Swap type parameter order in OneAnd

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

@@ -10,10 +10,10 @@ import scala.collection.mutable.ListBuffer
  * used to represent a List which is guaranteed to not be empty:
  *
  * {{{
- * type NonEmptyList[A] = OneAnd[A, List]
+ * type NonEmptyList[A] = OneAnd[List, A]
  * }}}
  */
-final case class OneAnd[A, F[_]](head: A, tail: F[A]) {
+final case class OneAnd[F[_], A](head: A, tail: F[A]) {
 
   /**
    * Combine the head and tail into a single `F[A]` value.
@@ -32,7 +32,7 @@ final case class OneAnd[A, F[_]](head: A, tail: F[A]) {
   /**
    * Append another OneAnd to this
    */
-  def combine(other: OneAnd[A, F])(implicit F: MonadCombine[F]): OneAnd[A, F] =
+  def combine(other: OneAnd[F, A])(implicit F: MonadCombine[F]): OneAnd[F, A] =
     OneAnd(head, F.combine(tail, F.combine(F.pure(other.head), other.tail)))
 
   /**
@@ -69,11 +69,11 @@ final case class OneAnd[A, F[_]](head: A, tail: F[A]) {
    * Typesafe equality operator.
    *
    * This method is similar to == except that it only allows two
-   * OneAnd[A, F] values to be compared to each other, and uses
+   * OneAnd[F, A] values to be compared to each other, and uses
    * equality provided by Eq[_] instances, rather than using the
    * universal equality provided by .equals.
    */
-  def ===(that: OneAnd[A, F])(implicit A: Eq[A], FA: Eq[F[A]]): Boolean =
+  def ===(that: OneAnd[F, A])(implicit A: Eq[A], FA: Eq[F[A]]): Boolean =
     A.eqv(head, that.head) && FA.eqv(tail, that.tail)
 
   /**
@@ -89,41 +89,41 @@ final case class OneAnd[A, F[_]](head: A, tail: F[A]) {
 
 trait OneAndInstances extends OneAndLowPriority1 {
 
-  implicit def oneAndEq[A, F[_]](implicit A: Eq[A], FA: Eq[F[A]]): Eq[OneAnd[A, F]] =
-    new Eq[OneAnd[A, F]]{
-      def eqv(x: OneAnd[A, F], y: OneAnd[A, F]): Boolean = x === y
+  implicit def oneAndEq[A, F[_]](implicit A: Eq[A], FA: Eq[F[A]]): Eq[OneAnd[F, A]] =
+    new Eq[OneAnd[F, A]]{
+      def eqv(x: OneAnd[F, A], y: OneAnd[F, A]): Boolean = x === y
     }
 
-  implicit def oneAndShow[A, F[_]](implicit A: Show[A], FA: Show[F[A]]): Show[OneAnd[A, F]] =
-    Show.show[OneAnd[A, F]](_.show)
+  implicit def oneAndShow[A, F[_]](implicit A: Show[A], FA: Show[F[A]]): Show[OneAnd[F, A]] =
+    Show.show[OneAnd[F, A]](_.show)
 
-  implicit def oneAndSemigroupK[F[_]: MonadCombine]: SemigroupK[OneAnd[?, F]] =
-    new SemigroupK[OneAnd[?, F]] {
-      def combine[A](a: OneAnd[A, F], b: OneAnd[A, F]): OneAnd[A, F] =
+  implicit def oneAndSemigroupK[F[_]: MonadCombine]: SemigroupK[OneAnd[F, ?]] =
+    new SemigroupK[OneAnd[F, ?]] {
+      def combine[A](a: OneAnd[F, A], b: OneAnd[F, A]): OneAnd[F, A] =
         a combine b
     }
 
-  implicit def oneAndSemigroup[F[_]: MonadCombine, A]: Semigroup[OneAnd[A, F]] =
+  implicit def oneAndSemigroup[F[_]: MonadCombine, A]: Semigroup[OneAnd[F, A]] =
     oneAndSemigroupK.algebra
 
-  implicit def oneAndFoldable[F[_]](implicit foldable: Foldable[F]): Foldable[OneAnd[?,F]] =
-    new Foldable[OneAnd[?,F]] {
-      override def foldLeft[A, B](fa: OneAnd[A, F], b: B)(f: (B, A) => B): B =
+  implicit def oneAndFoldable[F[_]](implicit foldable: Foldable[F]): Foldable[OneAnd[F, ?]] =
+    new Foldable[OneAnd[F, ?]] {
+      override def foldLeft[A, B](fa: OneAnd[F, A], b: B)(f: (B, A) => B): B =
         fa.foldLeft(b)(f)
-      override def foldRight[A, B](fa: OneAnd[A, F], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
+      override def foldRight[A, B](fa: OneAnd[F, A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
         fa.foldRight(lb)(f)
-      override def isEmpty[A](fa: OneAnd[A, F]): Boolean = false
+      override def isEmpty[A](fa: OneAnd[F, A]): Boolean = false
     }
 
