restrict traverse_ and friends to require Unit

When using `traverse_` or `sequence_` to evaluate some applicative
effect `G[B]` within the context of a foldable structure, any `B` value
is thrown away. Per the scaladoc for `traverse_`, these functions expect
that the `G[B]` is primarily a side effect or action.  The scala
convention for expressing this expectation is to require a `Unit`
parameter.

Teach `traverse_`, `sequence_`, and their parallel and nonempty
analogues to follow this convention by accepting only `G[Unit]` instead
of simply any `G[B]`.

core/src/main/scala/cats/Foldable.scala

@@ -569,50 +569,45 @@ trait Foldable[F[_]] extends UnorderedFoldable[F] with FoldableNFunctions[F] { s
   /**
    * Traverse `F[A]` using `Applicative[G]`.
    *
-   * `A` values will be mapped into `G[B]` and combined using
+   * `A` values will be mapped into `G[Unit]` and combined using
    * `Applicative#map2`.
    *
    * For example:
    *
    * {{{
    * scala> import cats.implicits._
-   * scala> def parseInt(s: String): Option[Int] = Either.catchOnly[NumberFormatException](s.toInt).toOption
+   * scala> def checkInt(s: String): Option[Unit] = Either.catchOnly[NumberFormatException]{ s.toInt ; () }.toOption
    * scala> val F = Foldable[List]
-   * scala> F.traverse_(List("333", "444"))(parseInt)
+   * scala> F.traverse_(List("333", "444"))(checkInt)
    * res0: Option[Unit] = Some(())
-   * scala> F.traverse_(List("333", "zzz"))(parseInt)
+   * scala> F.traverse_(List("333", "zzz"))(checkInt)
    * res1: Option[Unit] = None
    * }}}
    *
    * This method is primarily useful when `G[_]` represents an action
-   * or effect, and the specific `A` aspect of `G[A]` is not otherwise
-   * needed.
+   * or effect. It is equivalent to `foldMapA` using the Unit monoid.
    */
-  def traverse_[G[_], A, B](fa: F[A])(f: A => G[B])(implicit G: Applicative[G]): G[Unit] =
-    foldRight(fa, Always(G.pure(()))) { (a, acc) =>
-      G.map2Eval(f(a), acc) { (_, _) =>
-        ()
-      }
-    }.value
+  def traverse_[G[_], A](fa: F[A])(f: A => G[Unit])(implicit G: Applicative[G]): G[Unit] =
+    foldMapA(fa)(f)
 
   /**
-   * Sequence `F[G[A]]` using `Applicative[G]`.
+   * Sequence `F[G[Unit]]` using `Applicative[G]`.
    *
-   * This is similar to `traverse_` except it operates on `F[G[A]]`
+   * This is similar to `traverse_` except it operates on `F[G[Unit]]`
    * values, so no additional functions are needed.
    *
    * For example:
    *
    * {{{
    * scala> import cats.implicits._
    * scala> val F = Foldable[List]
-   * scala> F.sequence_(List(Option(1), Option(2), Option(3)))
+   * scala> F.sequence_(List(Option(()), Option(()), Option(())))
    * res0: Option[Unit] = Some(())
-   * scala> F.sequence_(List(Option(1), None, Option(3)))
+   * scala> F.sequence_(List(Option(()), None, Option(())))
    * res1: Option[Unit] = None
    * }}}
    */
-  def sequence_[G[_]: Applicative, A](fga: F[G[A]]): G[Unit] =
+  def sequence_[G[_]: Applicative](fga: F[G[Unit]]): G[Unit] =
     traverse_(fga)(identity)
 
   /**
@@ -1051,10 +1046,10 @@ object Foldable {
       typeClassInstance.foldMapM[G, A, B](self)(f)(G, B)
     def foldMapA[G[_], B](f: A => G[B])(implicit G: Applicative[G], B: Monoid[B]): G[B] =
       typeClassInstance.foldMapA[G, A, B](self)(f)(G, B)
-    def traverse_[G[_], B](f: A => G[B])(implicit G: Applicative[G]): G[Unit] =
-      typeClassInstance.traverse_[G, A, B](self)(f)(G)
-    def sequence_[G[_], B](implicit ev$1: A <:< G[B], ev$2: Applicative[G]): G[Unit] =
-      typeClassInstance.sequence_[G, B](self.asInstanceOf[F[G[B]]])
+    def traverse_[G[_]](f: A => G[Unit])(implicit G: Applicative[G]): G[Unit] =
+      typeClassInstance.traverse_[G, A](self)(f)(G)
+    def sequence_[G[_]](implicit ev$1: A <:< G[Unit], ev$2: Applicative[G]): G[Unit] =
+      typeClassInstance.sequence_[G](self.asInstanceOf[F[G[Unit]]])
     def foldK[G[_], B](implicit ev$1: A <:< G[B], G: MonoidK[G]): G[B] =
       typeClassInstance.foldK[G, B](self.asInstanceOf[F[G[B]]])(G)
     def find(f: A => Boolean): Option[A] = typeClassInstance.find[A](self)(f)

