backported typelevel#3088 (adding bifold to Bifoldable typeclass)…

… to scala_2.11

core/src/main/scala/cats/syntax/all.scala

@@ -95,4 +95,4 @@ trait AllSyntaxBinCompat5 extends ParallelBitraverseSyntax
 
 trait AllSyntaxBinCompat6 extends ParallelUnorderedTraverseSyntax
 
-trait AllSyntaxBinCompat7 extends FunctorSyntaxBinCompat0
+trait AllSyntaxBinCompat7 extends FunctorSyntaxBinCompat0 with BiFoldableSyntaxBinCompat0

core/src/main/scala/cats/syntax/bifoldable.scala

@@ -1,4 +1,29 @@
 package cats
 package syntax
 
+import scala.language.implicitConversions
+
 trait BifoldableSyntax extends Bifoldable.ToBifoldableOps
+
+private[syntax] trait BiFoldableSyntaxBinCompat0 {
+  implicit final def catsSyntaxBiFoldOps[F[_, _], A, B](fa: F[A, B]): BifoldBifoldableOps[F, A, B] =
+    new BifoldBifoldableOps[F, A, B](fa)
+}
+
+final class BifoldBifoldableOps[F[_, _], A, B](private val fab: F[A, B]) extends AnyVal {
+
+  /**
+   * Collapse the structure to a tuple2, given each type has an available [[cats.Monoid]]
+   *
+   * {{{
+   * scala> import cats.implicits._
+   * scala> Either.left[Int, String](5).bifold == ((5, ""))
+   * res0: Boolean = true
+   * }}}
+   *
+   */
+  def bifold(implicit F: Bifoldable[F], A: Monoid[A], B: Monoid[B]): (A, B) = {
+    import cats.instances.tuple._
+    F.bifoldMap(fab)((_, B.empty), (A.empty, _))
+  }
+}

core/src/main/scala/cats/syntax/package.scala

@@ -9,7 +9,7 @@ package object syntax {
   object arrow extends ArrowSyntax
   object arrowChoice extends ArrowChoiceSyntax
   object bifunctor extends BifunctorSyntax
-  object bifoldable extends BifoldableSyntax
+  object bifoldable extends BifoldableSyntax with BiFoldableSyntaxBinCompat0
   object binested extends BinestedSyntax
   object bitraverse extends BitraverseSyntax with BitraverseSyntaxBinCompat0
   @deprecated("use cats.syntax.semigroupal instead", "1.0.0-RC1")

tests/src/test/scala/cats/tests/BifoldableSuite.scala

@@ -10,4 +10,9 @@ class BifoldableSuite extends CatsSuite {
 
   checkAll("Either compose Either", BifoldableTests(eitherComposeEither).bifoldable[Int, Int, Int])
   checkAll("Bifoldable[Either compose Either]", SerializableTests.serializable(eitherComposeEither))
+
+  test("bifold works for 2 monoids") {
+    Either.right[Int, String]("something").bifold should ===((0, "something"))
+    Either.left[Int, String](5).bifold should ===((5, ""))
+  }
 }