Add Traverse and Comonad instances for Tuple2.

core/src/main/scala/cats/instances/tuple.scala

@@ -21,4 +21,22 @@ sealed trait Tuple2Instances {
       s"(${aShow.show(f._1)},${bShow.show(f._2)})"
     }
   }
+
+  implicit def catsStdInstancesForTuple2[X]: Traverse[(X, ?)] with Comonad[(X, ?)] =
+    new Traverse[(X, ?)] with Comonad[(X, ?)] {
+      def traverse[G[_], A, B](fa: (X, A))(f: A => G[B])(implicit G: Applicative[G]): G[(X, B)] =
+        G.map(f(fa._2))((fa._1, _))
+
+      def foldLeft[A, B](fa: (X, A), b: B)(f: (B, A) => B): B = f(b, fa._2)
+
+      def foldRight[A, B](fa: (X, A), lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] = f(fa._2, lb)
+
+      override def map[A, B](fa: (X, A))(f: A => B): (X, B) = (fa._1, f(fa._2))
+
+      def coflatMap[A, B](fa: (X, A))(f: ((X, A)) => B): (X, B) = (fa._1, f(fa))
+
+      def extract[A](fa: (X, A)): A = fa._2
+
+      override def coflatten[A](fa: (X, A)): (X, (X, A)) = (fa._1, fa)
+    }
 }

tests/src/test/scala/cats/tests/TupleTests.scala

@@ -1,12 +1,18 @@
 package cats
 package tests
 
-import cats.laws.discipline.{BitraverseTests, SerializableTests}
+import cats.laws.discipline.{BitraverseTests, ComonadTests, SerializableTests, TraverseTests}
 
 class TupleTests extends CatsSuite {
   checkAll("Tuple2", BitraverseTests[Tuple2].bitraverse[Option, Int, Int, Int, String, String, String])
   checkAll("Bitraverse[Tuple2]", SerializableTests.serializable(Bitraverse[Tuple2]))
 
+  checkAll("Tuple2[String, Int] with Option", TraverseTests[(String, ?)].traverse[Int, Int, Int, Int, Option, Option])
+  checkAll("Traverse[(String, ?)]", SerializableTests.serializable(Traverse[(String, ?)]))
+
+  checkAll("Tuple2[String, Int]", ComonadTests[(String, ?)].comonad[Int, Int, Int])
+  checkAll("Comonad[(String, ?)]", SerializableTests.serializable(Comonad[(String, ?)]))
+
   test("eqv") {
     val eq = Eq[(Int, Long)]
     forAll { t: (Int, Long) => eq.eqv(t, t) should === (true) }