From 98b68089730d0ba8e308ba293b91a3dedf244b9b Mon Sep 17 00:00:00 2001
From: Andy Scott <andy.g.scott@gmail.com>
Date: Thu, 6 Jul 2017 23:58:41 -0700
Subject: [PATCH] Add indexed to Traverse

---
 core/src/main/scala/cats/Traverse.scala         | 12 ++++++++++++
 tests/src/test/scala/cats/tests/ListTests.scala |  6 ++++++
 2 files changed, 18 insertions(+)

diff --git a/core/src/main/scala/cats/Traverse.scala b/core/src/main/scala/cats/Traverse.scala
index 24015db6b96..f7fb664f2e7 100644
--- a/core/src/main/scala/cats/Traverse.scala
+++ b/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
 }
diff --git a/tests/src/test/scala/cats/tests/ListTests.scala b/tests/src/test/scala/cats/tests/ListTests.scala
index 308e0672789..ffa5c342c09 100644
--- a/tests/src/test/scala/cats/tests/ListTests.scala
+++ b/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)
+    }
+  }
 }