From 5471bd4544d8065185e850ed07dc8e254a3a68b6 Mon Sep 17 00:00:00 2001
From: "Ross A. Baker" <ross@rossabaker.com>
Date: Fri, 4 Dec 2015 21:59:01 -0500
Subject: [PATCH] Add XorT.orElse

---
 core/src/main/scala/cats/data/XorT.scala        |  9 +++++++++
 tests/src/test/scala/cats/tests/XorTTests.scala | 15 +++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/core/src/main/scala/cats/data/XorT.scala b/core/src/main/scala/cats/data/XorT.scala
index 7736113826..6bc5c4a23c 100644
--- a/core/src/main/scala/cats/data/XorT.scala
+++ b/core/src/main/scala/cats/data/XorT.scala
@@ -29,6 +29,15 @@ final case class XorT[F[_], A, B](value: F[A Xor B]) {
     }
   }
 
+  def orElse[AA >: A, BB >: B](default: => XorT[F, AA, BB])(implicit F: Monad[F]): XorT[F, AA, BB] = {
+    XorT(F.flatMap(value) { xor =>
+      xor match {
+        case Xor.Left(_) => default.value
+        case _ => F.pure(xor)
+      }
+    })
+  }
+
   def recover(pf: PartialFunction[A, B])(implicit F: Functor[F]): XorT[F, A, B] =
     XorT(F.map(value)(_.recover(pf)))
 
diff --git a/tests/src/test/scala/cats/tests/XorTTests.scala b/tests/src/test/scala/cats/tests/XorTTests.scala
index 323888f833..067b9da785 100644
--- a/tests/src/test/scala/cats/tests/XorTTests.scala
+++ b/tests/src/test/scala/cats/tests/XorTTests.scala
@@ -163,6 +163,21 @@ class XorTTests extends CatsSuite {
     }
   }
 
+  test("orElse with Id consistent with Xor orElse") {
+    forAll { (xort: XorT[Id, String, Int], fallback: XorT[Id, String, Int]) =>
+      xort.orElse(fallback).value should === (xort.value.orElse(fallback.value))
+    }
+  }
+
+  test("orElse evaluates effect only once") {
+    forAll { (xor: String Xor Int, fallback: XorT[Eval, String, Int]) =>
+      var evals = 0
+      val xort = (XorT(Eval.always { evals += 1; xor }) orElse fallback)
+      xort.value.value
+      evals should === (1)
+    }
+  }
+
   test("forall with Id consistent with Xor forall") {
     forAll { (xort: XorT[Id, String, Int], f: Int => Boolean) =>
       xort.forall(f) should === (xort.value.forall(f))