Cats-1567 Replace Split with Commutative Arrow. Introduces Commutative Monad.

core/src/main/scala/cats/CommutativeCoflatMap.scala

@@ -0,0 +1,14 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * Commutative CoflatMap.
+ *
+ * Further than a CoflatMap, which just allows composition of dependent effectful functions,
+ * in a Commutative CoflatMap those functions can be composed in any order, which guarantees
+ * that their effects do not interfere.
+ *
+ * Must obey the laws defined in cats.laws.CommutativeCoflatMapLaws.
+ */
+@typeclass trait CommutativeCoflatMap[F[_]] extends CoflatMap[F]

core/src/main/scala/cats/CommutativeComonad.scala

@@ -0,0 +1,14 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * Commutative Comonad.
+ *
+ * Further than a Comonad, which just allows composition of dependent effectful functions,
+ * in a Commutative Comonad those functions can be composed in any order, which guarantees
+ * that their effects do not interfere.
+ *
+ * Must obey the laws defined in cats.laws.CommutativeComonadLaws.
+ */
+@typeclass trait CommutativeComonad[F[_]] extends Comonad[F] with CommutativeCoflatMap[F]

core/src/main/scala/cats/CommutativeFlatMap.scala

@@ -0,0 +1,14 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * Commutative FlatMap.
+ *
+ * Further than a FlatMap, which just allows composition of dependent effectful functions,
+ * in a Commutative FlatMap those functions can be composed in any order, which guarantees
+ * that their effects do not interfere.
+ *
+ * Must obey the laws defined in cats.laws.CommutativeFlatMapLaws.
+ */
+@typeclass trait CommutativeFlatMap[F[_]] extends FlatMap[F]

core/src/main/scala/cats/CommutativeMonad.scala

@@ -0,0 +1,14 @@
+package cats
+
+import simulacrum.typeclass
+
+/**
+ * Commutative Monad.
+ *
+ * Further than a Monad, which just allows composition of dependent effectful functions,
+ * in a Commutative Monad those functions can be composed in any order, which guarantees
+ * that their effects do not interfere.
+ *
+ * Must obey the laws defined in cats.laws.CommutativeMonadLaws.
+ */
+@typeclass trait CommutativeMonad[F[_]] extends Monad[F] with CommutativeFlatMap[F]

core/src/main/scala/cats/arrow/Arrow.scala

@@ -5,10 +5,16 @@ import cats.functor.Strong
 
 import simulacrum.typeclass
 
-@typeclass trait Arrow[F[_, _]] extends Split[F] with Strong[F] with Category[F] { self =>
+/**
+ * Must obey the laws defined in cats.laws.ArrowLaws.
+ */
+@typeclass trait Arrow[F[_, _]] extends Category[F] with Strong[F] { self =>
 
   /**
-   * Lift a function into the context of an Arrow
+   * Lift a function into the context of an Arrow.
+   *
+   * In the reference articles "Arrows are Promiscuous...", and in the corresponding Haskell
+   * library `Control.Arrow`, this function is called `arr`.
    */
   def lift[A, B](f: A => B): F[A, B]
 
@@ -20,6 +26,25 @@ import simulacrum.typeclass
     compose(swap, compose(first[A, B, C](fa), swap))
   }
 
-  override def split[A, B, C, D](f: F[A, B], g: F[C, D]): F[(A, C), (B, D)] =
+  /**
+   * Create a new computation `F` that splits its input between `f` and `g`
+   * and combines the output of each.
+   *
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> import cats.arrow.Arrow
+   * scala> val toLong: Int => Long = _.toLong
+   * scala> val toDouble: Float => Double = _.toDouble
+   * scala> val f: ((Int, Float)) => (Long, Double) = Arrow[Function1].split(toLong, toDouble)
+   * scala> f((3, 4.0f))
+   * res0: (Long, Double) = (3,4.0)
+   * }}}
+   *
+   * Note that the arrow laws do not guarantee the non-interference between the _effects_ of
+   *  `f` and `g` in the context of F. This means that `f *** g` may not be equivalent to `g *** f`.
+   */
+  @simulacrum.op("***", alias = true)
+  def split[A, B, C, D](f: F[A, B], g: F[C, D]): F[(A, C), (B, D)] =
     andThen(first(f), second(g))
 }