-  implicit def oneAndMonad[F[_]](implicit monad: MonadCombine[F]): Monad[OneAnd[?, F]] =
-    new Monad[OneAnd[?, F]] {
-      override def map[A, B](fa: OneAnd[A,F])(f: A => B): OneAnd[B, F] =
+  implicit def oneAndMonad[F[_]](implicit monad: MonadCombine[F]): Monad[OneAnd[F, ?]] =
+    new Monad[OneAnd[F, ?]] {
+      override def map[A, B](fa: OneAnd[F, A])(f: A => B): OneAnd[F, B] =
         OneAnd(f(fa.head), monad.map(fa.tail)(f))
 
-      def pure[A](x: A): OneAnd[A, F] =
+      def pure[A](x: A): OneAnd[F, A] =
         OneAnd(x, monad.empty)
 
-      def flatMap[A, B](fa: OneAnd[A, F])(f: A => OneAnd[B, F]): OneAnd[B, F] = {
+      def flatMap[A, B](fa: OneAnd[F, A])(f: A => OneAnd[F, B]): OneAnd[F, B] = {
         val end = monad.flatMap(fa.tail) { a =>
           val fa = f(a)
           monad.combine(monad.pure(fa.head), fa.tail)
@@ -135,10 +135,10 @@ trait OneAndInstances extends OneAndLowPriority1 {
 }
 
 trait OneAndLowPriority0 {
-  implicit val nelComonad: Comonad[OneAnd[?, List]] =
-    new Comonad[OneAnd[?, List]] {
+  implicit val nelComonad: Comonad[OneAnd[List, ?]] =
+    new Comonad[OneAnd[List, ?]] {
 
-      def coflatMap[A, B](fa: OneAnd[A, List])(f: OneAnd[A, List] => B): OneAnd[B, List] = {
+      def coflatMap[A, B](fa: OneAnd[List, A])(f: OneAnd[List, A] => B): OneAnd[List, B] = {
         @tailrec def consume(as: List[A], buf: ListBuffer[B]): List[B] =
           as match {
             case Nil => buf.toList
@@ -147,18 +147,18 @@ trait OneAndLowPriority0 {
         OneAnd(f(fa), consume(fa.tail, ListBuffer.empty))
       }
 
-      def extract[A](fa: OneAnd[A, List]): A =
+      def extract[A](fa: OneAnd[List, A]): A =
         fa.head
 
-      def map[A, B](fa: OneAnd[A, List])(f: A => B): OneAnd[B, List] =
+      def map[A, B](fa: OneAnd[List, A])(f: A => B): OneAnd[List, B] =
         OneAnd(f(fa.head), fa.tail.map(f))
     }
 }
 
 trait OneAndLowPriority1 extends OneAndLowPriority0 {
-  implicit def oneAndFunctor[F[_]](implicit F: Functor[F]): Functor[OneAnd[?, F]] =
-    new Functor[OneAnd[?, F]] {
-      def map[A, B](fa: OneAnd[A, F])(f: A => B): OneAnd[B, F] =
+  implicit def oneAndFunctor[F[_]](implicit F: Functor[F]): Functor[OneAnd[F, ?]] =
+    new Functor[OneAnd[F, ?]] {
+      def map[A, B](fa: OneAnd[F, A])(f: A => B): OneAnd[F, B] =
         OneAnd(f(fa.head), F.map(fa.tail)(f))
     }
 }

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

@@ -1,15 +1,15 @@
 package cats
 
 package object data {
-  type NonEmptyList[A] = OneAnd[A, List]
-  type NonEmptyVector[A] = OneAnd[A, Vector]
-  type NonEmptyStream[A] = OneAnd[A, Stream]
+  type NonEmptyList[A] = OneAnd[List, A]
+  type NonEmptyVector[A] = OneAnd[Vector, A]
+  type NonEmptyStream[A] = OneAnd[Stream, A]
   type ValidatedNel[E, A] = Validated[NonEmptyList[E], A]
 
   def NonEmptyList[A](head: A, tail: List[A] = Nil): NonEmptyList[A] =
     OneAnd(head, tail)
   def NonEmptyList[A](head: A, tail: A*): NonEmptyList[A] =
-    OneAnd[A, List](head, tail.toList)
+    OneAnd[List, A](head, tail.toList)
 
   def NonEmptyVector[A](head: A, tail: Vector[A] = Vector.empty): NonEmptyVector[A] =
     OneAnd(head, tail)

laws/src/main/scala/cats/laws/discipline/Arbitrary.scala

@@ -14,7 +14,7 @@ object arbitrary {
   implicit def constArbitrary[A, B](implicit A: Arbitrary[A]): Arbitrary[Const[A, B]] =
     Arbitrary(A.arbitrary.map(Const[A, B]))
 
