Experiment: use immutable List for optimized traverse

To see if this is more performant than an immutable vector
typelevel · Nov 10, 2023 · e3ae584 · e3ae584
1 parent 8cc742e
commit e3ae584
Show file tree

Hide file tree

Showing 3 changed files with 6 additions and 6 deletions.
diff --git a/core/src/main/scala/cats/Traverse.scala b/core/src/main/scala/cats/Traverse.scala
@@ -287,14 +287,14 @@ object Traverse {
 
   private[cats] def traverseDirectly[G[_], A, B](
     fa: IterableOnce[A]
-  )(f: A => G[B])(implicit G: StackSafeMonad[G]): G[Vector[B]] = {
-    fa.iterator.foldLeft(G.pure(Vector.empty[B])) { case (accG, a) =>
+  )(f: A => G[B])(implicit G: StackSafeMonad[G]): G[List[B]] = {
+    G.map(fa.iterator.foldLeft(G.pure(List.empty[B])) { case (accG, a) =>
       G.flatMap(accG) { acc =>
         G.map(f(a)) { b =>
-          acc :+ b
+          b :: acc
         }
       }
-    }
+    })(_.reverse)
   }
 
   private[cats] def traverse_Directly[G[_], A, B](

diff --git a/core/src/main/scala/cats/instances/list.scala b/core/src/main/scala/cats/instances/list.scala
@@ -122,7 +122,7 @@ trait ListInstances extends cats.kernel.instances.ListInstances {
         if (fa.isEmpty) G.pure(Nil)
         else
           G match {
-            case x: StackSafeMonad[G] => G.map(Traverse.traverseDirectly[G, A, B](fa)(f)(x))(_.toList)
+            case x: StackSafeMonad[G] => Traverse.traverseDirectly[G, A, B](fa)(f)(x)
             case _ =>
               G.map(Chain.traverseViaChain {
                 val as = collection.mutable.ArrayBuffer[A]()

diff --git a/core/src/main/scala/cats/instances/vector.scala b/core/src/main/scala/cats/instances/vector.scala
@@ -126,7 +126,7 @@ trait VectorInstances extends cats.kernel.instances.VectorInstances {
 
       final override def traverse[G[_], A, B](fa: Vector[A])(f: A => G[B])(implicit G: Applicative[G]): G[Vector[B]] =
         G match {
-          case x: StackSafeMonad[G] => Traverse.traverseDirectly(fa)(f)(x)
+          case x: StackSafeMonad[G] => x.map(Traverse.traverseDirectly(fa)(f)(x))(_.toVector)
           case _                    => G.map(Chain.traverseViaChain(fa)(f))(_.toVector)
         }