Replace Split with Commutative Arrow. Continuing #1719

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

@@ -1,7 +1,7 @@
 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 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)
-
-    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))
-      })
+  implicit val catsDataCommutativeArrowForCokleisliId: CommutativeArrow[Cokleisli[Id, ?, ?]] =
+    new CokleisliArrow[Id] with CommutativeArrow[Cokleisli[Id, ?, ?]] {
+      def F: Comonad[Id] = Comonad[Id]
   }
 
+  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 +79,32 @@ private[data] sealed abstract class CokleisliInstances0 {
     }
 }
 
-private trait CokleisliArrow[F[_]] extends Arrow[Cokleisli[F, ?, ?]] with CokleisliSplit[F] with CokleisliProfunctor[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 +123,14 @@ 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}
 
 /**
@@ -81,7 +81,13 @@ private[data] sealed trait KleisliFunctions {
 }
 
 private[data] sealed abstract class KleisliInstances extends KleisliInstances0 {
+  implicit def catsDataCommutativeMonadForKleisli[F[_], A, B](implicit F0: CommutativeMonad[F]): CommutativeMonad[Kleisli[F, A, ?]] =
+    new KleisliMonad[F, A] with CommutativeMonad[Kleisli[F, A, ?]] {
+      implicit def F: Monad[F] = F0
+    }
+}
 
+private[data] sealed abstract class KleisliInstances0 extends KleisliInstances1 {
   implicit def catsDataMonoidForKleisli[F[_], A, B](implicit FB0: Monoid[F[B]]): Monoid[Kleisli[F, A, B]] =
     new KleisliMonoid[F, A, B] { def FB: Monoid[F[B]] = FB0 }
 
@@ -91,11 +97,11 @@ 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]
@@ -115,25 +121,28 @@ private[data] sealed abstract class KleisliInstances extends KleisliInstances0 {
     new KleisliApplicativeError[F, A, E] { def F: ApplicativeError[F, E] = AE }
 }
 
-private[data] sealed abstract class KleisliInstances0 extends KleisliInstances1 {
+private[data] sealed abstract class KleisliInstances1 extends KleisliInstances2 {
+  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 }
 }
 
-private[data] sealed abstract class KleisliInstances1 extends KleisliInstances2 {
+private[data] sealed abstract class KleisliInstances2 extends KleisliInstances3 {
   implicit def catsDataMonadReaderForKleisli[F[_], A](implicit M: Monad[F]): MonadReader[Kleisli[F, A, ?], A] =
     new KleisliMonadReader[F, A] { def F: Monad[F] = M }
 }
 
-private[data] sealed abstract class KleisliInstances2 extends KleisliInstances3 {
+private[data] sealed abstract class KleisliInstances3 extends KleisliInstances4 {
   implicit def catsDataChoiceForKleisli[F[_]](implicit M: Monad[F]): Choice[Kleisli[F, ?, ?]] =
     new KleisliChoice[F] { def F: Monad[F] = M }
 
   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 }
@@ -148,30 +157,30 @@ private[data] sealed abstract class KleisliInstances2 extends KleisliInstances3
     Compose[Kleisli[F, ?, ?]].algebraK
 }
 
-private[data] sealed abstract class KleisliInstances3 extends KleisliInstances4 {
+private[data] sealed abstract class KleisliInstances4 extends KleisliInstances5 {
   implicit def catsDataApplicativeForKleisli[F[_], A](implicit A: Applicative[F]): Applicative[Kleisli[F, A, ?]] =
     new KleisliApplicative[F, A] { def F: Applicative[F] = A }
 }
 
-private[data] sealed abstract class KleisliInstances4 extends KleisliInstances5 {
+private[data] sealed abstract class KleisliInstances5 extends KleisliInstances6 {
   implicit def catsDataApplyForKleisli[F[_], A](implicit A: Apply[F]): Apply[Kleisli[F, A, ?]] =
     new KleisliApply[F, A] { def F: Apply[F] = A }
 }
 
-private[data] sealed abstract class KleisliInstances5 {
+private[data] sealed abstract class KleisliInstances6 {
   implicit def catsDataFunctorForKleisli[F[_], A](implicit F0: Functor[F]): Functor[Kleisli[F, A, ?]] =
     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))) }