Port EmptyK to dotty (#448)

* Port EmptyK to dotty

* Fix formatting

* Support EmptyK for sum types

* Refactor EmptyKSuite

Co-authored-by: Georgi Krastev <joro.kr.21@gmail.com>

build.sbt

@@ -44,7 +44,10 @@ lazy val commonSettings = Seq(
   ),
   libraryDependencies ++= (CrossVersion.partialVersion(scalaVersion.value) match {
     case Some((3, _)) =>
-      Seq("org.typelevel" %%% "shapeless3-deriving" % shapeless3Version)
+      Seq(
+        "org.typelevel" %%% "shapeless3-deriving" % shapeless3Version,
+        "org.typelevel" %%% "shapeless3-test" % shapeless3Version % Test
+      )
     case _ =>
       Seq(
         "com.chuusai" %%% "shapeless" % shapeless2Version,

core/src/main/scala-3/cats/derived/DerivedEmptyK.scala

@@ -0,0 +1,44 @@
+package cats.derived
+
+import alleycats.{Empty, EmptyK, Pure}
+import cats.derived.util.Kinds
+import shapeless3.deriving.{Const, K1}
+
+import scala.annotation.implicitNotFound
+import scala.compiletime.summonInline
+import scala.util.NotGiven
+
+@implicitNotFound("""Could not derive an instance of EmptyK[F] where F = ${F}.
+Make sure that F[_] satisfies one of the following conditions:
+  * it is a constant type λ[x => T] where T: Empty
+  * it is a nested type λ[x => G[H[x]]] where G: EmptyK
+  * it is a nested type λ[x => G[H[x]]] where G: Pure and H: EmptyK
+  * it is a generic case class where all fields have an EmptyK instance
+  * it is a generic sealed trait where exactly one subtype has an EmptyK instance
+
+Note: using kind-projector notation - https://github.com/typelevel/kind-projector""")
+type DerivedEmptyK[F[_]] = Derived[EmptyK[F]]
+object DerivedEmptyK:
+  type Or[F[_]] = Derived.Or[EmptyK[F]]
+  inline def apply[F[_]]: EmptyK[F] =
+    import DerivedEmptyK.given
+    summonInline[DerivedEmptyK[F]].instance
+
+  given [T](using T: Empty[T]): DerivedEmptyK[Const[T]] =
+    new EmptyK[Const[T]]:
+      def empty[A] = T.empty
+
+  given [F[_], G[_]](using F: Or[F]): DerivedEmptyK[[x] =>> F[G[x]]] =
+    new EmptyK[[x] =>> F[G[x]]]:
+      def empty[A] = F.unify.empty
+
+  given [F[_], G[_]](using NotGiven[Or[F]])(using F: Pure[F], G: Or[G]): DerivedEmptyK[[x] =>> F[G[x]]] =
+    new EmptyK[[x] =>> F[G[x]]]:
+      def empty[A] = F.pure(G.unify.empty)
+
+  given product[F[_]](using inst: K1.ProductInstances[Or, F]): DerivedEmptyK[F] =
+    new EmptyK[F]:
+      def empty[A]: F[A] = inst.unify.construct([f[_]] => (E: EmptyK[f]) => E.empty[A])
+
+  inline given coproduct[F[_]](using gen: K1.CoproductGeneric[F]): DerivedEmptyK[F] =
+    Kinds.summonOne1[Or, gen.MirroredElemTypes, F].unify

core/src/main/scala-3/cats/derived/package.scala

@@ -17,6 +17,7 @@ extension (x: CommutativeMonoid.type) inline def derived[A]: CommutativeMonoid[A
 extension (x: Show.type) inline def derived[A]: Show[A] = DerivedShow[A]
 extension (x: Applicative.type) inline def derived[F[_]]: Applicative[F] = DerivedApplicative[F]
 extension (x: Apply.type) inline def derived[F[_]]: Apply[F] = DerivedApply[F]
+extension (x: EmptyK.type) inline def derived[F[_]]: EmptyK[F] = DerivedEmptyK[F]
 extension (x: Foldable.type) inline def derived[F[_]]: Foldable[F] = DerivedFoldable[F]
 extension (x: Functor.type) inline def derived[F[_]]: Functor[F] = DerivedFunctor[F]
 extension (x: Reducible.type) inline def derived[F[_]]: Reducible[F] = DerivedReducible[F]
@@ -25,7 +26,6 @@ extension (x: NonEmptyTraverse.type) inline def derived[F[_]]: NonEmptyTraverse[
 
 object semiauto
     extends ContravariantDerivation,
-      EmptyKDerivation,
       InvariantDerivation,
       MonoidKDerivation,
       PartialOrderDerivation,
@@ -42,6 +42,7 @@ object semiauto
   inline def commutativeMonoid[A]: CommutativeMonoid[A] = DerivedCommutativeMonoid[A]
   inline def applicative[F[_]]: Applicative[F] = DerivedApplicative[F]
   inline def apply[F[_]]: Apply[F] = DerivedApply[F]
+  inline def emptyK[F[_]]: EmptyK[F] = DerivedEmptyK[F]
   inline def foldable[F[_]]: Foldable[F] = DerivedFoldable[F]
   inline def functor[F[_]]: Functor[F] = DerivedFunctor[F]
   inline def reducible[F[_]]: Reducible[F] = DerivedReducible[F]
@@ -83,6 +84,9 @@ object auto:
   object apply:
     inline given [F[_]](using NotGiven[Apply[F]]): Apply[F] = DerivedApply[F]
 
