Add kind-projector migration page

Author: adpi2

_overviews/scala3-migration/options-new.md

@@ -4,7 +4,7 @@ type: section
 description: This chapter lists all the new compiler options in Scala 3
 num: 23
 previous-page: options-lookup
-next-page:
+next-page: plugin-intro
 ---
 
 The current page only contains the options that were added in Scala 3.0.x.

_overviews/scala3-migration/plugin-intro.md

@@ -0,0 +1,11 @@
+---
+title: Compiler Plugins
+type: chapter
+description: This section shows how to migrate from using Scala 2 compiler plugins
+num: 24
+previous-page: options-new
+next-page: plugin-kind-projector
+---
+
+Scala 3 includes some features that were previously provided by a compiler plugin.
+This chapter gives more detail on how to migrate from using a specific compiler plugin to Scala 3.

_overviews/scala3-migration/plugin-kind-projector.md

@@ -0,0 +1,131 @@
+---
+title: Kind Projector Migration
+type: section
+description: This section shows how to migrate from the kind-projector plugin to Scala 3 kind-projector syntax
+num: 25
+previous-page: plugin-intro
+next-page:
+---
+
+In the future, Scala 3 will use the `_` underscore symbol for placeholders in type lambdas---just as the underscore is currently used for placeholders in (ordinary) term-level lambdas.
+
+```scala
+
+```
+
+The new type lambda syntax is not enabled by default, to enable it, use a compiler flag `-Ykind-projector:underscores`. Note that enabling underscore type lambdas will disable usage of `_` as a wildcard, you will only be able to write wildcards using the `?` symbol. 
+
+If you wish to cross-compile a project for Scala 2 & Scala 3 while using underscore type lambdas for both, you may do so starting with [kind-projector](https://github.com/typelevel/kind-projector) version `0.13.0` and up and Scala 2 versions `2.13.6` and `2.12.14`.
+To enable it, add the compiler flags `-Xsource:3 -P:kind-projector:underscore-placeholders` to your build.
+As in Scala 3, this will disable usage of `_` as a wildcard, however, the flag `-Xsource:3` will allow you to replace it with the `?` symbol. 
+
+The following `sbt` configuration will set up the correct flags to cross-compile with new syntax:
+
+```scala
+ThisBuild / scalacOptions ++= {
+  CrossVersion.partialVersion(scalaVersion.value) match {
+    case Some((3, _)) => Seq("-Ykind-projector:underscores")
+    case Some((2, 13)) => Seq("-Xsource:3", "-P:kind-projector:underscore-placeholders"))
+  }
+}
+```
+
+## Migrating to New Syntax
+
+To use underscores for type-lambdas in existing kind-projector enabled code, replace `*` or `?` type lambda placeholders with `_`.
+
+In turn, you will also have to rewrite all usages of `_` as the wildcard to use `?` symbol.
+
+For example the following usage of the wildcard:
+
+```scala
+def getWidget(widgets: Set[_ <: Widget], name: String): Option[Widget] = widgets.find(_.name == name) 
+```
+
+Must be rewritten to:
+
+```scala
+def getWidget(widgets: Set[? <: Widget], name: String): Option[Widget] = widgets.find(_.name == name) 
+```
+
+And the following usages of kind-projector's `*` placeholder:
+
+```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]
+```
+
+Must be rewritten to:
+
+```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]
+```
+
+## Compiling Existing Code
+
+Even without migrating to underscore type lambdas, you will likely be able to compile most of it with Scala 3 without changes.
+
+Use the flag `-Ykind-projector` to enable support for `*`-based type lambdas (without enabling underscore type lambdas), the following forms will now compile:
+
+```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]
+```
+
+## Rewriting Incompatible Constructs
+
+Scala 3's `-Ykind-projector` & `-Ykind-projector:underscores` implement only a subset of `kind-projector` syntax, in particular they do not implement:
+
+* higher-kinded type lambda placeholders
+* higher-kinded named type lambda parameters
+* The `Lambda` keyword (`λ` is still supported)
+
+You must rewrite ALL of the following forms:
+
+```scala
+// classic
+EitherT[*[_], Int, *]    // equivalent to: type R[F[_], B] = EitherT[F, Int, B]
+// underscores
+EitherT[_[_], Int, _]    // equivalent to: type R[F[_], B] = EitherT[F, Int, B]
+// named λ
+λ[(F[_], A) => EitherT[F, Int, A]]
+// named Lambda
+Lambda[(F[_], A) => EitherT[F, Int, A]]
+```
+
+Into the following long-form to cross-compile with Scala 3:
+
+```scala
+type MyLambda[F[_], A] = EitherT[F, Int, A]
+MyLambda
+```
+
+Alternatively you may use Scala 3's [Native Type Lambdas](https://dotty.epfl.ch/docs/reference/new-types/type-lambdas.html) if you do not need to cross-compile:
+
+```scala
+[F[_], A] =>> EitherT[F, Int, A]
+```
+
+For `Lambda` you must rewrite the following form:
+
+```scala
+Lambda[(`+E`, `+A`) => Either[E, A]]
+```
+
+To the following to cross-compile:
+
+```scala
+λ[(`+E`, `+A`) => Either[E, A]]
+```
+
+Or alternatively to Scala 3 type lambdas:
+
+```scala
+[E, A] =>> Either[E, A]
+```
+
+Note: Scala 3 type lambdas no longer need `-` or `+` variance markers on parameters, these are now inferred.

_overviews/scala3-migration/tutorial-prerequisites.md

@@ -115,6 +115,8 @@ AdditionalLy, we now have the following features that make `kind-projector` not
 - [Polymorphic Functions](http://dotty.epfl.ch/docs/reference/new-types/polymorphic-function-types.html)
 - [Kind Polymorphism](http://dotty.epfl.ch/docs/reference/other-new-features/kind-polymorphism.html)
 
+You can learn more about the Kind Projector migration in its [dedicated page](plugin-kind-projector.html).
+
 ## Runtime reflection
 
 `scala-reflect` will not be ported to Scala 3 because it exposes Scala 2 compiler internals that do not exist in Scala 3.