Add option to use underscore symbol _ instead of * to define anon…

…ymous type lambdas

The syntax roughly follows the [proposed new syntax for wildcards and placeholders](https://dotty.epfl.ch/docs/reference/changed-features/wildcards.html#migration-strategy) for Scala 3.2+ and is designed to allow cross-compilation of libraries between Scala 2 and Scala 3 while using the new Scala 3 syntax for both versions.

To enable this mode, add `-P:kind-projector:underscore-placeholders` to your scalac command-line. In sbt you may do this as follows:

```scala
ThisBuild / scalacOptions += "-P:kind-projector:underscore-placeholders"
```

This mode is designed to be used with scalac versions `2.12.14`+ and `2.13.6`+, these versions add an the ability to use `?` as the existential type wildcard ([scala/scala#9560](scala/scala#9560)), allowing to repurpose the underscore without losing the ability to write existential types. It is not advised that you use this mode with older versions of scalac or without `-Xsource:3` flag, since you will lose the underscore syntax entirely.

Here are a few examples:

```scala
Tuple2[_, Double]        // equivalent to: type R[A] = Tuple2[A, Double]
Either[Int, +_]          // equivalent to: type R[+A] = Either[Int, A]
Function2[-_, Long, +_]  // equivalent to: type R[-A, +B] = Function2[A, Long, B]
EitherT[_[_], Int, _]    // equivalent to: type R[F[_], B] = EitherT[F, Int, B]
```

Examples with `-Xsource:3`'s `?`-wildcard:

```scala
Tuple2[_, ?]        // equivalent to: type R[A] = Tuple2[A, x] forSome { type x }
Either[?, +_]          // equivalent to: type R[+A] = Either[x, A] forSome { type x }
Function2[-_, ?, +_]  // equivalent to: type R[-A, +B] = Function2[A, x, B] forSome { type x }
EitherT[_[_], ?, _]    // equivalent to: type R[F[_], B] = EitherT[F, x, B] forSome { type x }
```

README.md

@@ -131,6 +131,37 @@ lambda is only used in the body once, and in the same order. For more
 complex type lambda expressions, you will need to use the function
 syntax.
 
+#### Inline Underscore Syntax
+
+Since version `0.13.0` kind-projector adds an option to use underscore symbol `_` instead of `*` to define anonymous type lambdas.
+The syntax roughly follows the [proposed new syntax for wildcards and placeholders](https://dotty.epfl.ch/docs/reference/changed-features/wildcards.html#migration-strategy) for Scala 3.2+ and is designed to allow cross-compilation of libraries between Scala 2 and Scala 3 while using the new Scala 3 syntax for both versions.
+
+To enable this mode, add `-P:kind-projector:underscore-placeholders` to your scalac command-line. In sbt you may do this as follows:
+
+```scala
+ThisBuild / scalacOptions += "-P:kind-projector:underscore-placeholders"
+```
+
+This mode is designed to be used with scalac versions `2.12.14`+ and `2.13.6`+, these versions add an the ability to use `?` as the existential type wildcard ([scala/scala#9560](https://github.com/scala/scala/pull/9560)), allowing to repurpose the underscore without losing the ability to write existential types. It is not advised that you use this mode with older versions of scalac or without `-Xsource:3` flag, since you will lose the underscore syntax entirely.
+
+Here are a few examples:
+
+```scala
+Tuple2[_, Double]        // equivalent to: type R[A] = Tuple2[A, Double]
+Either[Int, +_]          // equivalent to: type R[+A] = Either[Int, A]
+Function2[-_, Long, +_]  // equivalent to: type R[-A, +B] = Function2[A, Long, B]
+EitherT[_[_], Int, _]    // equivalent to: type R[F[_], B] = EitherT[F, Int, B]
+```
+
+Examples with `-Xsource:3`'s `?`-wildcard:
+
+```scala
+Tuple2[_, ?]        // equivalent to: type R[A] = Tuple2[A, x] forSome { type x }
+Either[?, +_]          // equivalent to: type R[+A] = Either[x, A] forSome { type x }
+Function2[-_, ?, +_]  // equivalent to: type R[-A, +B] = Function2[A, x, B] forSome { type x }
+EitherT[_[_], ?, _]    // equivalent to: type R[F[_], B] = EitherT[F, x, B] forSome { type x }
+```
+
 ### Function Syntax
 
