From c4b4134246ffa500070c5098ad38f2a3f2fe4867 Mon Sep 17 00:00:00 2001
From: ImLiar <whoisliar@gmail.com>
Date: Fri, 11 Mar 2016 00:11:55 +0300
Subject: [PATCH] "ensure" method for XorT

---
 .gitignore                                    |  1 +
 core/src/main/scala/cats/data/Xor.scala       |  4 ++--
 core/src/main/scala/cats/data/XorT.scala      |  2 ++
 .../src/test/scala/cats/tests/XorTTests.scala | 24 +++++++++++++++++++
 4 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/.gitignore b/.gitignore
index 4c19d3d8e4..4581b4d5be 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,3 +16,4 @@ TAGS
 .sbtrc
 *.sublime-project
 *.sublime-workspace
+tests.iml
diff --git a/core/src/main/scala/cats/data/Xor.scala b/core/src/main/scala/cats/data/Xor.scala
index b086575a13..ef778bc37a 100644
--- a/core/src/main/scala/cats/data/Xor.scala
+++ b/core/src/main/scala/cats/data/Xor.scala
@@ -59,8 +59,8 @@ sealed abstract class Xor[+A, +B] extends Product with Serializable {
 
   def exists(f: B => Boolean): Boolean = fold(_ => false, f)
 
-  def ensure[AA >: A](ifLeft: => AA)(f: B => Boolean): AA Xor B =
-    fold(_ => this, b => if (f(b)) this else Xor.Left(ifLeft))
+  def ensure[AA >: A](onFailure: => AA)(f: B => Boolean): AA Xor B =
+    fold(_ => this, b => if (f(b)) this else Xor.Left(onFailure))
 
   def toIor: A Ior B = fold(Ior.left, Ior.right)
 
diff --git a/core/src/main/scala/cats/data/XorT.scala b/core/src/main/scala/cats/data/XorT.scala
index d7ff05bf80..71d4a97c28 100644
--- a/core/src/main/scala/cats/data/XorT.scala
+++ b/core/src/main/scala/cats/data/XorT.scala
@@ -51,6 +51,8 @@ final case class XorT[F[_], A, B](value: F[A Xor B]) {
 
   def exists(f: B => Boolean)(implicit F: Functor[F]): F[Boolean] = F.map(value)(_.exists(f))
 
+  def ensure[AA >: A](onFailure: => AA)(f: B => Boolean)(implicit F: Functor[F]): XorT[F, AA, B] = XorT(F.map(value)(_.ensure(onFailure)(f)))
+
   def toEither(implicit F: Functor[F]): F[Either[A, B]] = F.map(value)(_.toEither)
 
   def toOption(implicit F: Functor[F]): OptionT[F, B] = OptionT(F.map(value)(_.toOption))
diff --git a/tests/src/test/scala/cats/tests/XorTTests.scala b/tests/src/test/scala/cats/tests/XorTTests.scala
index f75b14a713..7ea4e9b343 100644
--- a/tests/src/test/scala/cats/tests/XorTTests.scala
+++ b/tests/src/test/scala/cats/tests/XorTTests.scala
@@ -251,4 +251,28 @@ class XorTTests extends CatsSuite {
       x.toEither.map(_.right.toOption) should === (x.toOption.value)
     }
   }
+
+  test("ensure on left is identity") {
+    forAll { (x: XorT[Id, String, Int], s: String, p: Int => Boolean) =>
+      if (x.isLeft) {
+        x.ensure(s)(p) should === (x)
+      }
+    }
+  }
+
+  test("ensure on right is identity if predicate satisfied") {
+    forAll { (x: XorT[Id, String, Int], s: String, p: Int => Boolean) =>
+      if (x.isRight && p(x getOrElse 0)) {
+        x.ensure(s)(p) should === (x)
+      }
+    }
+  }
+
+  test("ensure should fail if predicate not satisfied") {
+    forAll { (x: XorT[Id, String, Int], s: String, p: Int => Boolean) =>
+      if (x.isRight && !p(x getOrElse 0)) {
+        x.ensure(s)(p) should === (XorT.left[Id, String, Int](s))
+      }
+    }
+  }
 }