core/src/main/scala/cats/Parallel.scala

@@ -263,7 +263,7 @@ object Parallel extends ParallelArityFunctions2 {
    * Like `Foldable[A].sequence_`, but uses the applicative instance
    * corresponding to the Parallel instance instead.
    */
-  def parSequence_[T[_]: Foldable, M[_], A](tma: T[M[A]])(implicit P: Parallel[M]): M[Unit] = {
+  def parSequence_[T[_]: Foldable, M[_]](tma: T[M[Unit]])(implicit P: Parallel[M]): M[Unit] = {
     val fu: P.F[Unit] = Foldable[T].traverse_(tma)(P.parallel.apply(_))(P.applicative)
     P.sequential(fu)
   }
@@ -272,9 +272,9 @@ object Parallel extends ParallelArityFunctions2 {
    * Like `Foldable[A].traverse_`, but uses the applicative instance
    * corresponding to the Parallel instance instead.
    */
-  def parTraverse_[T[_]: Foldable, M[_], A, B](
+  def parTraverse_[T[_]: Foldable, M[_], A](
     ta: T[A]
-  )(f: A => M[B])(implicit P: Parallel[M]): M[Unit] = {
+  )(f: A => M[Unit])(implicit P: Parallel[M]): M[Unit] = {
     val gtb: P.F[Unit] = Foldable[T].traverse_(ta)(a => P.parallel(f(a)))(P.applicative)
     P.sequential(gtb)
   }
@@ -348,8 +348,8 @@ object Parallel extends ParallelArityFunctions2 {
    * Like `Reducible[A].nonEmptySequence_`, but uses the apply instance
    * corresponding to the Parallel instance instead.
    */
-  def parNonEmptySequence_[T[_]: Reducible, M[_], A](
-    tma: T[M[A]]
+  def parNonEmptySequence_[T[_]: Reducible, M[_]](
+    tma: T[M[Unit]]
   )(implicit P: NonEmptyParallel[M]): M[Unit] = {
     val fu: P.F[Unit] = Reducible[T].nonEmptyTraverse_(tma)(P.parallel.apply(_))(P.apply)
     P.sequential(fu)
@@ -359,9 +359,9 @@ object Parallel extends ParallelArityFunctions2 {
    * Like `Reducible[A].nonEmptyTraverse_`, but uses the apply instance
    * corresponding to the Parallel instance instead.
    */
-  def parNonEmptyTraverse_[T[_]: Reducible, M[_], A, B](
+  def parNonEmptyTraverse_[T[_]: Reducible, M[_], A](
     ta: T[A]
-  )(f: A => M[B])(implicit P: NonEmptyParallel[M]): M[Unit] = {
+  )(f: A => M[Unit])(implicit P: NonEmptyParallel[M]): M[Unit] = {
     val gtb: P.F[Unit] = Reducible[T].nonEmptyTraverse_(ta)(a => P.parallel(f(a)))(P.apply)
     P.sequential(gtb)
   }

core/src/main/scala/cats/Reducible.scala

@@ -200,7 +200,7 @@ trait Reducible[F[_]] extends Foldable[F] { self =>
    * Traverse `F[A]` using `Apply[G]`.
    *
    * `A` values will be mapped into `G[B]` and combined using
-   * `Apply#map2`.
+   * `Apply#map2`. This is equivalent to `reduceMapA`.
    *
    * This method is similar to [[Foldable.traverse_]]. There are two
    * main differences:
@@ -213,10 +213,8 @@ trait Reducible[F[_]] extends Foldable[F] { self =>
    * available for `G` and want to take advantage of short-circuiting
    * the traversal.
    */
-  def nonEmptyTraverse_[G[_], A, B](fa: F[A])(f: A => G[B])(implicit G: Apply[G]): G[Unit] = {
-    val f1 = f.andThen(G.void)
-    reduceRightTo(fa)(f1)((x, y) => G.map2Eval(f1(x), y)((_, b) => b)).value
-  }
+  def nonEmptyTraverse_[G[_], A](fa: F[A])(f: A => G[Unit])(implicit G: Apply[G]): G[Unit] =
+    reduceMapA(fa)(f)
 