 The more powerful syntax to use is the function syntax. This syntax

build.sbt

@@ -18,7 +18,7 @@ inThisBuild {
       "2.13.2",
       "2.13.3",
       "2.13.4",
-      "2.13.5"
+      "2.13.5",
     ),
     organization := "org.typelevel",
     licenses += ("MIT", url("http://opensource.org/licenses/MIT")),
@@ -63,7 +63,7 @@ inThisBuild {
 
 val HasScalaVersion = {
   object Matcher {
-    def unapply(versionString: String) = 
+    def unapply(versionString: String) =
       versionString.takeWhile(ch => ch != '-').split('.').toList.map(str => scala.util.Try(str.toInt).toOption) match {
         case List(Some(epoch), Some(major), Some(minor)) => Some((epoch, major, minor))
         case _ => None
@@ -84,6 +84,11 @@ def hasNewParser(versionString: String) = versionString match {
   case _ => false
 }
 
+def hasNewPlugin(versionString: String) = versionString match {
+  case HasScalaVersion(2, 10, _) => false
+  case _ => true
+}
+
 lazy val `kind-projector` = project
   .in(file("."))
   .settings(
@@ -100,6 +105,7 @@ lazy val `kind-projector` = project
         val suffices =
           (if (hasNewParser(sv)) "-newParser" else "-oldParser") ::
           (if (hasNewReporting(sv)) "-newReporting" else "-oldReporting") ::
+          (if (hasNewPlugin(sv)) "-newPlugin" else "-oldPlugin") ::
           Nil
         suffices.map(suffix => file(dir.getPath + suffix))
       }
@@ -127,7 +133,7 @@ lazy val `kind-projector` = project
     Test / scalacOptions ++= (scalaVersion.value match {
       case HasScalaVersion(2, 13, n) if n >= 2 => List("-Wconf:src=WarningSuppression.scala:error")
       case _                                   => Nil
-    }),
+    }) ++ List("-P:kind-projector:underscore-placeholders"),
     console / initialCommands := "import d_m._",
     Compile / console / scalacOptions := Seq("-language:_", "-Xplugin:" + (Compile / packageBin).value),
     Test / console / scalacOptions := (Compile / console / scalacOptions).value,

src/main/scala-newPlugin/PluginOptionsCompat.scala

@@ -0,0 +1,10 @@
+package d_m
+
+import scala.tools.nsc.plugins.Plugin
+
+trait PluginOptionsCompat {
+  def pluginOptions(plugin: Plugin) = plugin.options
+}
+
+//compatibility stub
+trait PluginCompat

src/main/scala-oldPlugin/PluginOptionsCompat.scala

@@ -0,0 +1,16 @@
+package d_m
+
+import scala.tools.nsc.plugins.Plugin
+
+trait PluginOptionsCompat {
+  def pluginOptions(plugin: Plugin) = plugin.asInstanceOf[PluginCompat].options
+}
+
+trait PluginCompat extends Plugin {
+  var options: List[String] = _
+  override def processOptions(options: List[String], error: String => Unit): Unit = {
+    this.options = options
+    init(options, error)
+  }
+  def init(options: List[String], error: String => Unit): Boolean
+}

src/main/scala/KindProjector.scala

@@ -12,14 +12,28 @@ import nsc.ast.TreeDSL
 import scala.reflect.NameTransformer
 import scala.collection.mutable
 
