[A, B](fa: Kleisli[F, D, A], fb: Kleisli[F, D, B]): Kleisli[F, D, (A, B)] =
+    Kleisli(d => F.product(fa.run(d), fb.run(d)))
+}
+
 private[data] trait KleisliMonadError[F[_], A, E] extends MonadError[Kleisli[F, A, ?], E] with KleisliApplicativeError[F, A, E] with KleisliMonad[F, A] {
   def F: MonadError[F, E]
 }

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

@@ -43,6 +43,11 @@ private[data] sealed abstract class NestedInstances extends NestedInstances0 {
     new NestedNonEmptyTraverse[F, G] {
       val FG: NonEmptyTraverse[λ[α => F[G[α]]]] = NonEmptyTraverse[F].compose[G]
     }
+
+  implicit def catsDataContravariantMonoidalForApplicativeForNested[F[_]: Applicative, G[_]: ContravariantMonoidal]: ContravariantMonoidal[Nested[F, G, ?]] =
+    new NestedContravariantMonoidal[F, G] {
+      val FG: ContravariantMonoidal[λ[α => F[G[α]]]] = Applicative[F].composeContravariantMonoidal[G]
+    }
 }
 
 private[data] sealed abstract class NestedInstances0 extends NestedInstances1 {
@@ -263,3 +268,15 @@ private[data] trait NestedContravariant[F[_], G[_]] extends Contravariant[Nested
   def contramap[A, B](fga: Nested[F, G, A])(f: B => A): Nested[F, G, B] =
     Nested(FG.contramap(fga.value)(f))
 }
+
+private[data] trait NestedContravariantMonoidal[F[_], G[_]] extends ContravariantMonoidal[Nested[F, G, ?]] {
+  def FG: ContravariantMonoidal[λ[α => F[G[α]]]]
+
+  def unit[A]: Nested[F, G, A] = Nested(FG.unit)
+
+  def contramap[A, B](fa: Nested[F, G, A])(f: B => A): Nested[F, G, B] =
+    Nested(FG.contramap(fa.value)(f))
+
+  def product[A, B](fa: Nested[F, G, A], fb: Nested[F, G, B]): Nested[F, G, (A, B)] =
+    Nested(FG.product(fa.value, fb.value))
+}

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

@@ -221,6 +221,9 @@ private[data] sealed abstract class OptionTInstances0 extends OptionTInstances1
   implicit def catsDataMonadErrorForOptionT[F[_], E](implicit F0: MonadError[F, E]): MonadError[OptionT[F, ?], E] =
     new OptionTMonadError[F, E] { implicit val F = F0 }
 
+  implicit def catsDataContravariantMonoidalForOptionT[F[_]](implicit F0: ContravariantMonoidal[F]): ContravariantMonoidal[OptionT[F, ?]] =
+    new OptionTContravariantMonoidal[F] { implicit val F = F0 }
+
   implicit def catsDataSemigroupKForOptionT[F[_]](implicit F0: Monad[F]): SemigroupK[OptionT[F, ?]] =
     new OptionTSemigroupK[F] { implicit val F = F0 }
 
@@ -281,6 +284,23 @@ private trait OptionTMonadError[F[_], E] extends MonadError[OptionT[F, ?], E] wi
     OptionT(F.handleErrorWith(fa.value)(f(_).value))
 }
 
+private trait OptionTContravariantMonoidal[F[_]] extends ContravariantMonoidal[OptionT[F, ?]] {
+  def F: ContravariantMonoidal[F]
+
+  override def unit[A]: OptionT[F, A] = OptionT (F.unit)
+
+  override def contramap[A, B](fa: OptionT[F, A])(f: B => A): OptionT[F, B] =
+    OptionT(F.contramap(fa.value)(_ map f))
+
+  override def product[A, B](fa: OptionT[F, A], fb: OptionT[F, B]): OptionT[F, (A, B)] =
+    OptionT(F.contramap(F.product(fa.value, fb.value))(
+        (t: Option[(A, B)]) => t match {
+          case Some((x, y)) => (Some(x), Some(y))
+          case None => (None, None)
+        }
+      ))
+}
+
 private[data] trait OptionTFoldable[F[_]] extends Foldable[OptionT[F, ?]] {
   implicit def F: Foldable[F]
 

