Add mapWithIndex to Traverse; implement indexed in terms of mapWithIndex

typelevel · Jul 7, 2017 · 8c6e3e6 · 8c6e3e6
1 parent 4413058
commit 8c6e3e6
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diff --git a/core/src/main/scala/cats/Traverse.scala b/core/src/main/scala/cats/Traverse.scala
@@ -108,5 +108,8 @@ import simulacrum.typeclass
    * `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
+    mapWithIndex(fa)((a, i) => (a, i))
+
+  def mapWithIndex[A, B](fa: F[A])(f: (A, Int) => B): F[B] =
+    traverse(fa)(a => State((s: Int) => (s + 1, f(a, s)))).runA(0).value
 }
diff --git a/tests/src/test/scala/cats/tests/TraverseTests.scala b/tests/src/test/scala/cats/tests/TraverseTests.scala
@@ -14,6 +14,12 @@ abstract class TraverseCheck[F[_]: Traverse](name: String)(implicit ArbFInt: Arb
     }
   }
 
+  test(s"Traverse[$name].mapWithIndex") {
+    forAll { (fa: F[Int]) =>
+      fa.mapWithIndex((a, i) => (a, i)).toList should === (fa.toList.zipWithIndex)
+    }
+  }
+
 }
 
 class TraverseListCheck extends TraverseCheck[List]("list")