-  implicit def oneAndArbitrary[F[_], A](implicit A: Arbitrary[A], F: Arbitrary[F[A]]): Arbitrary[OneAnd[A, F]] =
+  implicit def oneAndArbitrary[F[_], A](implicit A: Arbitrary[A], F: Arbitrary[F[A]]): Arbitrary[OneAnd[F, A]] =
     Arbitrary(F.arbitrary.flatMap(fa => A.arbitrary.map(a => OneAnd(a, fa))))
 
   implicit def xorArbitrary[A, B](implicit A: Arbitrary[A], B: Arbitrary[B]): Arbitrary[A Xor B] =

tests/src/test/scala/cats/tests/ListWrapper.scala

@@ -91,9 +91,9 @@ object ListWrapper {
       def synthesize[A: Arbitrary]: Arbitrary[ListWrapper[A]] = implicitly
     }
 
-  implicit val listWrapperOneAndArbitraryK: ArbitraryK[OneAnd[?, ListWrapper]] =
-    new ArbitraryK[OneAnd[?, ListWrapper]] {
-      def synthesize[A: Arbitrary]: Arbitrary[OneAnd[A, ListWrapper]] = implicitly
+  implicit val listWrapperOneAndArbitraryK: ArbitraryK[OneAnd[ListWrapper, ?]] =
+    new ArbitraryK[OneAnd[ListWrapper, ?]] {
+      def synthesize[A: Arbitrary]: Arbitrary[OneAnd[ListWrapper, A]] = implicitly
     }
 
   implicit def listWrapperEq[A: Eq]: Eq[ListWrapper[A]] = Eq.by(_.list)

tests/src/test/scala/cats/tests/OneAndTests.scala

@@ -11,27 +11,27 @@ import cats.laws.discipline.arbitrary.{evalArbitrary, oneAndArbitrary}
 import scala.util.Random
 
 class OneAndTests extends CatsSuite {
-  checkAll("OneAnd[Int, List]", OrderLaws[OneAnd[Int, List]].eqv)
+  checkAll("OneAnd[List, Int]", OrderLaws[OneAnd[List, Int]].eqv)
 
   // Test instances that have more general constraints
   {
     implicit val functor = ListWrapper.functor
-    checkAll("OneAnd[Int, ListWrapper]", FunctorTests[OneAnd[?, ListWrapper]].functor[Int, Int, Int])
-    checkAll("Functor[OneAnd[A, ListWrapper]]", SerializableTests.serializable(Functor[OneAnd[?, ListWrapper]]))
+    checkAll("OneAnd[ListWrapper, Int]", FunctorTests[OneAnd[ListWrapper, ?]].functor[Int, Int, Int])
+    checkAll("Functor[OneAnd[ListWrapper, A]]", SerializableTests.serializable(Functor[OneAnd[ListWrapper, ?]]))
   }
 
   {
     implicit val monadCombine = ListWrapper.monadCombine
-    checkAll("OneAnd[Int, ListWrapper]", SemigroupKTests[OneAnd[?, ListWrapper]].semigroupK[Int])
-    checkAll("OneAnd[Int, List]", GroupLaws[OneAnd[Int, List]].semigroup)
-    checkAll("SemigroupK[OneAnd[A, ListWrapper]]", SerializableTests.serializable(SemigroupK[OneAnd[?, ListWrapper]]))
-    checkAll("Semigroup[NonEmptyList[Int]]", SerializableTests.serializable(Semigroup[OneAnd[Int, List]]))
+    checkAll("OneAnd[ListWrapper, Int]", SemigroupKTests[OneAnd[ListWrapper, ?]].semigroupK[Int])
+    checkAll("OneAnd[List, Int]", GroupLaws[OneAnd[List, Int]].semigroup)
+    checkAll("SemigroupK[OneAnd[ListWrapper, A]]", SerializableTests.serializable(SemigroupK[OneAnd[ListWrapper, ?]]))
+    checkAll("Semigroup[NonEmptyList[Int]]", SerializableTests.serializable(Semigroup[OneAnd[List, Int]]))
   }
 
   {
     implicit val foldable = ListWrapper.foldable
-    checkAll("OneAnd[Int, ListWrapper]", FoldableTests[OneAnd[?, ListWrapper]].foldable[Int, Int])
-    checkAll("Foldable[OneAnd[A, ListWrapper]]", SerializableTests.serializable(Foldable[OneAnd[?, ListWrapper]]))
+    checkAll("OneAnd[ListWrapper, Int]", FoldableTests[OneAnd[ListWrapper, ?]].foldable[Int, Int])
+    checkAll("Foldable[OneAnd[ListWrapper, A]]", SerializableTests.serializable(Foldable[OneAnd[ListWrapper, ?]]))
   }
 
   {