Add lefts and rights functions to MonadPlus

They were already present as private functions in `separate`.

(cherry picked from commit 7d865da)

core/src/main/scala/scalaz/syntax/MonadPlusSyntax.scala

@@ -20,6 +20,12 @@ final class MonadPlusOps[F[_],A] private[syntax](val self: F[A])(implicit val F:
     F.unite[T, B](ftb)
   }
 
+  final def lefts[G[_, _], B, C](implicit ev: A === G[B, C], G: Bifoldable[G]): F[B] =
+    F.bind(ev.subst(self))(b => G.leftFoldable.foldMap(b)(F.point(_))(F.monoid[B]))
+
+  final def rights[G[_, _], B, C](implicit ev: A === G[B, C], G: Bifoldable[G]): F[C] =
+    F.bind(ev.subst(self))(c => G.rightFoldable.foldMap(c)(F.point(_))(F.monoid[C]))
+
   final def separate[G[_, _], B, C](implicit ev: A === G[B, C], G: Bifoldable[G]): (F[B], F[C]) =
     F.separate(ev.subst(self))
 

tests/src/test/scala/scalaz/MonadPlusTest.scala

@@ -14,6 +14,32 @@ object MonadPlusTest extends SpecLite {
     MonadPlus[List].uniteU(List(\/.right(1), \/.left("a"), \/.right(2))) must_===(List(1, 2))
   }
 
+  "lefts" in {
+    import \&/._
+    import syntax.monadPlus._
+
+    List(\/.right(1), \/.left("a"), \/.right(2)).lefts must_===(List("a"))
+
+    List(1 -> "a", 2 -> "b").lefts must_===(List(1, 2))
+
+    Vector[Int \&/ String](This(1), Both(3, "a"), That("b")).lefts must_===(Vector(1, 3))
+
+    Stream(Success(1), Failure("a"), Success(2)).lefts must_===(Stream("a"))
+  }
+
+  "rights" in {
+    import \&/._
+    import syntax.monadPlus._
+
+    List(\/.right(1), \/.left("a"), \/.right(2)).rights must_===(List(1, 2))
+
+    List(1 -> "a", 2 -> "b").rights must_===(List("a", "b"))
+
+    Vector[Int \&/ String](This(1), Both(3, "a"), That("b")).rights must_===(Vector("a", "b"))
+
+    Stream(Success(1), Failure("a"), Success(2)).rights must_===(Stream(1, 2))
+  }
+
   "separate" in {
     import \&/._
     import syntax.monadPlus._