-class KindProjector(val global: Global) extends Plugin {
+class KindProjector(val global: Global) extends Plugin with PluginCompat {
   val name = "kind-projector"
   val description = "Expand type lambda syntax"
+
+  override val optionsHelp = Some(Seq(
+    "-P:kind-projector:underscore-placeholders - treat underscore as a type lambda placeholder,",
+    "disables Scala 2 wildcards, you must separately enable `-Xsource:3` option to be able to",
+    "write wildcards using `?` symbol").mkString(" "))
+
+
+  override def init(options: List[String], error: String => Unit): Boolean = {
+    if (options.exists(_ != "underscore-placeholders")) {
+      error(s"Error: $name takes no options except `-P:kind-projector:underscore-placeholders`, but got ${options.mkString(",")}")
+    }
+    true
+  }
+
   val components = new KindRewriter(this, global) :: Nil
 }
 
 class KindRewriter(plugin: Plugin, val global: Global)
-    extends PluginComponent with Transform with TypingTransformers with TreeDSL with ReportingCompat {
+    extends PluginComponent with Transform with TypingTransformers with TreeDSL with PluginOptionsCompat with ReportingCompat {
 
   import global._
 
@@ -32,6 +46,10 @@ class KindRewriter(plugin: Plugin, val global: Global)
   lazy val useAsciiNames: Boolean =
     System.getProperty("kp:genAsciiNames") == "true"
 
+  lazy val useUnderscoresForTypeLambda: Boolean = {
+    pluginOptions(plugin).contains("underscore-placeholders")
+  }
+
   def newTransformer(unit: CompilationUnit): MyTransformer =
     new MyTransformer(unit)
 
@@ -54,15 +72,28 @@ class KindRewriter(plugin: Plugin, val global: Global)
     val InvPlaceholder = newTypeName("$times")
     val CoPlaceholder = newTypeName("$plus$times")
     val ContraPlaceholder = newTypeName("$minus$times")
- 
+
     val TermLambda1 = TypeLambda1.toTermName
     val TermLambda2 = TypeLambda2.toTermName
 
+    object InvPlaceholderScala3 {
+      def apply(n: Name): Boolean = n match { case InvPlaceholderScala3() => true; case _ => false }
+      def unapply(t: TypeName): Boolean = t.startsWith("_$") && t.drop(2).decoded.forall(_.isDigit)
+    }
+    val CoPlaceholderScala3 = newTypeName("$plus_")
+    val ContraPlaceholderScala3 = newTypeName("$minus_")
+
     object Placeholder {
       def unapply(name: TypeName): Option[Variance] = name match {
         case InvPlaceholder => Some(Invariant)
         case CoPlaceholder => Some(Covariant)
         case ContraPlaceholder => Some(Contravariant)
+        case _ if useUnderscoresForTypeLambda => name match {
+          case InvPlaceholderScala3() => Some(Invariant)
+          case CoPlaceholderScala3 => Some(Covariant)
+          case ContraPlaceholderScala3 => Some(Contravariant)
+          case _ => None
+        }
         case _ => None
       }
     }
@@ -248,9 +279,20 @@ class KindRewriter(plugin: Plugin, val global: Global)
           case (ExistentialTypeTree(AppliedTypeTree(Ident(Placeholder(variance)), ps), _), i) =>
             (Ident(newParamName(i)), Some(Left((variance, ps.map(makeComplexTypeParam)))))
           case (a, i) =>
-            (super.transform(a), None)
+            // Using super.transform in existential type case in underscore mode
+            // skips the outer `ExistentialTypeTree` (reproduces in nested.scala test)
+            // and produces invalid trees where the unused underscore variables are not cleaned up
+            // by the current transformer
+            // I do not know why! Using `this.transform` instead works around the issue,
+            // however it seems to have worked correctly all this time non-underscore mode, so
+            // we keep calling super.transform to not change anything for existing code in classic mode.
+            val transformedA =
+              if (useUnderscoresForTypeLambda) this.transform(a)
+              else super.transform(a)
+            (transformedA, None)
         }
 
