"ensure" method for XorT

.gitignore

@@ -16,3 +16,4 @@ TAGS
 .sbtrc
 *.sublime-project
 *.sublime-workspace
+tests.iml

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)
 

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))

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))
+      }
+    }
+  }
 }