Scala 3 support

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        scala: [2.12.16, 2.13.8]
+        scala: [2.12.16, 2.13.8, 3.2.0]
         java: [temurin@8]
         project: [rootJS, rootJVM, rootNative]
     runs-on: ${{ matrix.os }}
@@ -114,7 +114,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        scala: [2.13.8]
+        scala: [3.2.0]
         java: [temurin@8]
     runs-on: ${{ matrix.os }}
     steps:
@@ -211,6 +211,36 @@ jobs:
           tar xf targets.tar
           rm targets.tar
 
+      - name: Download target directories (3.2.0, rootJS)
+        uses: actions/download-artifact@v2
+        with:
+          name: target-${{ matrix.os }}-${{ matrix.java }}-3.2.0-rootJS
+
+      - name: Inflate target directories (3.2.0, rootJS)
+        run: |
+          tar xf targets.tar
+          rm targets.tar
+
+      - name: Download target directories (3.2.0, rootJVM)
+        uses: actions/download-artifact@v2
+        with:
+          name: target-${{ matrix.os }}-${{ matrix.java }}-3.2.0-rootJVM
+
+      - name: Inflate target directories (3.2.0, rootJVM)
+        run: |
+          tar xf targets.tar
+          rm targets.tar
+
+      - name: Download target directories (3.2.0, rootNative)
+        uses: actions/download-artifact@v2
+        with:
+          name: target-${{ matrix.os }}-${{ matrix.java }}-3.2.0-rootNative
+
+      - name: Inflate target directories (3.2.0, rootNative)
+        run: |
+          tar xf targets.tar
+          rm targets.tar
+
       - name: Import signing key
         if: env.PGP_SECRET != '' && env.PGP_PASSPHRASE == ''
         run: echo $PGP_SECRET | base64 -di | gpg --import

.gitignore

@@ -32,3 +32,10 @@ metals.sbt
 .*.swp
 .swo
 .*.swo
+
+# VS Code
+.bloop
+.bsp
+.metals
+.vscode
+**/metals.sbt

.mergify.yml

@@ -18,6 +18,9 @@ pull_request_rules:
   - status-success=Build and Test (ubuntu-latest, 2.13.8, temurin@8, rootJS)
   - status-success=Build and Test (ubuntu-latest, 2.13.8, temurin@8, rootJVM)
   - status-success=Build and Test (ubuntu-latest, 2.13.8, temurin@8, rootNative)
+  - status-success=Build and Test (ubuntu-latest, 3.2.0, temurin@8, rootJS)
+  - status-success=Build and Test (ubuntu-latest, 3.2.0, temurin@8, rootJVM)
+  - status-success=Build and Test (ubuntu-latest, 3.2.0, temurin@8, rootNative)
   actions:
     merge: {}
 - name: Label core PRs

.scalafmt.conf

@@ -10,4 +10,7 @@ rewrite.redundantBraces.stringInterpolation = true
 rewrite.redundantBraces.methodBodies = true
 rewrite.redundantBraces.generalExpressions = true
 rewriteTokens = { "⇒": "=>", "→": "->", "←": "<-" }
-fileOverride { "glob:**/*.sbt" { runner.dialect = scala212 } }
+fileOverride {
+  "glob:**/*.sbt" { runner.dialect = scala212 },
+  "glob:**/scala-3/**" { runner.dialect = scala3 }
+}

README.md

@@ -24,6 +24,8 @@ libraryDependencies += "org.typelevel" %% "kittens" % "latestVersion" // indicat
 [![Scala.js](http://scala-js.org/assets/badges/scalajs-1.5.0.svg)](http://scala-js.org)
 [![Latest version](https://img.shields.io/maven-central/v/org.typelevel/kittens_2.12.svg)](https://maven-badges.herokuapp.com/maven-central/org.typelevel/kittens_2.12)
 
+## Scala 2
+
 Instance derivations are available for the following type classes:
 
 * `Eq`, `PartialOrder`, `Order`, `Hash`
@@ -210,7 +212,109 @@ implicit val showFoo: Show[Foo] =  semiauto.show
 
 This way the native instance for `Show[List]` would be used.
 
