Merge pull request #521 from drostron/xor-monoid-instance

add xorMonoid
typelevel · Sep 10, 2015 · a89d71e · a89d71e
2 parents eb5f431 + 86dbe1c
commit a89d71e
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diff --git a/core/src/main/scala/cats/data/Xor.scala b/core/src/main/scala/cats/data/Xor.scala
@@ -156,7 +156,13 @@ sealed abstract class XorInstances extends XorInstances1 {
       def show(f: A Xor B): String = f.show
     }
 
-  implicit def xorInstances[A]: Traverse[A Xor ?] with MonadError[Xor, A ]=
+  implicit def xorMonoid[A, B](implicit A: Semigroup[A], B: Monoid[B]): Monoid[A Xor B] =
+    new Monoid[A Xor B] {
+      def empty: A Xor B = Xor.Right(B.empty)
+      def combine(x: A Xor B, y: A Xor B): A Xor B = x combine y
+    }
+
+  implicit def xorInstances[A]: Traverse[A Xor ?] with MonadError[Xor, A ] =
     new Traverse[A Xor ?] with MonadError[Xor, A] {
       def traverse[F[_]: Applicative, B, C](fa: A Xor B)(f: B => F[C]): F[A Xor C] = fa.traverse(f)
       def foldLeft[B, C](fa: A Xor B, c: C)(f: (C, B) => C): C = fa.foldLeft(c)(f)

diff --git a/tests/shared/src/test/scala/cats/tests/XorTests.scala b/tests/shared/src/test/scala/cats/tests/XorTests.scala
@@ -3,6 +3,7 @@ package tests
 
 import cats.data.Xor
 import cats.data.Xor._
+import cats.laws.discipline.arbitrary.xorArbitrary
 import cats.laws.discipline.{TraverseTests, MonadErrorTests, SerializableTests}
 import org.scalacheck.{Arbitrary, Gen}
 import org.scalacheck.Prop._
@@ -12,6 +13,8 @@ import org.scalacheck.Arbitrary._
 import scala.util.Try
 
 class XorTests extends CatsSuite {
+  checkAll("Xor[String, Int]", algebra.laws.GroupLaws[Xor[String, Int]].monoid)
+
   checkAll("Xor[String, Int]", MonadErrorTests[Xor, String].monadError[Int, Int, Int])
   checkAll("MonadError[Xor, String]", SerializableTests.serializable(MonadError[Xor, String]))