core/src/main/scala/cats/arrow/CommutativeArrow.scala

@@ -0,0 +1,13 @@
+package cats
+package arrow
+
+import simulacrum.typeclass
+
+/**
+  * In a Commutative Arrow F[_, _], the split operation (or `***`) is commutative,
+  * which means that there is non-interference between the effect of the paired arrows.
+  *
+  * Must obey the laws in CommutativeArrowLaws
+  */
+@typeclass trait CommutativeArrow[F[_, _]] extends Arrow[F]
+

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

@@ -1,9 +1,9 @@
 package cats
 package data
 
-import cats.arrow.{Arrow, Category, Compose, Split}
+import cats.arrow.{Arrow, Category, CommutativeArrow, Compose}
 import cats.functor.{Contravariant, Profunctor}
-import cats.{CoflatMap, Comonad, Functor, Monad}
+import cats.{CoflatMap, Comonad, CommutativeComonad, Functor, Monad}
 import scala.annotation.tailrec
 
 /**
@@ -45,37 +45,27 @@ object Cokleisli extends CokleisliInstances {
 }
 
 private[data] sealed abstract class CokleisliInstances extends CokleisliInstances0 {
-  implicit def catsDataArrowForCokleisli[F[_]](implicit ev: Comonad[F]): Arrow[Cokleisli[F, ?, ?]] =
-    new CokleisliArrow[F] { def F: Comonad[F] = ev }
-
-  implicit def catsDataMonadForCokleisli[F[_], A]: Monad[Cokleisli[F, A, ?]] = new Monad[Cokleisli[F, A, ?]] {
-    def pure[B](x: B): Cokleisli[F, A, B] =
-      Cokleisli.pure(x)
+  implicit def catsDataCommutativeArrowForCokleisli[F[_]](implicit ev: CommutativeComonad[F]): CommutativeArrow[Cokleisli[F, ?, ?]] =
+    new CokleisliCommutativeArrow[F] { def F: CommutativeComonad[F] = ev }
 
-    def flatMap[B, C](fa: Cokleisli[F, A, B])(f: B => Cokleisli[F, A, C]): Cokleisli[F, A, C] =
-      fa.flatMap(f)
+  implicit val catsDataCommutativeArrowForCokleisliId: CommutativeArrow[Cokleisli[Id, ?, ?]] =
+    catsDataCommutativeArrowForCokleisli[Id]
 
-    override def map[B, C](fa: Cokleisli[F, A, B])(f: B => C): Cokleisli[F, A, C] =
-      fa.map(f)
-
-    def tailRecM[B, C](b: B)(fn: B => Cokleisli[F, A, Either[B, C]]): Cokleisli[F, A, C] =
-      Cokleisli({ (fa: F[A]) =>
-        @tailrec
-        def loop(c: Cokleisli[F, A, Either[B, C]]): C = c.run(fa) match {
-          case Right(c) => c
-          case Left(bb) => loop(fn(bb))
-        }
-        loop(fn(b))
-      })
-  }
+  implicit def catsDataMonadForCokleisli[F[_], A]: Monad[Cokleisli[F, A, ?]] =
+    new CokleisliMonad[F, A]
 
   implicit def catsDataMonoidKForCokleisli[F[_]](implicit ev: Comonad[F]): MonoidK[λ[α => Cokleisli[F, α, α]]] =
     Category[Cokleisli[F, ?, ?]].algebraK
 }
 