core/src/main/scala/cats/data/Tuple2K.scala

@@ -29,17 +29,22 @@ private[data] sealed abstract class Tuple2KInstances extends Tuple2KInstances0 {
     def F: Show[F[A]] = FF
     def G: Show[G[A]] = GF
   }
-  implicit def catsDataContravariantForTuple2K[F[_], G[_]](implicit FC: Contravariant[F], GC: Contravariant[G]): Contravariant[λ[α => Tuple2K[F, G, α]]] = new Tuple2KContravariant[F, G] {
-    def F: Contravariant[F] = FC
-    def G: Contravariant[G] = GC
-  }
+  implicit def catsDataContravariantMonoidalForTuple2k[F[_], G[_]](implicit FD: ContravariantMonoidal[F], GD: ContravariantMonoidal[G]): ContravariantMonoidal[λ[α => Tuple2K[F, G, α]]] =
+    new Tuple2KContravariantMonoidal[F, G] {
+      def F: ContravariantMonoidal[F] = FD
+      def G: ContravariantMonoidal[G] = GD
+    }
 }
 
 private[data] sealed abstract class Tuple2KInstances0 extends Tuple2KInstances1 {
   implicit def catsDataTraverseForTuple2K[F[_], G[_]](implicit FF: Traverse[F], GF: Traverse[G]): Traverse[λ[α => Tuple2K[F, G, α]]] = new Tuple2KTraverse[F, G] {
     def F: Traverse[F] = FF
     def G: Traverse[G] = GF
   }
+  implicit def catsDataContravariantForTuple2K[F[_], G[_]](implicit FC: Contravariant[F], GC: Contravariant[G]): Contravariant[λ[α => Tuple2K[F, G, α]]] = new Tuple2KContravariant[F, G] {
+    def F: Contravariant[F] = FC
+    def G: Contravariant[G] = GC
+  }
   implicit def catsDataEqForTuple2K[F[_], G[_], A](implicit FF: Eq[F[A]], GG: Eq[G[A]]): Eq[Tuple2K[F, G, A]] = new Eq[Tuple2K[F, G, A]] {
     def eqv(x: Tuple2K[F, G, A], y: Tuple2K[F, G, A]): Boolean =
       FF.eqv(x.first, y.first) && GG.eqv(x.second, y.second)
@@ -123,6 +128,16 @@ private[data] sealed trait Tuple2KContravariant[F[_], G[_]] extends Contravarian
   def contramap[A, B](fa: Tuple2K[F, G, A])(f: B => A): Tuple2K[F, G, B] = Tuple2K(F.contramap(fa.first)(f), G.contramap(fa.second)(f))
 }
 
+private[data] sealed trait Tuple2KContravariantMonoidal[F[_], G[_]] extends ContravariantMonoidal[λ[α => Tuple2K[F, G, α]]] {
+  def F: ContravariantMonoidal[F]
+  def G: ContravariantMonoidal[G]
+  def unit[A]: Tuple2K[F, G, A] = Tuple2K(F.unit, G.unit)
+  def product[A, B](fa: Tuple2K[F, G, A], fb: Tuple2K[F, G, B]): Tuple2K[F, G, (A, B)] =
+    Tuple2K(F.product(fa.first, fb.first), G.product(fa.second, fb.second))
+  def contramap[A, B](fa: Tuple2K[F, G, A])(f: B => A): Tuple2K[F, G, B] =
+    Tuple2K(F.contramap(fa.first)(f), G.contramap(fa.second)(f))
+}
+
 private[data] sealed trait Tuple2KApply[F[_], G[_]] extends Apply[λ[α => Tuple2K[F, G, α]]] with Tuple2KFunctor[F, G] {
   def F: Apply[F]
   def G: Apply[G]