-### Type class support matrix
+## Scala 3
+
+We also offer 3 methods of derivation for Scala 3. All of them have the same behaviour wrt to recursively defining instances: 
+1. Instances will always be recursively instantiated if necessary
+2. Subject to the same type constructor field limitation as the Scala 2 auto and manual semi derivations
+
+### `derives` syntax (recommended)
+
+Kittens for scala 3 supports Scala 3's [derivation syntax](https://docs.scala-lang.org/scala3/reference/contextual/derivation.html). 
+
+``` scala
+import cats.derived.*
+
+// No instances declared for Name
+case class Name(value: String)
+case class Person(name: Name, age: Int) derives Eq, Show
+
+enum CList[+A] derives Functor:
+  case CNil
+  case CCons(head: A, tail: CList[A])
+```
+
+### semiauto derivation
+
+This looks similar to `semiauto` for Scala 2.
+
+``` scala
+import cats.derived.semiauto
+
+// No instances declared for Name
+case class Name(value: String)
+case class Person(name: Name, age: Int)
+
+object Person:
+  given Eq[Person] = semiauto.eq
+  given Show[Person] = semiauto.show
+
+enum CList[+A]:
+  case CNil
+  case CCons(head: A, tail: CList[A])
+
+object CList:
+  given Functor[CList] = semiauto.functor
+```
+
+As with Scala 2, you can combine `auto` and `semiauto` to avoid the type constructor field limitation:
+
+``` scala
+import cats.derived.*
+
+case class Name(value: String)
+case class Person(name: Name, age: Int)
+
+case class People(people: List[Person])
+object People:
+  given Show[People] =
+    import auto.show.given
+    // Uses the correct List instance despite deriving an instance for Person automatically
+    semiauto.show
+```
+
+`
+
+### auto derivation
+
+This looks similar to `auto` for Scala 2.
+
+``` scala
+import cats.derived.auto.eq.given
+import cats.derived.auto.show.given
+import cats.derived.auto.functor.given
+
+case class Name(value: String)
+case class Person(name: Name, age: Int)
+
+enum CList[+A]:
+  case CNil
+  case CCons(head: A, tail: CList[A])
+```
+
+### Caveats
+
+#### Nested type constructors
+
+We are [currently](https://github.com/typelevel/kittens/issues/473) unable to
+derive instances for nested type constructors, such as `Functor[[x] =>>
+List[Set[x]]]`.
+
+#### Stack safety
+
+Our derived instances are not stack-safe. This is a departure from the behaviour for Scala 2 because we didn't want to incur the performance penalty of trampolining all instances in `cats.Eval`. If your data-type is recursive or _extremely_ large then you may want to write instances by hand instead.
+
+#### Missing features
+
+Kittens for Scala 3 is built on top of [Shapeless
+3](https://github.com/typelevel/shapeless-3) which has a completely different
+API than [Shapeless 2](https://github.com/milessabin/shapeless) so we don't
+support features like `Sequence` and `Lift`.
+
+`ConsK` derivation is also not supported although we expect this to be
+[added](https://github.com/typelevel/kittens/issues/489) in a future release.
+
+## Type class support matrix
 
 Legend:
 - `∀` - all must satisfy a constraint
@@ -277,4 +381,5 @@ Kittens is built with SBT 1.x, and its master branch is built with Scala 2.13 by
 + Miles Sabin <miles@milessabin.com> [@milessabin](https://twitter.com/milessabin)
 + Qi Wang [Qi77Qi](http://github.com/Qi77Qi)
 + Kailuo Wang <kailuo.wang@gmail.com> [@kailuowang](https://twitter.com/kailuowang)
++ Tim Spence <timothywspence@gmail.com> [timwspence](https://twitter.com/timwspence)
 + Your name here :-)

build.sbt

@@ -1,42 +1,54 @@
 import sbt._
 
-ThisBuild / crossScalaVersions := Seq("2.12.16", "2.13.8")
-ThisBuild / scalaVersion := "2.13.8"
-ThisBuild / tlBaseVersion := "2.3"
+val scala212 = "2.12.16"
+val scala213 = "2.13.8"
+val scala3 = "3.2.0"
+
+ThisBuild / crossScalaVersions := Seq(scala212, scala213, scala3)
+ThisBuild / scalaVersion := scala3
+ThisBuild / tlBaseVersion := "3.0"
 ThisBuild / organization := "org.typelevel"
 
