Add stateTraverse and indexed to Traverse

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

@@ -1,5 +1,7 @@
 package cats
 
+import cats.data.State
+
 import simulacrum.typeclass
 
 /**
@@ -32,6 +34,17 @@ import simulacrum.typeclass
    */
   def traverse[G[_]: Applicative, A, B](fa: F[A])(f: A => G[B]): G[F[B]]
 
+  /**
+   * Traverse with a function returning state.
+   *
+   * Given a function which returns state, thread the state through
+   * the running of this function on all the values in F, returning
+   * F[B] in the state context.
+   */
+  def stateTraverse[S, A, B](fa: F[A])(f: A => State[S, B]): State[S, F[B]] =
+    State[S, F[B]](s =>
+      traverse[State[S, ?] , A, B](fa)(f).run(s).value)
+
   /**
    * A traverse followed by flattening the inner result.
    *
@@ -97,4 +110,14 @@ 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)] =
+    stateTraverse(fa)(a => State((s: Int) => (s + 1, (a, s)))).runA(0).value
 }

tests/src/test/scala/cats/tests/ListTests.scala

@@ -42,4 +42,10 @@ class ListTests extends CatsSuite {
       l.show should === (l.toString)
     }
   }
+
+  test("indexed consistent with zipWithIndex") {
+    forAll { (fa: List[String]) =>
+      Traverse[List].indexed(fa) should === (fa.zipWithIndex)
+    }
+  }
 }