   /**
    * Sequence `F[G[A]]` using `Apply[G]`.
@@ -225,7 +223,7 @@ trait Reducible[F[_]] extends Foldable[F] { self =>
    * an [[Apply]] instance for `G` instead of [[Applicative]]. See the
    * [[nonEmptyTraverse_]] documentation for a description of the differences.
    */
-  def nonEmptySequence_[G[_], A](fga: F[G[A]])(implicit G: Apply[G]): G[Unit] =
+  def nonEmptySequence_[G[_]](fga: F[G[Unit]])(implicit G: Apply[G]): G[Unit] =
     nonEmptyTraverse_(fga)(identity)
 
   def toNonEmptyList[A](fa: F[A]): NonEmptyList[A] =
@@ -399,10 +397,10 @@ object Reducible {
       typeClassInstance.reduceMapM[G, A, B](self)(f)(G, B)
     def reduceRightTo[B](f: A => B)(g: (A, Eval[B]) => Eval[B]): Eval[B] =
       typeClassInstance.reduceRightTo[A, B](self)(f)(g)
-    def nonEmptyTraverse_[G[_], B](f: A => G[B])(implicit G: Apply[G]): G[Unit] =
-      typeClassInstance.nonEmptyTraverse_[G, A, B](self)(f)(G)
-    def nonEmptySequence_[G[_], B](implicit ev$1: A <:< G[B], G: Apply[G]): G[Unit] =
-      typeClassInstance.nonEmptySequence_[G, B](self.asInstanceOf[F[G[B]]])(G)
+    def nonEmptyTraverse_[G[_]](f: A => G[Unit])(implicit G: Apply[G]): G[Unit] =
+      typeClassInstance.nonEmptyTraverse_[G, A](self)(f)(G)
+    def nonEmptySequence_[G[_]](implicit ev$1: A <:< G[Unit], G: Apply[G]): G[Unit] =
+      typeClassInstance.nonEmptySequence_[G](self.asInstanceOf[F[G[Unit]]])(G)
     def toNonEmptyList: NonEmptyList[A] = typeClassInstance.toNonEmptyList[A](self)
     def minimum(implicit A: Order[A]): A = typeClassInstance.minimum[A](self)(A)
     def maximum(implicit A: Order[A]): A = typeClassInstance.maximum[A](self)(A)

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

@@ -129,7 +129,7 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
       /**
        * This avoids making a very deep stack by building a tree instead
        */
-      override def traverse_[G[_], A, B](fa: List[A])(f: A => G[B])(implicit G: Applicative[G]): G[Unit] = {
+      override def traverse_[G[_], A](fa: List[A])(f: A => G[Unit])(implicit G: Applicative[G]): G[Unit] = {
         // the cost of this is O(size log size)
         // c(n) = n + 2 * c(n/2) = n + 2(n/2 log (n/2)) = n + n (logn - 1) = n log n
         // invariant: size >= 1
@@ -155,10 +155,7 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
             // failed. We do not use laziness to avoid
             // traversing fa, which we will do fully
             // in all cases.
-            Eval.always {
-              val gb = f(a)
-              G.void(gb)
-            }
+            Eval.always { f(a) }
           }
 
         val len = fa.length

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

@@ -137,7 +137,7 @@ trait VectorInstances extends cats.kernel.instances.VectorInstances {
       /**
        * This avoids making a very deep stack by building a tree instead
        */
-      override def traverse_[G[_], A, B](fa: Vector[A])(f: A => G[B])(implicit G: Applicative[G]): G[Unit] = {
+      override def traverse_[G[_], A](fa: Vector[A])(f: A => G[Unit])(implicit G: Applicative[G]): G[Unit] = {
         // the cost of this is O(size)
         // c(n) = 1 + 2 * c(n/2)
         // invariant: size >= 1
@@ -161,10 +161,7 @@ trait VectorInstances extends cats.kernel.instances.VectorInstances {
             // failed. We do not use laziness to avoid
             // traversing fa, which we will do fully
             // in all cases.
-            Eval.always {
-              val gb = f(a)
-              G.void(gb)
-            }
+            Eval.always { f(a) }
           }
 
