Enable breakout in functions reduceRightToOption and reduceRightTo.

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

@@ -3,7 +3,8 @@ package cats
 import scala.collection.mutable
 import cats.kernel.CommutativeMonoid
 import simulacrum.{noop, typeclass}
-import Foldable.sentinel
+import Foldable.{sentinel, Source}
+
 import scala.annotation.implicitNotFound
 
 /**
@@ -98,7 +99,7 @@ import scala.annotation.implicitNotFound
 
   def foldRightDefer[G[_]: Defer, A, B](fa: F[A], gb: G[B])(fn: (A, G[B]) => G[B]): G[B] =
     Defer[G].defer(
-      this.foldLeft(fa, (z: G[B]) => z) { (acc, elem) => z =>
+      foldLeft(fa, (z: G[B]) => z) { (acc, elem) => z =>
         Defer[G].defer(acc(fn(elem, z)))
       }(gb)
     )
@@ -109,13 +110,19 @@ import scala.annotation.implicitNotFound
       case (None, a)    => Some(f(a))
     }
 
-  def reduceRightToOption[A, B](fa: F[A])(f: A => B)(g: (A, Eval[B]) => Eval[B]): Eval[Option[B]] =
-    foldRight(fa, Now(Option.empty[B])) { (a, lb) =>
-      lb.flatMap {
-        case Some(b) => g(a, Now(b)).map(Some(_))
-        case None    => Later(Some(f(a)))
-      }
+  def reduceRightToOption[A, B](fa: F[A])(f: A => B)(g: (A, Eval[B]) => Eval[B]): Eval[Option[B]] = {
+    Source.fromFoldable(fa)(self).uncons match {
+      case Some((first, s)) =>
+        def loop(now: A, source: Source[A]): Eval[B] =
+          source.uncons match {
+            case Some((next, s)) => g(now, Eval.defer(loop(next, s.value)))
+            case None            => Eval.later(f(now))
+          }
+
+        Eval.defer(loop(first, s.value).map(Some(_)))
+      case None => Eval.now(None)
     }
+  }
 
   /**
    * Reduce the elements of this structure down to a single value by applying

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

@@ -1,5 +1,6 @@
 package cats
 
+import cats.Foldable.Source
 import cats.data.{Ior, NonEmptyList}
 import simulacrum.{noop, typeclass}
 import scala.annotation.implicitNotFound
@@ -386,14 +387,18 @@ abstract class NonEmptyReducible[F[_], G[_]](implicit G: Foldable[G]) extends Re
     G.foldLeft(ga, f(a))((b, a) => g(b, a))
   }
 
-  def reduceRightTo[A, B](fa: F[A])(f: A => B)(g: (A, Eval[B]) => Eval[B]): Eval[B] =
+  def reduceRightTo[A, B](fa: F[A])(f: A => B)(g: (A, Eval[B]) => Eval[B]): Eval[B] = {
+    def loop(now: A, source: Source[A]): Eval[B] =
+      source.uncons match {
+        case Some((next, s)) => g(now, Eval.defer(loop(next, s.value)))
+        case None            => Eval.later(f(now))
+      }
+
     Always(split(fa)).flatMap {
       case (a, ga) =>
-        G.reduceRightToOption(ga)(f)(g).flatMap {
-          case Some(b) => g(a, Now(b))
-          case None    => Later(f(a))
-        }
+        Eval.defer(loop(a, Foldable.Source.fromFoldable(ga)))
     }
+  }
 
   override def size[A](fa: F[A]): Long = {
     val (_, tail) = split(fa)

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

@@ -2,7 +2,6 @@ package cats
 package data
 
 import NonEmptyChainImpl.create
-import cats.{Order, Semigroup}
 import cats.kernel._
 import scala.collection.immutable.SortedMap
 

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

@@ -606,7 +606,7 @@ sealed abstract private[data] class NonEmptyListInstances extends NonEmptyListIn
 
       override def nonEmptyPartition[A, B, C](
         fa: NonEmptyList[A]
-      )(f: (A) => Either[B, C]): Ior[NonEmptyList[B], NonEmptyList[C]] = {
+      )(f: A => Either[B, C]): Ior[NonEmptyList[B], NonEmptyList[C]] = {
         val reversed = fa.reverse
         val lastIor = f(reversed.head) match {
           case Right(c) => Ior.right(NonEmptyList.one(c))

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

@@ -116,7 +116,7 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
 
       override def partitionEither[A, B, C](
         fa: List[A]
-      )(f: (A) => Either[B, C])(implicit A: Alternative[List]): (List[B], List[C]) =
+      )(f: A => Either[B, C])(implicit A: Alternative[List]): (List[B], List[C]) =
         fa.foldRight((List.empty[B], List.empty[C]))((a, acc) =>
           f(a) match {
             case Left(b)  => (b :: acc._1, acc._2)

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

@@ -10,6 +10,8 @@ import cats.syntax.option._
 import cats.syntax.reducible._
 import org.scalacheck.Arbitrary
 
+import scala.collection.mutable
+
 class ReducibleSuiteAdditional extends CatsSuite {
 
   test("Reducible[NonEmptyList].reduceLeftM stack safety") {
@@ -22,6 +24,12 @@ class ReducibleSuiteAdditional extends CatsSuite {
     actual should ===(Some(expected))
   }
 
+  test("Reducible[NonEmptyList].reduceRightTo stack safety") {
+    val n = 100000L
+    val actual = (1L to n).toList.toNel.get.reduceRightTo(identity) { case (a, r) => r.map(_ + a) }.value
+    actual should ===((1L to n).sum)
+  }
+
   // exists method written in terms of reduceRightTo
   def contains[F[_]: Reducible, A: Eq](as: F[A], goal: A): Eval[Boolean] =
     as.reduceRightTo(_ === goal) { (a, lb) =>
@@ -76,6 +84,15 @@ class ReducibleSuiteAdditional extends CatsSuite {
     assert(contains(large, 10000).value)
   }
 
+  test("Reducible[NonEmptyList].reduceMapA can breakout") {
+    val notAllEven = NonEmptyList.of(2, 4, 6, 9, 10, 12, 14)
+    val out = mutable.ListBuffer[Int]()
+
+    notAllEven.reduceMapA { a => out += a; if (a % 2 == 0) Some(a) else None }
+
+    out.toList should ===(List(2, 4, 6, 9))
+  }
+
   // A simple non-empty stream with lazy `foldRight` and `reduceRightTo` implementations.
   case class NES[A](h: A, t: Stream[A]) {
     def toStream: Stream[A] = h #:: t