+
         // for each placeholder, create a type parameter
         val innerTypes = xyz.collect {
           case (Ident(name), Some(Right(variance))) =>
@@ -331,6 +373,15 @@ class KindRewriter(plugin: Plugin, val global: Global)
         case AppliedTypeTree(Ident(TypeLambda2), AppliedTypeTree(target, a :: as) :: Nil) =>
           validateLambda(tree.pos, target, a, as)
 
+        // Either[_, Int] case (if `underscore-placeholders` is enabled)
+        case ExistentialTypeTree(AppliedTypeTree(t, as), params) if useUnderscoresForTypeLambda =>
+          val nonUnderscoreExistentials = params.filterNot {
+            case p: MemberDef => InvPlaceholderScala3(p.name)
+            case _ => true
+          }
+          val nt = atPos(tree.pos.makeTransparent)(handlePlaceholders(t, as))
+          if (nonUnderscoreExistentials.isEmpty) nt else ExistentialTypeTree(nt, nonUnderscoreExistentials)
+
         // Either[?, Int] case (if no ? present this is a noop)
         case AppliedTypeTree(t, as) =>
           atPos(tree.pos.makeTransparent)(handlePlaceholders(t, as))

src/test/scala/issue80.scala

@@ -17,4 +17,5 @@ object Coproduct {
           case Right(bx) => Coproduct(Right(g(bx)))
         }
     }
+  def test[X[_]]: Bifunctor[({ type L[F[_[_]], G[_[_]]] = Coproduct[F, G, X] })#L] = coproductBifunctor[X]
 }

src/test/scala/underscores/functor.scala

@@ -0,0 +1,13 @@
+package underscores
+
+trait Functor[M[_]] {
+  def fmap[A, B](fa: M[A])(f: A => B): M[B]
+}
+
+class EitherRightFunctor[L] extends Functor[Either[L, _]] {
+  def fmap[A, B](fa: Either[L, A])(f: A => B): Either[L, B] =
+    fa match {
+      case Right(a) => Right(f(a))
+      case Left(l) => Left(l)
+    }
+}

src/test/scala/underscores/issue80.scala

@@ -0,0 +1,23 @@
+package underscores
+
+trait ~~>[A[_[_]], B[_[_]]] {
+  def apply[X[_]](a: A[X]): B[X]
+}
+
+trait Bifunctor[F[_[_[_]], _[_[_]]]] {
+  def bimap[A[_[_]], B[_[_]], C[_[_]], D[_[_]]](fab: F[A, B])(f: A ~~> C, g: B ~~> D): F[C, D]
+}
+
+final case class Coproduct[A[_[_]], B[_[_]], X[_]](run: Either[A[X], B[X]])
+
+object Coproduct {
+  def coproductBifunctor[X[_]]: Bifunctor[Coproduct[_[_[_]], _[_[_]], X]] =
+    new Bifunctor[Coproduct[_[_[_]], _[_[_]], X]] {
+      def bimap[A[_[_]], B[_[_]], C[_[_]], D[_[_]]](abx: Coproduct[A, B, X])(f: A ~~> C, g: B ~~> D): Coproduct[C, D, X] =
+        abx.run match {
+          case Left(ax)  => Coproduct(Left(f(ax)))
+          case Right(bx) => Coproduct(Right(g(bx)))
+        }
+    }
+  def test[X[_]]: Bifunctor[({ type L[F[_[_]], G[_[_]]] = Coproduct[F, G, X] })#L] = coproductBifunctor[X]
+}