 val catsVersion = "2.8.0"
-val disciplineMunitVersion = "1.0.9"
+val munitVersion = "1.0.0-M6"
+val disciplineMunitVersion = "2.0.0-M3"
 val kindProjectorVersion = "0.13.2"
-val shapelessVersion = "2.3.9"
+val shapeless2Version = "2.3.8"
+val shapeless3Version = "3.1.0"
 
 lazy val commonSettings = Seq(
-  scalacOptions := Seq(
+  scalacOptions ++= Seq(
     "-feature",
     "-language:higherKinds",
     "-language:implicitConversions",
     "-unchecked",
     "-deprecation",
     "-Xfatal-warnings"
   ),
-  scalacOptions ++= (
-    CrossVersion.partialVersion(scalaVersion.value) match {
-      case Some((2, v)) if v <= 12 => Seq("-Ypartial-unification")
-      case _ => Seq.empty
-    }
-  ),
+  scalacOptions ++= CrossVersion.partialVersion(scalaVersion.value).toList.flatMap {
+    case (3, _) => List("-Xmax-inlines", "64")
+    case (2, 12) => List("-Ypartial-unification")
+    case _ => Nil
+  },
   resolvers ++= Resolver.sonatypeOssRepos("releases"),
   resolvers ++= Resolver.sonatypeOssRepos("snapshots"),
   libraryDependencies ++= Seq(
     "org.typelevel" %%% "cats-core" % catsVersion,
     "org.typelevel" %%% "alleycats-core" % catsVersion,
-    "com.chuusai" %%% "shapeless" % shapelessVersion,
     "org.typelevel" %%% "cats-testkit" % catsVersion % Test,
-    "org.scalameta" %%% "munit" % "0.7.29" % Test,
     "org.typelevel" %%% "discipline-munit" % disciplineMunitVersion % Test,
-    "org.scala-lang" % "scala-reflect" % scalaVersion.value % Test,
-    compilerPlugin(("org.typelevel" %% "kind-projector" % kindProjectorVersion).cross(CrossVersion.full))
+    "org.scalameta" %%% "munit" % munitVersion % Test
   ),
+  libraryDependencies ++= (CrossVersion.partialVersion(scalaVersion.value) match {
+    case Some((3, _)) =>
+      Seq("org.typelevel" %%% "shapeless3-deriving" % shapeless3Version)
+    case _ =>
+      Seq(
+        "com.chuusai" %%% "shapeless" % shapeless2Version,
+        "org.scala-lang" % "scala-reflect" % scalaVersion.value % Test,
+        compilerPlugin(("org.typelevel" %% "kind-projector" % kindProjectorVersion).cross(CrossVersion.full))
+      )
+  }),
   Test / parallelExecution := false
 )
 
@@ -74,14 +86,15 @@ ThisBuild / licenses := Seq(License.Apache2)
 ThisBuild / developers := List(
   Developer("milessabin", "Miles Sabin", "", url("http://milessabin.com/blog")),
   Developer("kailuowang", "Kai(luo) Wang", "kailuo.wang@gmail.com", url("http://kailuowang.com/")),
-  Developer("joroKr21", "Georgi Krastev", "joro.kr.21@gmail.com", url("https://twitter.com/Joro_Kr"))
+  Developer("joroKr21", "Georgi Krastev", "joro.kr.21@gmail.com", url("https://twitter.com/Joro_Kr")),
+  Developer("TimWSpence", "Tim Spence", "timothywspence@gmail.com", url("https://twitter.com/timwspence"))
 )
 
+ThisBuild / tlCiScalafmtCheck := true
 ThisBuild / tlCiReleaseBranches := Seq("master")
 ThisBuild / mergifyStewardConfig := Some(
   MergifyStewardConfig(
     author = "typelevel-steward[bot]",
     mergeMinors = true
   )
 )
-ThisBuild / tlCiScalafmtCheck := true