-private[data] sealed abstract class CokleisliInstances0 {
-  implicit def catsDataSplitForCokleisli[F[_]](implicit ev: CoflatMap[F]): Split[Cokleisli[F, ?, ?]] =
-    new CokleisliSplit[F] { def F: CoflatMap[F] = ev }
+private[data] sealed abstract class CokleisliInstances0 extends CokleisliInstances1 {
+  implicit def catsDataArrowForCokleisli[F[_]](implicit ev: Comonad[F]): Arrow[Cokleisli[F, ?, ?]] =
+    new CokleisliArrow[F] { def F: Comonad[F] = ev }
+}
+
+private[data] sealed abstract class CokleisliInstances1 {
+  implicit def catsDataComposeForCokleisli[F[_]](implicit ev: CoflatMap[F]): Compose[Cokleisli[F, ?, ?]] =
+    new CokleisliCompose[F] { def F: CoflatMap[F] = ev }
 
   implicit def catsDataProfunctorForCokleisli[F[_]](implicit ev: Functor[F]): Profunctor[Cokleisli[F, ?, ?]] =
     new CokleisliProfunctor[F] { def F: Functor[F] = ev }
@@ -89,7 +79,34 @@ private[data] sealed abstract class CokleisliInstances0 {
     }
 }
 
-private trait CokleisliArrow[F[_]] extends Arrow[Cokleisli[F, ?, ?]] with CokleisliSplit[F] with CokleisliProfunctor[F] {
+private[data] trait CokleisliCommutativeArrow[F[_]] extends CommutativeArrow[Cokleisli[F, ?, ?]] with CokleisliArrow[F] {
+  implicit def F: CommutativeComonad[F]
+}
+
+private[data] class CokleisliMonad[F[_], A] extends Monad[Cokleisli[F, A, ?]] {
+
+  def pure[B](x: B): Cokleisli[F, A, B] =
+    Cokleisli.pure(x)
+
+  def flatMap[B, C](fa: Cokleisli[F, A, B])(f: B => Cokleisli[F, A, C]): Cokleisli[F, A, C] =
+    fa.flatMap(f)
+
+  override def map[B, C](fa: Cokleisli[F, A, B])(f: B => C): Cokleisli[F, A, C] =
+    fa.map(f)
+
+  def tailRecM[B, C](b: B)(fn: B => Cokleisli[F, A, Either[B, C]]): Cokleisli[F, A, C] =
+    Cokleisli({ (fa: F[A]) =>
+      @tailrec
+      def loop(c: Cokleisli[F, A, Either[B, C]]): C = c.run(fa) match {
+        case Right(c) => c
+        case Left(bb) => loop(fn(bb))
+      }
+      loop(fn(b))
+    })
+
+}
+
+private trait CokleisliArrow[F[_]] extends Arrow[Cokleisli[F, ?, ?]] with CokleisliCompose[F] with CokleisliProfunctor[F] {
   implicit def F: Comonad[F]
 
   def lift[A, B](f: A => B): Cokleisli[F, A, B] =
@@ -108,17 +125,14 @@ private trait CokleisliArrow[F[_]] extends Arrow[Cokleisli[F, ?, ?]] with Coklei
     super[CokleisliProfunctor].dimap(fab)(f)(g)
 
   override def split[A, B, C, D](f: Cokleisli[F, A, B], g: Cokleisli[F, C, D]): Cokleisli[F, (A, C), (B, D)] =
-    super[CokleisliSplit].split(f, g)
+    Cokleisli(fac => f.run(F.map(fac)(_._1)) -> g.run(F.map(fac)(_._2)))
 }
 
-private trait CokleisliSplit[F[_]] extends Split[Cokleisli[F, ?, ?]] {
+private trait CokleisliCompose[F[_]] extends Compose[Cokleisli[F, ?, ?]] {
   implicit def F: CoflatMap[F]
 
   def compose[A, B, C](f: Cokleisli[F, B, C], g: Cokleisli[F, A, B]): Cokleisli[F, A, C] =
     f.compose(g)
-
-  def split[A, B, C, D](f: Cokleisli[F, A, B], g: Cokleisli[F, C, D]): Cokleisli[F, (A, C), (B, D)] =
-    Cokleisli(fac => f.run(F.map(fac)(_._1)) -> g.run(F.map(fac)(_._2)))
 }
 
 private trait CokleisliProfunctor[F[_]] extends Profunctor[Cokleisli[F, ?, ?]] {

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

@@ -1,7 +1,7 @@
 package cats
 package data
 
-import cats.arrow.{Arrow, Category, Choice, Compose, Split, FunctionK}
+import cats.arrow.{Arrow, Category, Choice, CommutativeArrow, Compose, FunctionK}
 import cats.functor.{Contravariant, Strong}
 
