Add indexed to Traverse

core/src/main/scala/cats/Traverse.scala

@@ -1,5 +1,7 @@
 package cats
 
+import cats.data.State
+
 import simulacrum.typeclass
 
 /**
@@ -97,4 +99,14 @@ import simulacrum.typeclass
 
   override def map[A, B](fa: F[A])(f: A => B): F[B] =
     traverse[Id, A, B](fa)(f)
+
+  /**
+   * Traverses through the structure F, pairing the values with
+   * assigned indices.
+   *
+   * The behavior is consistent with the Scala collection library's
+   * `zipWithIndex` for collections such as `List`.
+   */
+  def indexed[A](fa: F[A]): F[(A, Int)] =
+    traverse(fa)(a => State((s: Int) => (s + 1, (a, s)))).runA(0).value
 }

tests/src/test/scala/cats/tests/TraverseTests.scala

@@ -0,0 +1,25 @@
+package cats
+package tests
+
+class TraverseTestsAdditional extends CatsSuite {
+
+  def checkIndexedStackSafety[F[_]](fromRange: Range => F[Int])(implicit F: Traverse[F]): Unit = {
+    val res = F.indexed(fromRange(1 to 500000))
+    assert(F.forall(res) { case (a, i) => a == i + 1 })
+    ()
+  }
+
+  test("Traverse[List].indexed consistent with zipWithIndex") {
+    forAll { (fa: List[String]) =>
+      Traverse[List].indexed(fa) should === (fa.zipWithIndex)
+    }
+  }
+
+  test("Traverse[List].indexed stack safety") {
+    checkIndexedStackSafety[List](_.toList)
+  }
+
+  test("Traverse[Stream].indexed stack safety") {
+    checkIndexedStackSafety[Stream](_.toStream)
+  }
+}