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

@@ -0,0 +1,36 @@
+package cats.derived
+
+import shapeless3.deriving.*
+import scala.annotation.*
+import scala.compiletime.*
+
+@implicitNotFound("Could not derive an instance of ${A}")
+opaque type Derived[A] = A
+object Derived:
+  def apply[A](instance: A): Derived[A] = instance
+  extension [A](derived: Derived[A]) def instance: A = derived
+  given [A]: Conversion[A, Derived[A]] = apply
+
+  type Or0[F[_]] = [x] =>> Or[F[x]]
+  type Or1[F[_[_]]] = [x[_]] =>> Or[F[x]]
+  type Or11[F[_[_[_]]]] = [x[_[_]]] =>> Or[F[x]]
+  type Or2[F[_[_, _]]] = [x[_, _]] =>> Or[F[x]]
+
+  opaque type Or[A] = A
+  object Or extends OrInstances:
+    def apply[A](instance: A): Or[A] = instance
+    extension [A](or: Or[A]) def unify: A = or
+    extension [I[f[_], t] <: K0.Instances[f, t], F[_], T](inst: I[Or0[F], T])
+      @targetName("unifyK0") def unify: I[F, T] = inst
+    extension [I[f[_[_]], t[_]] <: K1.Instances[f, t], F[_[_]], T[_]](inst: I[Or1[F], T])
+      @targetName("unifyK1") def unify: I[F, T] = inst
+    extension [I[f[_[_[_]]], t[_[_]]] <: K11.Instances[f, t], F[_[_[_]]], T[_[_]]](inst: I[Or11[F], T])
+      @targetName("unifyK11") def unify: I[F, T] = inst
+    extension [I[f[_[_, _]], t[_, _]] <: K2.Instances[f, t], F[_[_, _]], T[_, _]](inst: I[Or2[F], T])
+      @targetName("unifyK2") def unify: I[F, T] = inst
+
+sealed abstract class OrInstances:
+  inline given [A]: Derived.Or[A] = summonFrom {
+    case instance: A => Derived.Or(instance)
+    case derived: Derived[A] => Derived.Or(derived.instance)
+  }

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

@@ -0,0 +1,39 @@
+package cats.derived
+
+import shapeless3.deriving.{Const, K1}
+import cats.{Applicative, Monoid}
+
+import scala.compiletime.*
+import shapeless3.deriving.{Continue, K0, Labelling}
+
+import scala.annotation.implicitNotFound
+import scala.deriving.Mirror
+
+@implicitNotFound("""Could not derive an instance of Applicative[F] where F = ${F}.
+Make sure that F[_] satisfies one of the following conditions:
+  * it is a constant type [x] =>> T where T: Monoid
+  * it is a nested type [x] =>> G[H[x]] where G: Applicative and H: Applicative
+  * it is a generic case class where all fields have an Applicative instance""")
+type DerivedApplicative[F[_]] = Derived[Applicative[F]]
+object DerivedApplicative:
+  type Or[F[_]] = Derived.Or[Applicative[F]]
+
+  inline def apply[F[_]]: Applicative[F] =
+    import DerivedApplicative.given
+    summonInline[DerivedApplicative[F]].instance
+
+  given [T](using T: Monoid[T]): DerivedApplicative[Const[T]] = new Applicative[Const[T]]:
+    def pure[A](x: A): Const[T][A] = T.empty
+    def ap[A, B](ff: T)(fa: T): Const[T][B] = T.combine(ff, fa)
+
+  given [F[_], G[_]](using F: Or[F], G: Or[G]): DerivedApplicative[[x] =>> F[G[x]]] =
+    F.unify.compose(G.unify)
+
+  given [F[_]](using inst: => K1.ProductInstances[Or, F]): DerivedApplicative[F] =
+    given K1.ProductInstances[Applicative, F] = inst.unify
+    new Product[Applicative, F] with DerivedApply.Product[Applicative, F] {}
+
+  trait Product[T[x[_]] <: Applicative[x], F[_]](using inst: K1.ProductInstances[T, F])
+      extends Applicative[F],
+        DerivedApply.Product[T, F]:
+    override def pure[A](x: A): F[A] = inst.construct([t[_]] => (apl: T[t]) => apl.pure[A](x))