src/test/scala/underscores/nested.scala

@@ -0,0 +1,13 @@
+package underscores
+
+// // From https://github.com/non/kind-projector/issues/20
+// import scala.language.higherKinds
+
+object KindProjectorWarnings {
+  trait Foo[F[_], A]
+  trait Bar[A, B]
+
+  def f[G[_]]: Unit = ()
+
+  f[Foo[Bar[Int, _], _]] // shadowing warning
+}

src/test/scala/underscores/polylambda.scala

@@ -0,0 +1,67 @@
+package underscores
+
+trait ~>[-F[_], +G[_]] {
+  def apply[A](x: F[A]): G[A]
+}
+trait ~>>[-F[_], +G[_]] {
+  def dingo[B](x: F[B]): G[B]
+}
+final case class Const[A, B](getConst: A)
+
+class PolyLambdas {
+  type ToSelf[F[_]] = F ~> F
+
+  val kf1 = Lambda[Option ~> Vector](_.iterator.toVector)
+
+  val kf2 = λ[Vector ~> Option] {
+    case Vector(x) => Some(x)
+    case _         => None
+  }
+
+  val kf3 = λ[ToSelf[Vector]](_.reverse)
+
+  val kf4 = λ[Option ~>> Option].dingo(_ flatMap (_ => None))
+
+  val kf5 = λ[Map[_, Int] ~> Map[_, Long]](_.map { case (k, v) => (k, v.toLong) }.toMap)
+
+  val kf6 = λ[ToSelf[Map[_, Int]]](_.map { case (k, v) => (k, v * 2) }.toMap)
+
+  implicit class FGOps[F[_], A](x: F[A]) {
+    def ntMap[G[_]](kf: F ~> G): G[A] = kf(x)
+  }
+
+  // Scala won't infer the unary type constructor alias from a
+  // tuple. I'm not sure how it even could, so we'll let it slide.
+  type PairWithInt[A] = (A, Int)
+  def mkPair[A](x: A, y: Int): PairWithInt[A] = x -> y
+  val pairMap = λ[ToSelf[PairWithInt]] { case (k, v) => (k, v * 2) }
+  val tupleTakeFirst = λ[λ[A => (A, Int)] ~> List](x => List(x._1))
+
+  // All these formulations should be equivalent.
+  def const1[A]                              = λ[ToSelf[Const[A, _]]](x => x)
+  def const2[A] : ToSelf[Const[A, _]]        = λ[Const[A, _] ~> Const[A, _]](x => x)
+  def const3[A] : Const[A, _] ~> Const[A, _] = λ[ToSelf[Const[A, _]]](x => x)
+  def const4[A]                              = λ[Const[A, _] ~> Const[A, _]](x => x)
+  def const5[A] : ToSelf[Const[A, _]]        = λ[ToSelf[λ[B => Const[A, B]]]](x => x)
+  def const6[A] : Const[A, _] ~> Const[A, _] = λ[ToSelf[λ[B => Const[A, B]]]](x => x)
+
+  @org.junit.Test
+  def polylambda(): Unit = {
+    assert(kf1(None) == Vector())
+    assert(kf1(Some("a")) == Vector("a"))
+    assert(kf1(Some(5d)) == Vector(5d))
+    assert(kf2(Vector(5)) == Some(5))
+    assert(kf3(Vector(1, 2)) == Vector(2, 1))
+    assert(kf4.dingo(Some(5)) == None)
+    assert(kf5(Map("a" -> 5)) == Map("a" -> 5))
+    assert(kf6(Map("a" -> 5)) == Map("a" -> 10))
+
+    assert((mkPair("a", 1) ntMap pairMap) == ("a" -> 2))
+    assert((mkPair(Some(true), 1) ntMap pairMap) == (Some(true) -> 2))
+
+    assert(mkPair('a', 1).ntMap(tupleTakeFirst) == List('a'))
+    // flatten works, whereas it would be a static error in the
+    // line above. That's pretty poly!
+    assert(mkPair(Some(true), 1).ntMap(tupleTakeFirst).flatten == List(true))
+  }
+}