From 4939f83485c2f476aef7ba7cb8872aed2c49c707 Mon Sep 17 00:00:00 2001
From: takayahilton <takayahilton@gmail.com>
Date: Tue, 18 Aug 2020 22:18:44 +0900
Subject: [PATCH] Avoid all evaluation of NonEmptyLazyList#reduceRightTo

---
 .../scala-2.13+/cats/data/NonEmptyLazyList.scala  |  9 ++++++---
 core/src/main/scala/cats/instances/function.scala |  8 +++-----
 .../cats/tests/NonEmptyLazyListSuite.scala        | 15 +++++++++++++++
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/core/src/main/scala-2.13+/cats/data/NonEmptyLazyList.scala b/core/src/main/scala-2.13+/cats/data/NonEmptyLazyList.scala
index 2e2b64d088..71e8a87a8d 100644
--- a/core/src/main/scala-2.13+/cats/data/NonEmptyLazyList.scala
+++ b/core/src/main/scala-2.13+/cats/data/NonEmptyLazyList.scala
@@ -484,9 +484,12 @@ sealed abstract private[data] class NonEmptyLazyListInstances extends NonEmptyLa
       def reduceLeftTo[A, B](fa: NonEmptyLazyList[A])(f: A => B)(g: (B, A) => B): B = fa.reduceLeftTo(f)(g)
 
       def reduceRightTo[A, B](fa: NonEmptyLazyList[A])(f: A => B)(g: (A, cats.Eval[B]) => cats.Eval[B]): cats.Eval[B] =
-        Eval.defer(fa.reduceRightTo(a => Eval.later(f(a))) { (a, b) =>
-          Eval.defer(g(a, b))
-        })
+        fa.tail match {
+          case LazyList() => Eval.later(f(fa.head))
+          case head +: tail =>
+            val nell = NonEmptyLazyList.fromLazyListPrepend(head, tail)
+            g(fa.head, Eval.defer(reduceRightTo(nell)(f)(g)))
+        }
 
       private val alignInstance = Align[LazyList].asInstanceOf[Align[NonEmptyLazyList]]
 
diff --git a/core/src/main/scala/cats/instances/function.scala b/core/src/main/scala/cats/instances/function.scala
index feed2f8b4a..8221821bd8 100644
--- a/core/src/main/scala/cats/instances/function.scala
+++ b/core/src/main/scala/cats/instances/function.scala
@@ -117,11 +117,9 @@ sealed private[instances] trait Function1Instances extends Function1Instances0 {
       def unit: Unit => R = Function.const(Monoid[R].empty)
       def contramap[A, B](fa: A => R)(f: B => A): B => R =
         fa.compose(f)
-      def product[A, B](fa: A => R, fb: B => R): ((A, B)) => R =
-        (ab: (A, B)) =>
-          ab match {
-            case (a, b) => Monoid[R].combine(fa(a), fb(b))
-          }
+      def product[A, B](fa: A => R, fb: B => R): ((A, B)) => R = {
+        case (a, b) => Monoid[R].combine(fa(a), fb(b))
+      }
     }
 
   implicit def catsStdMonadForFunction1[T1]: Monad[T1 => *] =
diff --git a/tests/src/test/scala-2.13+/cats/tests/NonEmptyLazyListSuite.scala b/tests/src/test/scala-2.13+/cats/tests/NonEmptyLazyListSuite.scala
index 9c0459f4c7..1957e3a1d8 100644
--- a/tests/src/test/scala-2.13+/cats/tests/NonEmptyLazyListSuite.scala
+++ b/tests/src/test/scala-2.13+/cats/tests/NonEmptyLazyListSuite.scala
@@ -16,6 +16,8 @@ import cats.laws.discipline.arbitrary._
 import cats.syntax.either._
 import cats.syntax.foldable._
 import cats.syntax.eq._
+import cats.Reducible
+import cats.Eval
 import org.scalacheck.Prop._
 
 class NonEmptyLazyListSuite extends NonEmptyCollectionSuite[LazyList, NonEmptyLazyList, NonEmptyLazyListOps] {
@@ -164,6 +166,19 @@ class NonEmptyLazyListSuite extends NonEmptyCollectionSuite[LazyList, NonEmptyLa
       assert(ci.toNev === (NonEmptyVector.fromVectorUnsafe(Vector.empty[Int] ++ ci.toList.toVector)))
     }
   }
+
+  test("Avoid all evaluation of NonEmptyLazyList#reduceRightTo") {
+    val sum = implicitly[Reducible[NonEmptyLazyList]]
+      .reduceRightTo(
+        NonEmptyLazyList
+          .fromLazyListPrepend(1, LazyList.from(2))
+      )(identity) { (elem, acc) =>
+        if (elem <= 100) acc.map(_ + elem) else Eval.later(0)
+      }
+      .value
+
+    (1 to 100).sum === sum
+  }
 }
 
 class ReducibleNonEmptyLazyListSuite extends ReducibleSuite[NonEmptyLazyList]("NonEmptyLazyList") {