From b0f871b5a65054b9cfc130a88d3b966a33893275 Mon Sep 17 00:00:00 2001
From: Rutvik Patel <heyrutvik@gmail.com>
Date: Wed, 20 Jun 2018 01:50:56 +0530
Subject: [PATCH] 2228-add MonadError instance for optionT where F is a Monad
 (#2299)

* added MonadError instance for OptionT where F is a Monad

* test added

* removed duplicate code using OptionTMonad[F]
---
 core/src/main/scala/cats/data/OptionT.scala       | 15 +++++++++++++++
 .../src/test/scala/cats/tests/OptionTSuite.scala  |  7 +++++++
 2 files changed, 22 insertions(+)

diff --git a/core/src/main/scala/cats/data/OptionT.scala b/core/src/main/scala/cats/data/OptionT.scala
index 74fb633fb5..30976b1a59 100644
--- a/core/src/main/scala/cats/data/OptionT.scala
+++ b/core/src/main/scala/cats/data/OptionT.scala
@@ -254,6 +254,9 @@ private[data] sealed abstract class OptionTInstances1 extends OptionTInstances2
 
   implicit def catsDataEqForOptionT[F[_], A](implicit F0: Eq[F[Option[A]]]): Eq[OptionT[F, A]] =
     new OptionTEq[F, A] { implicit val F = F0 }
+
+  implicit def catsDataMonadErrorMonadForOptionT[F[_]](implicit F0: Monad[F]): MonadError[OptionT[F, ?], Unit] =
+    new OptionTMonadErrorMonad[F] { implicit val F = F0 }
 }
 
 private[data] sealed abstract class OptionTInstances2 extends OptionTInstances3 {
@@ -287,6 +290,18 @@ private[data] trait OptionTMonad[F[_]] extends Monad[OptionT[F, ?]] {
     )))
 }
 
+private[data] trait OptionTMonadErrorMonad[F[_]] extends MonadError[OptionT[F, ?], Unit] with OptionTMonad[F] {
+  implicit def F: Monad[F]
+
+  override def raiseError[A](e: Unit): OptionT[F, A] = OptionT.none
+
+  override def handleErrorWith[A](fa: OptionT[F, A])(f: Unit => OptionT[F, A]): OptionT[F, A] =
+    OptionT(F.flatMap(fa.value) {
+      case s @ Some(_) => F.pure(s)
+      case None => f(()).value
+    })
+}
+
 private trait OptionTMonadError[F[_], E] extends MonadError[OptionT[F, ?], E] with OptionTMonad[F] {
   override def F: MonadError[F, E]
 
diff --git a/tests/src/test/scala/cats/tests/OptionTSuite.scala b/tests/src/test/scala/cats/tests/OptionTSuite.scala
index 32e3458fda..8ba0935b61 100644
--- a/tests/src/test/scala/cats/tests/OptionTSuite.scala
+++ b/tests/src/test/scala/cats/tests/OptionTSuite.scala
@@ -130,6 +130,13 @@ class OptionTSuite extends CatsSuite {
     checkAll("Semigroup[OptionT[ListWrapper, Int]]", SerializableTests.serializable(Semigroup[OptionT[ListWrapper, Int]]))
   }
 
+  {
+    // MonadError instance where F has a Monad
+    implicit val F = ListWrapper.monad
+    checkAll("OptionT[ListWrapper, Int]", MonadErrorTests[OptionT[ListWrapper, ?], Unit].monadError[Int, Int, Int])
+    checkAll("MonadError[OptionT[List, ?]]", SerializableTests.serializable(MonadError[OptionT[ListWrapper, ?], Unit]))
+  }
+
   test("fold and cata consistent") {
     forAll { (o: OptionT[List, Int], s: String, f: Int => String) =>
       o.fold(s)(f) should === (o.cata(s, f))