         val len = fa.length

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

@@ -47,7 +47,8 @@ private[syntax] trait FoldableSyntaxBinCompat1 {
 }
 
 final class NestedFoldableOps[F[_], G[_], A](private val fga: F[G[A]]) extends AnyVal {
-  def sequence_(implicit F: Foldable[F], G: Applicative[G]): G[Unit] = F.sequence_(fga)
+  def sequence_(implicit F: Foldable[F], G: Applicative[G], ev: A <:< Unit): G[Unit] =
+    F.sequence_(fga.asInstanceOf[F[G[Unit]]])
 
   /**
    * @see [[Foldable.foldK]].

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

@@ -111,7 +111,7 @@ trait ParallelTraverseSyntax {
   implicit final def catsSyntaxParallelTraverse_[T[_]: Foldable, A](ta: T[A]): ParallelTraversable_Ops[T, A] =
     new ParallelTraversable_Ops(ta)
 
-  implicit final def catsSyntaxParallelSequence_[T[_]: Foldable, M[_], A](tma: T[M[A]]): ParallelSequence_Ops[T, M, A] =
+  implicit final def catsSyntaxParallelSequence_[T[_]: Foldable, M[_]](tma: T[M[Unit]]): ParallelSequence_Ops[T, M] =
     new ParallelSequence_Ops(tma)
 }
 
@@ -189,7 +189,7 @@ final class ParallelSequenceFilterOps[T[_], M[_], A](private val tmoa: T[M[Optio
 }
 
 final class ParallelTraversable_Ops[T[_], A](private val ta: T[A]) extends AnyVal {
-  def parTraverse_[M[_], B](f: A => M[B])(implicit T: Foldable[T], P: Parallel[M]): M[Unit] =
+  def parTraverse_[M[_]](f: A => M[Unit])(implicit T: Foldable[T], P: Parallel[M]): M[Unit] =
     Parallel.parTraverse_(ta)(f)
 }
 
@@ -217,7 +217,7 @@ final class ParallelSequenceOps1[T[_], M[_], A](private val tma: T[M[A]]) extend
     Parallel.parSequence(tma)
 }
 
-final class ParallelSequence_Ops[T[_], M[_], A](private val tma: T[M[A]]) extends AnyVal {
+final class ParallelSequence_Ops[T[_], M[_]](private val tma: T[M[Unit]]) extends AnyVal {
   def parSequence_(implicit T: Foldable[T], P: Parallel[M]): M[Unit] =
     Parallel.parSequence_(tma)
 }

docs/nomenclature.md

@@ -115,8 +115,8 @@ Like the previous section, we use the `E` for the error parameter type.
 | `F[A] => (A => G[B]) => G[B]` | `foldMapM` | `G: Monad` and `B: Monoid`
 | `F[A] => (A => B) => Option[B]` | `collectFirst` | The `A => B` is a `PartialFunction`
 | `F[A] => (A => Option[B]) => Option[B]` | `collectFirstSome` |
-| `F[A] => (A => G[B]) => G[Unit]` | `traverse_` | `G: Applicative`
-| `F[G[A]] => G[Unit]` | `sequence_` | `G: Applicative`
+| `F[A] => (A => G[Unit]) => G[Unit]` | `traverse_` | `G: Applicative`
+| `F[G[Unit]] => G[Unit]` | `sequence_` | `G: Applicative`
 | `F[A] => (A => Either[B, C] => (F[B], F[C])` | `partitionEither` | `G: Applicative`
 
 ### Reducible

docs/typeclasses/foldable.md

@@ -52,10 +52,10 @@ Foldable[List].nonEmpty(List(1,2))
 Foldable[Option].toList(Option(1))
 Foldable[Option].toList(None)
 
-def parseInt(s: String): Option[Int] = scala.util.Try(Integer.parseInt(s)).toOption
+def checkInt(s: String): Option[Unit] = scala.util.Try{ Integer.parseInt(s) ; () }.toOption
 
