From 2e4b52f66426f3d4a8372526742f8c1b09edffd7 Mon Sep 17 00:00:00 2001
From: Luka Jacobowitz <luka.jacobowitz@gmail.com>
Date: Fri, 5 Jun 2020 15:44:19 -0400
Subject: [PATCH] Override map2Eval and combineKEval for monad transformers

---
 core/src/main/scala/cats/data/EitherT.scala   |  6 +++++
 core/src/main/scala/cats/data/Kleisli.scala   |  8 +++++++
 core/src/main/scala/cats/data/OptionT.scala   |  6 +++++
 core/src/main/scala/cats/data/WriterT.scala   |  3 +++
 .../test/scala/cats/tests/KleisliSuite.scala  | 22 ++++++++++++++++---
 5 files changed, 42 insertions(+), 3 deletions(-)

diff --git a/core/src/main/scala/cats/data/EitherT.scala b/core/src/main/scala/cats/data/EitherT.scala
index 16f242a743..f921e40b37 100644
--- a/core/src/main/scala/cats/data/EitherT.scala
+++ b/core/src/main/scala/cats/data/EitherT.scala
@@ -1040,6 +1040,12 @@ private[data] trait EitherTSemigroupK[F[_], L] extends SemigroupK[EitherT[F, L,
       case l @ Left(_)  => y.value
       case r @ Right(_) => F.pure(r)
     })
+
+  override def combineKEval[A](x: EitherT[F, L, A], y: Eval[EitherT[F, L, A]]): Eval[EitherT[F, L, A]] =
+    Eval.now(EitherT(F.flatMap(x.value) {
+      case l @ Left(_)  => y.value.value
+      case r @ Right(_) => F.pure(r: Either[L, A])
+    }))
 }
 
 private[data] trait EitherTFunctor[F[_], L] extends Functor[EitherT[F, L, *]] {
diff --git a/core/src/main/scala/cats/data/Kleisli.scala b/core/src/main/scala/cats/data/Kleisli.scala
index d88556791a..dd3afc56b8 100644
--- a/core/src/main/scala/cats/data/Kleisli.scala
+++ b/core/src/main/scala/cats/data/Kleisli.scala
@@ -541,6 +541,9 @@ sealed private[data] trait KleisliSemigroupK[F[_], A] extends SemigroupK[Kleisli
 
   override def combineK[B](x: Kleisli[F, A, B], y: Kleisli[F, A, B]): Kleisli[F, A, B] =
     Kleisli(a => F.combineK(x.run(a), y.run(a)))
+
+  override def combineKEval[B](x: Kleisli[F, A, B], y: Eval[Kleisli[F, A, B]]): Eval[Kleisli[F, A, B]] =
+    Eval.now(Kleisli(a => F.combineKEval(x.run(a), y.map(_.run(a))).value))
 }
 
 sealed private[data] trait KleisliMonoidK[F[_], A] extends MonoidK[Kleisli[F, A, *]] with KleisliSemigroupK[F, A] {
@@ -622,6 +625,11 @@ private[data] trait KleisliApply[F[_], A] extends Apply[Kleisli[F, A, *]] with K
   override def ap[B, C](f: Kleisli[F, A, B => C])(fa: Kleisli[F, A, B]): Kleisli[F, A, C] =
     fa.ap(f)
 
+  override def map2Eval[B, C, Z](fa: Kleisli[F, A, B], fb: Eval[Kleisli[F, A, C]])(
+    f: (B, C) => Z
+  ): Eval[Kleisli[F, A, Z]] =
+    Eval.now(Kleisli(a => F.map2Eval(fa.run(a), fb.map(_.run(a)))(f).value))
+
   override def product[B, C](fb: Kleisli[F, A, B], fc: Kleisli[F, A, C]): Kleisli[F, A, (B, C)] =
     Kleisli(a => F.product(fb.run(a), fc.run(a)))
 }
diff --git a/core/src/main/scala/cats/data/OptionT.scala b/core/src/main/scala/cats/data/OptionT.scala
index 16cff75a22..6c57e10560 100644
--- a/core/src/main/scala/cats/data/OptionT.scala
+++ b/core/src/main/scala/cats/data/OptionT.scala
@@ -540,6 +540,12 @@ private[data] trait OptionTSemigroupK[F[_]] extends SemigroupK[OptionT[F, *]] {
   implicit def F: Monad[F]
 
   def combineK[A](x: OptionT[F, A], y: OptionT[F, A]): OptionT[F, A] = x.orElse(y)
+
+  override def combineKEval[A](x: OptionT[F, A], y: Eval[OptionT[F, A]]): Eval[OptionT[F, A]] =
+    Eval.now(OptionT(F.flatMap(x.value) {
+      case oa @ Some(_) => F.pure(oa)
+      case None         => y.value.value
+    }))
 }
 
 private[data] trait OptionTMonoidK[F[_]] extends MonoidK[OptionT[F, *]] with OptionTSemigroupK[F] {
diff --git a/core/src/main/scala/cats/data/WriterT.scala b/core/src/main/scala/cats/data/WriterT.scala
index cf3da05942..bb309a9c74 100644
--- a/core/src/main/scala/cats/data/WriterT.scala
+++ b/core/src/main/scala/cats/data/WriterT.scala
@@ -704,6 +704,9 @@ sealed private[data] trait WriterTSemigroupK[F[_], L] extends SemigroupK[WriterT
 
   def combineK[A](x: WriterT[F, L, A], y: WriterT[F, L, A]): WriterT[F, L, A] =
     WriterT(F0.combineK(x.run, y.run))
+
+  override def combineKEval[A](x: WriterT[F, L, A], y: Eval[WriterT[F, L, A]]): Eval[WriterT[F, L, A]] =
+    F0.combineKEval(x.run, y.map(_.run)).map(WriterT(_))
 }
 
 sealed private[data] trait WriterTMonoidK[F[_], L] extends MonoidK[WriterT[F, L, *]] with WriterTSemigroupK[F, L] {
diff --git a/tests/src/test/scala/cats/tests/KleisliSuite.scala b/tests/src/test/scala/cats/tests/KleisliSuite.scala
index 91c269c9e6..6a59890b02 100644
--- a/tests/src/test/scala/cats/tests/KleisliSuite.scala
+++ b/tests/src/test/scala/cats/tests/KleisliSuite.scala
@@ -9,9 +9,7 @@ import cats.laws.discipline.arbitrary._
 import cats.laws.discipline.eq._
 import cats.laws.discipline.SemigroupalTests.Isomorphisms
 import cats.laws.discipline.{DeferTests, MonoidKTests, SemigroupKTests}
-import cats.syntax.flatMap._
-import cats.syntax.functor._
-import cats.syntax.traverse._
+import cats.implicits._
 import cats.platform.Platform
 import cats.tests.Helpers.CSemi
 
@@ -321,6 +319,24 @@ class KleisliSuite extends CatsSuite {
     result.run(()).value
   }
 
+  test("map2Eval is lazy") {
+    var count = 0
+    val l = Kleisli { (n: Int) => count += 1; Option.empty[String] }
+
+    l.map2Eval(Eval.now(l))(_ + _).value.run(0)
+
+    count shouldBe 1
+  }
+
+  test("combineKEval is lazy") {
+    var count = 0
+    val l = Kleisli { (n: Int) => count += 1; Option(n.toString) }
+
+    l.combineKEval(Eval.now(l)).value.run(0)
+
+    count shouldBe 1
+  }
+
   test("auto contravariant") {
     trait A1
     trait A2