From 35bbb0957658db626fe1699b047aa840872cff52 Mon Sep 17 00:00:00 2001
From: peterneyens <peter.neyens@gmail.com>
Date: Tue, 18 Oct 2016 13:03:46 +0200
Subject: [PATCH] Add MonadError instance for StateT.

---
 core/src/main/scala/cats/data/StateT.scala    | 20 ++++++++++++++++---
 .../test/scala/cats/tests/StateTTests.scala   | 12 ++++++++++-
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/core/src/main/scala/cats/data/StateT.scala b/core/src/main/scala/cats/data/StateT.scala
index e6bd6f166d..65150e3a2a 100644
--- a/core/src/main/scala/cats/data/StateT.scala
+++ b/core/src/main/scala/cats/data/StateT.scala
@@ -174,14 +174,19 @@ private[data] sealed trait StateTInstances1 extends StateTInstances2 {
     new StateTMonadCombine[F, S] { implicit def F = F0 }
 }
 
-private[data] sealed trait StateTInstances2 {
-  implicit def catsDataMonadForStateT[F[_], S](implicit F0: Monad[F]): Monad[StateT[F, S, ?]] =
-    new StateTMonad[F, S] { implicit def F = F0 }
+private[data] sealed trait StateTInstances2 extends StateTInstances3 {
+  implicit def catsDataMonadErrorForStateT[F[_], S, E](implicit F0: MonadError[F, E]): MonadError[StateT[F, S, ?], E] =
+    new StateTMonadError[F, S, E] { implicit def F = F0 }
 
   implicit def catsDataSemigroupKForStateT[F[_], S](implicit F0: Monad[F], G0: SemigroupK[F]): SemigroupK[StateT[F, S, ?]] =
     new StateTSemigroupK[F, S] { implicit def F = F0; implicit def G = G0 }
 }
 
+private[data] sealed trait StateTInstances3 {
+  implicit def catsDataMonadForStateT[F[_], S](implicit F0: Monad[F]): Monad[StateT[F, S, ?]] =
+    new StateTMonad[F, S] { implicit def F = F0 }
+}
+
 // To workaround SI-7139 `object State` needs to be defined inside the package object
 // together with the type alias.
 private[data] abstract class StateFunctions {
@@ -258,3 +263,12 @@ private[data] sealed trait StateTMonadCombine[F[_], S] extends MonadCombine[Stat
 
   def empty[A]: StateT[F, S, A] = liftT[F, A](F.empty[A])
 }
+
+private[data] sealed trait StateTMonadError[F[_], S, E] extends StateTMonad[F, S] with MonadError[StateT[F, S, ?], E] {
+  implicit def F: MonadError[F, E]
+
+  def raiseError[A](e: E): StateT[F, S, A] = StateT.lift(F.raiseError(e))
+
+  def handleErrorWith[A](fa: StateT[F, S, A])(f: E => StateT[F, S, A]): StateT[F, S, A] =
+    StateT(s => F.handleErrorWith(fa.run(s))(e => f(e).run(s)))
+}
diff --git a/tests/src/test/scala/cats/tests/StateTTests.scala b/tests/src/test/scala/cats/tests/StateTTests.scala
index 62696864e9..8d1e876a92 100644
--- a/tests/src/test/scala/cats/tests/StateTTests.scala
+++ b/tests/src/test/scala/cats/tests/StateTTests.scala
@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{State, StateT}
+import cats.data.{State, StateT, EitherT}
 import cats.kernel.instances.tuple._
 import cats.laws.discipline._
 import cats.laws.discipline.eq._
@@ -260,6 +260,16 @@ class StateTTests extends CatsSuite {
     checkAll("State[Long, ?]", MonadTests[State[Long, ?]].monad[Int, Int, Int])
     checkAll("Monad[State[Long, ?]]", SerializableTests.serializable(Monad[State[Long, ?]]))
   }
+
+  {
+    // F has a MonadError
+    implicit val iso = CartesianTests.Isomorphisms.invariant[StateT[Option, Int, ?]]
+    implicit val eqEitherTFA: Eq[EitherT[StateT[Option, Int , ?], Unit, Int]] = EitherT.catsDataEqForEitherT[StateT[Option, Int , ?], Unit, Int]
+    
+    checkAll("StateT[Option, Int, Int]", MonadErrorTests[StateT[Option, Int , ?], Unit].monadError[Int, Int, Int])
+    checkAll("MonadError[StateT[Option, Int , ?], Unit]", SerializableTests.serializable(MonadError[StateT[Option, Int , ?], Unit]))
+  }
+
 }
 
 object StateTTests extends StateTTestsInstances {