 /**
@@ -91,11 +91,11 @@ private[data] sealed abstract class KleisliInstances extends KleisliInstances0 {
   implicit val catsDataMonoidKForKleisliId: MonoidK[λ[α => Kleisli[Id, α, α]]] =
     catsDataMonoidKForKleisli[Id]
 
-  implicit def catsDataArrowForKleisli[F[_]](implicit M: Monad[F]): Arrow[Kleisli[F, ?, ?]] =
-    new KleisliArrow[F] { def F: Monad[F] = M }
+  implicit def catsDataCommutativeArrowForKleisli[F[_]](implicit M: CommutativeMonad[F]): CommutativeArrow[Kleisli[F, ?, ?]] =
+    new KleisliCommutativeArrow[F] {def F: CommutativeMonad[F] = M }
 
-  implicit val catsDataArrowForKleisliId: Arrow[Kleisli[Id, ?, ?]] =
-    catsDataArrowForKleisli[Id]
+  implicit val catsDataCommutativeArrowForKleisliId: CommutativeArrow[Kleisli[Id, ?, ?]] =
+    catsDataCommutativeArrowForKleisli[Id]
 
   implicit def catsDataMonadReaderForKleisliId[A]: MonadReader[Kleisli[Id, A, ?], A] =
     catsDataMonadReaderForKleisli[Id, A]
@@ -116,6 +116,9 @@ private[data] sealed abstract class KleisliInstances extends KleisliInstances0 {
 }
 
 private[data] sealed abstract class KleisliInstances0 extends KleisliInstances1 {
+  implicit def catsDataArrowForKleisli[F[_]](implicit M: Monad[F]): Arrow[Kleisli[F, ?, ?]] =
+    new KleisliArrow[F] { def F: Monad[F] = M }
+
   implicit def catsDataMonadErrorForKleisli[F[_], A, E](implicit ME: MonadError[F, E]): MonadError[Kleisli[F, A, ?], E] =
     new KleisliMonadError[F, A, E] { def F: MonadError[F, E] = ME }
 }
@@ -132,8 +135,8 @@ private[data] sealed abstract class KleisliInstances2 extends KleisliInstances3
   implicit val catsDataChoiceForKleisliId: Choice[Kleisli[Id, ?, ?]] =
     catsDataChoiceForKleisli[Id]
 
-  implicit def catsDataSplitForKleisli[F[_]](implicit FM: FlatMap[F]): Split[Kleisli[F, ?, ?]] =
-    new KleisliSplit[F] { def F: FlatMap[F] = FM }
+  implicit def catsDataComposeForKleisli[F[_]](implicit FM: FlatMap[F]): Compose[Kleisli[F, ?, ?]] =
+    new KleisliCompose[F] { def F: FlatMap[F] = FM }
 
   implicit def catsDataStrongForKleisli[F[_]](implicit F0: Functor[F]): Strong[Kleisli[F, ?, ?]] =
     new KleisliStrong[F] { def F: Functor[F] = F0 }
@@ -163,15 +166,15 @@ private[data] sealed abstract class KleisliInstances5 {
     new KleisliFunctor[F, A] { def F: Functor[F] = F0 }
 }
 
-private trait KleisliArrow[F[_]] extends Arrow[Kleisli[F, ?, ?]] with KleisliSplit[F] with KleisliStrong[F] with KleisliCategory[F] {
+private trait KleisliCommutativeArrow[F[_]] extends CommutativeArrow[Kleisli[F, ?, ?]] with KleisliArrow[F] {
+  implicit def F: CommutativeMonad[F]
+}
+
+private trait KleisliArrow[F[_]] extends Arrow[Kleisli[F, ?, ?]] with KleisliCategory[F] with KleisliStrong[F]  {
   implicit def F: Monad[F]
 
   def lift[A, B](f: A => B): Kleisli[F, A, B] =
     Kleisli(a => F.pure(f(a)))
-}
-
-private trait KleisliSplit[F[_]] extends Split[Kleisli[F, ?, ?]] with KleisliCompose[F] {
-  implicit def F: FlatMap[F]
 
   override def split[A, B, C, D](f: Kleisli[F, A, B], g: Kleisli[F, C, D]): Kleisli[F, (A, C), (B, D)] =
     Kleisli{ case (a, c) => F.flatMap(f.run(a))(b => F.map(g.run(c))(d => (b, d))) }

