From df0bc8556c1c27fa0375721a98edaf8b359c0e0c Mon Sep 17 00:00:00 2001
From: Oscar Boykin <oscar@stripe.com>
Date: Sat, 18 Jun 2016 13:25:58 -1000
Subject: [PATCH] Add instance of MonadRec[Try]

---
 core/src/main/scala/cats/std/try.scala         | 12 ++++++++++--
 tests/src/test/scala/cats/tests/TryTests.scala |  3 +++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/core/src/main/scala/cats/std/try.scala b/core/src/main/scala/cats/std/try.scala
index d10d919fd8..3cdfc607cb 100644
--- a/core/src/main/scala/cats/std/try.scala
+++ b/core/src/main/scala/cats/std/try.scala
@@ -6,12 +6,13 @@ import TryInstances.castFailure
 
 import scala.util.control.NonFatal
 import scala.util.{Failure, Success, Try}
+import scala.annotation.tailrec
 
 trait TryInstances extends TryInstances1 {
 
   // scalastyle:off method.length
-  implicit def catsStdInstancesForTry: MonadError[Try, Throwable] with CoflatMap[Try] with Traverse[Try] =
-    new TryCoflatMap with MonadError[Try, Throwable] with Traverse[Try] {
+  implicit def catsStdInstancesForTry: MonadError[Try, Throwable] with CoflatMap[Try] with Traverse[Try] with MonadRec[Try] =
+    new TryCoflatMap with MonadError[Try, Throwable] with Traverse[Try] with MonadRec[Try] {
       def pure[A](x: A): Try[A] = Success(x)
 
       override def pureEval[A](x: Eval[A]): Try[A] = x match {
@@ -57,6 +58,13 @@ trait TryInstances extends TryInstances1 {
           case f: Failure[_] => G.pure(castFailure[B](f))
         }
 
+      @tailrec final def tailRecM[B, C](b: B)(f: B => Try[(B Xor C)]): Try[C] =
+        f(b) match {
+          case f: Failure[_] => castFailure[C](f)
+          case Success(Xor.Left(b1)) => tailRecM(b1)(f)
+          case Success(Xor.Right(c)) => Success(c)
+        }
+
       def handleErrorWith[A](ta: Try[A])(f: Throwable => Try[A]): Try[A] =
         ta.recoverWith { case t => f(t) }
 
diff --git a/tests/src/test/scala/cats/tests/TryTests.scala b/tests/src/test/scala/cats/tests/TryTests.scala
index 1d79a4ea35..9ded67b731 100644
--- a/tests/src/test/scala/cats/tests/TryTests.scala
+++ b/tests/src/test/scala/cats/tests/TryTests.scala
@@ -21,6 +21,9 @@ class TryTests extends CatsSuite {
   checkAll("Try[Int] with Option", TraverseTests[Try].traverse[Int, Int, Int, Int, Option, Option])
   checkAll("Traverse[Try]", SerializableTests.serializable(Traverse[Try]))
 
+  checkAll("Try", MonadRecTests[Try].monadRec[Int, Int, Int])
+  checkAll("MonadRec[Try]", SerializableTests.serializable(MonadRec[Try]))
+
   test("show") {
     forAll { fs: Try[String] =>
       fs.show should === (fs.toString)