+  object emptyK:
+    inline given [F[_]](using NotGiven[EmptyK[F]]): EmptyK[F] = DerivedEmptyK[F]
+
   object functor:
     inline given [F[_]](using NotGiven[Functor[F]]): Functor[F] = DerivedFunctor[F]
 

core/src/main/scala-3/cats/derived/util/Kinds.scala

@@ -1,16 +1,22 @@
 package cats.derived.util
 
-import shapeless3.deriving.K0.LiftP
+import shapeless3.deriving._
 
 import scala.compiletime.*
 import scala.util.NotGiven
 
 object Kinds:
   inline def summonOne0[F[_], T, U]: F[U] =
-    summonOneFrom[LiftP[F, T]].asInstanceOf[F[U]]
+    summonOneFrom[K0.LiftP[F, T]].asInstanceOf[F[U]]
+
+  inline def summonOne1[F[_[_]], T[_], U[_]]: F[U] =
+    summonOneFrom[K1.LiftP[F, T]].asInstanceOf[F[U]]
 
   inline def summonNone0[F[_], T]: Unit =
-    summonNoneFrom[LiftP[F, T]]
+    summonNoneFrom[K0.LiftP[F, T]]
+
+  inline def summonNone1[F[_[_]], T[_]]: Unit =
+    summonNoneFrom[K1.LiftP[F, T]]
 
   transparent inline def summonOneFrom[T <: Tuple]: Any =
     inline erasedValue[T] match

core/src/test/scala-3/cats/derived/EmptyKSuite.scala

@@ -0,0 +1,82 @@
+/*
+ * Copyright (c) 2015 Miles Sabin
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.derived
+
+import alleycats.{EmptyK, Pure}
+import alleycats.std.all.*
+import cats.data.NonEmptyList
+import cats.laws.discipline.SerializableTests
+import cats.syntax.all.*
+import shapeless3.test.illTyped
+
+import scala.compiletime.summonInline
+
+class EmptyKSuite extends KittensSuite:
+  import EmptyKSuite.*
+  import TestDefns.*
+
+  given Pure[Box] with
+    def pure[A](a: A) = Box(a)
+
+  inline def emptyK[F[_]] =
+    summonInline[EmptyK[F]].empty
+
+  inline def testEmptyK(context: String): Unit =
+    test(s"$context.EmptyK[LOption]")(assert(emptyK[LOption] == Nil))
+    test(s"$context.EmptyK[PList]")(assert(emptyK[PList] == (Nil, Nil)))
+    test(s"$context.EmptyK[CaseClassWOption]")(assert(emptyK[CaseClassWOption] == CaseClassWOption(None)))
+    test(s"$context.EmptyK[NelOption]")(assert(emptyK[NelOption] == NonEmptyList.one(None)))
+    test(s"$context.EmptyK[IList]")(assert(emptyK[IList] == INil()))
+    test(s"$context.EmptyK[Snoc]")(assert(emptyK[Snoc] == SNil()))
+    test(s"$context.EmptyK respects existing instances")(assert(emptyK[BoxColor] == Box(Color(255, 255, 255))))
+    checkAll(s"$context.EmptyK is Serializable", SerializableTests.serializable(summonInline[EmptyK[LOption]]))
+
+  locally {
+    import auto.emptyK.given
+    testEmptyK("auto")
+  }
+
+  locally {
+    import semiInstances.given
+    testEmptyK("semiauto")
+  }
+
+end EmptyKSuite
+
+object EmptyKSuite:
+  import TestDefns.*
+
+  type LOption[A] = List[Option[A]]
+  type PList[A] = (List[A], List[A])
+  type NelOption[A] = NonEmptyList[Option[A]]
+  type BoxColor[A] = Box[Color[A]]
+
+  object semiInstances:
+    given EmptyK[LOption] = semiauto.emptyK
+    given EmptyK[PList] = semiauto.emptyK
+    given EmptyK[CaseClassWOption] = semiauto.emptyK
+    given EmptyK[NelOption] = semiauto.emptyK
+    given EmptyK[IList] = semiauto.emptyK
+    given EmptyK[Snoc] = semiauto.emptyK
+    given EmptyK[BoxColor] = semiauto.emptyK
+
+  final case class Color[A](r: Int, g: Int, b: Int)
+  object Color:
+    given EmptyK[Color] with
+      def empty[A] = Color(255, 255, 255)
+
+end EmptyKSuite

core/src/test/scala-3/cats/derived/EmptyKTests.scala

@@ -3,8 +3,7 @@ package cats.derived
 import alleycats.*
 import alleycats.std.all.*
 import cats.*
-import cats.derived.semiauto.*
-import cats.derived.semiauto.given
+import cats.derived.*
 
 class EmptyKTests { //
   case class Foo[A](i: String, l: List[A]) derives EmptyK