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

@@ -367,8 +367,8 @@ object NonEmptyList extends NonEmptyListInstances {
 private[data] sealed trait NonEmptyListInstances extends NonEmptyListInstances0 {
 
   implicit val catsDataInstancesForNonEmptyList: SemigroupK[NonEmptyList] with Reducible[NonEmptyList]
-      with Comonad[NonEmptyList] with NonEmptyTraverse[NonEmptyList] with Monad[NonEmptyList] =
-    new NonEmptyReducible[NonEmptyList, List] with SemigroupK[NonEmptyList] with Comonad[NonEmptyList]
+      with CommutativeComonad[NonEmptyList] with NonEmptyTraverse[NonEmptyList] with Monad[NonEmptyList] =
+    new NonEmptyReducible[NonEmptyList, List] with SemigroupK[NonEmptyList] with CommutativeComonad[NonEmptyList]
       with Monad[NonEmptyList] with NonEmptyTraverse[NonEmptyList] {
 
       def combineK[A](a: NonEmptyList[A], b: NonEmptyList[A]): NonEmptyList[A] =

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

@@ -5,8 +5,8 @@ import scala.annotation.tailrec
 
 trait OptionInstances extends cats.kernel.instances.OptionInstances {
 
-  implicit val catsStdInstancesForOption: TraverseFilter[Option] with MonadError[Option, Unit] with MonadCombine[Option] with Monad[Option] with CoflatMap[Option] with Alternative[Option] =
-    new TraverseFilter[Option] with MonadError[Option, Unit]  with MonadCombine[Option] with Monad[Option] with CoflatMap[Option] with Alternative[Option] {
+  implicit val catsStdInstancesForOption: TraverseFilter[Option] with MonadError[Option, Unit] with MonadCombine[Option] with CommutativeMonad[Option] with CoflatMap[Option] with Alternative[Option] =
+    new TraverseFilter[Option] with MonadError[Option, Unit] with MonadCombine[Option] with CommutativeMonad[Option] with CoflatMap[Option] with Alternative[Option] {
 
       def empty[A]: Option[A] = None
 

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

@@ -32,8 +32,8 @@ package object cats {
  * encodes pure unary function application.
  */
   type Id[A] = A
-  implicit val catsInstancesForId: Bimonad[Id] with Monad[Id] with NonEmptyTraverse[Id] =
-    new Bimonad[Id] with Monad[Id] with NonEmptyTraverse[Id] {
+  implicit val catsInstancesForId: Bimonad[Id] with CommutativeMonad[Id] with CommutativeComonad[Id] with NonEmptyTraverse[Id] =
+    new Bimonad[Id] with CommutativeMonad[Id] with CommutativeComonad[Id] with NonEmptyTraverse[Id] {
       def pure[A](a: A): A = a
       def extract[A](a: A): A = a
       def flatMap[A, B](a: A)(f: A => B): B = f(a)

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

@@ -5,6 +5,7 @@ trait AllSyntax
     extends ApplicativeSyntax
     with ApplicativeErrorSyntax
     with ApplySyntax
+    with ArrowSyntax
     with BifunctorSyntax
     with BifoldableSyntax
     with BitraverseSyntax
@@ -38,7 +39,6 @@ trait AllSyntax
     with SemigroupSyntax
     with SemigroupKSyntax
     with ShowSyntax
-    with SplitSyntax
     with StrongSyntax
     with TraverseFilterSyntax
     with TraverseSyntax

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

@@ -0,0 +1,6 @@
+package cats
+package syntax
+
+import cats.arrow.Arrow
+
+trait ArrowSyntax extends Arrow.ToArrowOps