-Foldable[List].traverse_(List("1", "2"))(parseInt)
-Foldable[List].traverse_(List("1", "A"))(parseInt)
+Foldable[List].traverse_(List("1", "2"))(checkInt)
+Foldable[List].traverse_(List("1", "A"))(checkInt)
 Foldable[List].sequence_(List(Option(1), Option(2)))
 Foldable[List].sequence_(List(Option(1), None))
 

docs/typeclasses/reducible.md

@@ -38,7 +38,7 @@ Reducible[NonEmptyList].reduceLeftTo(NonEmptyList.of(1,2,3,4))(_.toString)((s,i)
 Reducible[NonEmptyList].reduceRightTo(NonEmptyList.of(1,2,3,4))(_.toString)((i,s) => Later(s.value + i)).value
 Reducible[NonEmptyList].nonEmptyIntercalate(NonEmptyList.of("a", "b", "c"), ", ")
 
-def countChars(s: String) = s.toCharArray.groupBy(identity).view.mapValues(_.length).toMap
+def countChars(s: String) = s.toCharArray.groupBy(identity).view.mapValues(x => { x.length ; () }).toMap
 
 Reducible[NonEmptyList].nonEmptyTraverse_(NonEmptyList.of("Hello", "World"))(countChars)
 Reducible[NonEmptyVector].nonEmptyTraverse_(NonEmptyVector.of("Hello", ""))(countChars)

laws/src/main/scala/cats/laws/ReducibleLaws.scala

@@ -53,10 +53,10 @@ trait ReducibleLaws[F[_]] extends FoldableLaws[F] {
   def reduceReduceLeftConsistent[B](fa: F[B])(implicit B: Semigroup[B]): IsEq[B] =
     fa.reduce <-> fa.reduceLeft(B.combine)
 
-  def traverseConsistent[G[_]: Applicative, A, B](fa: F[A], f: A => G[B]): IsEq[G[Unit]] =
+  def traverseConsistent[G[_]: Applicative, A](fa: F[A], f: A => G[Unit]): IsEq[G[Unit]] =
     fa.nonEmptyTraverse_(f) <-> fa.traverse_(f)
 
-  def sequenceConsistent[G[_]: Applicative, A](fa: F[G[A]]): IsEq[G[Unit]] =
+  def sequenceConsistent[G[_]: Applicative](fa: F[G[Unit]]): IsEq[G[Unit]] =
     fa.nonEmptySequence_ <-> fa.sequence_
 
   def sizeConsistent[A](fa: F[A]): IsEq[Long] =

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

@@ -44,13 +44,11 @@ trait NonEmptyTraverseTests[F[_]] extends TraverseTests[F] with ReducibleTests[F
     ArbYB: Arbitrary[Y[B]],
     ArbYC: Arbitrary[Y[C]],
     ArbFB: Arbitrary[F[B]],
-    ArbFM: Arbitrary[F[M]],
     ArbXM: Arbitrary[X[M]],
     ArbYM: Arbitrary[Y[M]],
-    ArbFGA: Arbitrary[F[G[A]]],
+    ArbGU: Arbitrary[G[Unit]],
+    ArbFGU: Arbitrary[F[G[Unit]]],
     ArbFXM: Arbitrary[F[X[M]]],
-    ArbGB: Arbitrary[G[B]],
-    ArbGM: Arbitrary[G[M]],
     CogenA: Cogen[A],
     CogenB: Cogen[B],
     CogenC: Cogen[C],

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

@@ -35,8 +35,8 @@ trait ReducibleTests[F[_]] extends FoldableTests[F] {
   def reducible[G[_]: Applicative, A: Arbitrary, B: Arbitrary](implicit
     ArbFA: Arbitrary[F[A]],
     ArbFB: Arbitrary[F[B]],
-    ArbFGA: Arbitrary[F[G[A]]],
-    ArbGB: Arbitrary[G[B]],
+    ArbFGU: Arbitrary[F[G[Unit]]],
+    ArbGU: Arbitrary[G[Unit]],
     CogenA: Cogen[A],
     CogenB: Cogen[B],
     EqG: Eq[G[Unit]],
@@ -58,8 +58,8 @@ trait ReducibleTests[F[_]] extends FoldableTests[F] {
         forAll(laws.reduceRightConsistentWithReduceRightOption[A] _),
       "reduce consistent with reduceLeft" ->
         forAll(laws.reduceReduceLeftConsistent[B] _),
-      "nonEmptyTraverse_ consistent with traverse_" -> forAll(laws.traverseConsistent[G, A, B] _),
-      "nonEmptySequence_ consistent with sequence_" -> forAll(laws.sequenceConsistent[G, A] _),
+      "nonEmptyTraverse_ consistent with traverse_" -> forAll(laws.traverseConsistent[G, A] _),
+      "nonEmptySequence_ consistent with sequence_" -> forAll(laws.sequenceConsistent[G] _),
       "size consistent with reduceMap" -> forAll(laws.sizeConsistent[A] _)
     )
 }

