Fix traverse mutability

typelevel · Sep 25, 2021 · a44fa6e · a44fa6e
1 parent c6fdb82
commit a44fa6e
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Showing 3 changed files with 23 additions and 11 deletions.
diff --git a/modules/deriving/src/main/scala/shapeless3/deriving/internals/erased.scala b/modules/deriving/src/main/scala/shapeless3/deriving/internals/erased.scala
@@ -160,18 +160,25 @@ private[shapeless3] final class ErasedProductInstancesN[K, FT](val mirror: Mirro
 
   final def erasedTraverse(x0: Any)(map: (Any, Any) => Any)(pure: Any => Any)(ap: (Any, Any) => Any)(f: (Any, Any) => Any) = {
     val n = is.length
+
+    def prepend(xs: List[Any])(x: Any) = x :: xs
+    def fromList(xs: List[Any]) =
+      val arr = new Array[Any](n)
+      @tailrec def toProduct(xs: List[Any], i: Int): ArrayProduct = xs match
+        case x :: xs => arr(i) = x; toProduct(xs, i - 1)
+        case Nil => new ArrayProduct(arr)
+      mirror.fromProduct(toProduct(xs, n - 1))
+
     if (n == 0) pure(x0)
     else {
       val x = toProduct(x0)
-      val arr = new Array[Any](n)
-      var acc = pure(())
+      var acc = pure(Nil)
       var i = 0
       while(i < n) {
-        val j = i // avoid capturing `i` when the applicative is lazy
-        acc = ap(map(acc, (_: Unit) => arr.update(j, _)), f(is(j), x.productElement(j)))
+        acc = ap(map(acc, prepend), f(is(i), x.productElement(i)))
         i = i+1
       }
-      map(acc, (_: Unit) => mirror.fromProduct(new ArrayProduct(arr)))
+      map(acc, fromList)
     }
   }
 

diff --git a/modules/deriving/src/test/scala/shapeless3/deriving/adts.scala b/modules/deriving/src/test/scala/shapeless3/deriving/adts.scala
@@ -38,6 +38,10 @@ object adts {
   sealed trait CList[+A] derives Eq, Show, Read, Functor, EmptyK, Traverse, Foldable
   case class CCons[+A](hd: A, tl: CList[A]) extends CList[A]
   case object CNil extends CList[Nothing]
+  object CList {
+    def apply[A](x: A, xs: A*): CCons[A] =
+      CCons(x, xs.foldRight[CList[A]](CNil)(CCons.apply))
+  }
 
   case class Order[F[_]](
     item: F[String],

diff --git a/modules/deriving/src/test/scala/shapeless3/deriving/deriving.scala b/modules/deriving/src/test/scala/shapeless3/deriving/deriving.scala
@@ -225,20 +225,21 @@ class DerivationTests {
     assert(v3.traverse(Nn)((x: Int) => List(x + 1)) == List(Nn))
 
     val v4 = Traverse[Const[CNil.type]]
-    assert(v4.traverse(CNil)(Option(_)) == Some(CNil))
+    assert(v4.traverse(CNil)(Option.apply) == Some(CNil))
     val v5 = Traverse[CCons]
-    assert(v5.traverse(CCons("foo", CCons("bar", CNil)))(Option(_)) == Some(CCons("foo", CCons("bar", CNil))))
+    assert(v5.traverse(CList("foo", "bar"))(Option.apply) == Some(CList("foo", "bar")))
     val v6 = Traverse[CList]
-    assert(v6.traverse(CCons("foo", CCons("bar", CNil)))(Option(_)) == Some(CCons("foo", CCons("bar", CNil))))
-    assert(v6.traverse(CNil)(Option(_)) == Some(CNil))
-    assert(v6.traverse(CCons("foo", CCons("bar", CNil)))(() => _).apply() == CCons("foo", CCons("bar", CNil)))
+    assert(v6.traverse(CList("foo", "bar"))(Option.apply) == Some(CList("foo", "bar")))
+    assert(v6.traverse(CNil)(Option.apply) == Some(CNil))
+    assert(v6.traverse(CList("foo", "bar"))(() => _).apply() == CList("foo", "bar"))
+    assert(v6.traverse(CList(1, 2))(x => List(x, x + 1)) == List(CList(1, 2), CList(1, 3), CList(2, 2), CList(2, 3)))
 
     val v7 = Traverse[OptE]
     assert(v7.traverse(SmE(1))((x: Int) => List(x + 1)) == List(SmE(2)))
     assert(v7.traverse(NnE)((x: Int) => List(x + 1)) == List(NnE))
 
     val v8 = Traverse[Phantom]
-    assert(v8.traverse(Phantom())(Option(_)) == Some(Phantom()))
+    assert(v8.traverse(Phantom())(Option.apply) == Some(Phantom()))
   }