tests/shared/src/test/scala-2.13+/cats/tests/ScalaVersionSpecific.scala

@@ -179,7 +179,7 @@ trait ScalaVersionSpecificRegressionSuite { self: RegressionSuite =>
     // shouldn't have ever evaluated validate(8)
     checkAndResetCount(3)
 
-    assert(LazyList(1, 2, 6, 8).traverse_(validate) === (Either.left("6 is greater than 5")))
+    assert(LazyList(1, 2, 6, 8).traverse_(validate(_).void) === (Either.left("6 is greater than 5")))
     checkAndResetCount(3)
   }
 }

tests/shared/src/test/scala/cats/tests/ParallelSuite.scala

@@ -79,23 +79,23 @@ class ParallelSuite
   }
 
   test("ParTraverse_ identity should be equivalent to parSequence_") {
-    forAll { (es: SortedSet[Either[String, Int]]) =>
-      assert(Parallel.parTraverse_(es)(identity) === (Parallel.parSequence_[SortedSet, Either[String, *], Int](es)))
+    forAll { (es: SortedSet[Either[String, Unit]]) =>
+      assert(Parallel.parTraverse_(es)(identity) === (Parallel.parSequence_[SortedSet, Either[String, *]](es)))
     }
   }
 
   test("ParTraverse_ syntax should be equivalent to Parallel.parTraverse_") {
-    forAll { (es: SortedSet[Either[String, Int]]) =>
+    forAll { (es: SortedSet[Either[String, Unit]]) =>
       assert(
-        Parallel.parTraverse_[SortedSet, Either[String, *], Either[String, Int], Int](es)(identity) === (es
+        Parallel.parTraverse_[SortedSet, Either[String, *], Either[String, Unit]](es)(identity) === (es
           .parTraverse_(identity))
       )
     }
   }
 
   test("ParSequence_ syntax should be equivalent to Parallel.parSequence_") {
-    forAll { (es: SortedSet[Either[String, Int]]) =>
-      assert(Parallel.parSequence_[SortedSet, Either[String, *], Int](es) === (es.parSequence_))
+    forAll { (es: SortedSet[Either[String, Unit]]) =>
+      assert(Parallel.parSequence_[SortedSet, Either[String, *]](es) === (es.parSequence_))
     }
   }
 
@@ -106,7 +106,7 @@ class ParallelSuite
   }
 
   test("ParNonEmptyTraverse_ identity should be equivalent to parNonEmptySequence_") {
-    forAll { (es: NonEmptyList[Either[String, Int]]) =>
+    forAll { (es: NonEmptyList[Either[String, Unit]]) =>
       assert(Parallel.parNonEmptyTraverse_(es)(identity) === (Parallel.parNonEmptySequence_(es)))
     }
   }
@@ -311,8 +311,8 @@ class ParallelSuite
   }
 
   test("ParReplicateA_ should be equivalent to fill parSequence_") {
-    forAll(Gen.choose(1, 20), Arbitrary.arbitrary[Either[String, String]]) {
-      (repetitions: Int, e: Either[String, String]) =>
+    forAll(Gen.choose(1, 20), Arbitrary.arbitrary[Either[String, Unit]]) {
+      (repetitions: Int, e: Either[String, Unit]) =>
         assert(Parallel.parReplicateA_(repetitions, e) === Parallel.parSequence_(List.fill(repetitions)(e)))
     }
   }

tests/shared/src/test/scala/cats/tests/ReducibleSuite.scala

@@ -238,7 +238,7 @@ abstract class ReducibleSuite[F[_]: Reducible](name: String)(implicit
     val notAllEven = fromValues(2, 4, 6, 9, 10, 12, 14)
     val out = mutable.ListBuffer[Int]()
 
-    notAllEven.nonEmptyTraverse_ { a => out += a; if (a % 2 == 0) Some(a) else None }
+    notAllEven.nonEmptyTraverse_ { a => out += a; if (a % 2 == 0) Some(()) else None }
 
     assert(out.toList === List(2, 4, 6, 9))
   }