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Add blogpost for 0.21.0-RC1 - (Feature Freeze)
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‎docs/blog/_posts/2019-12-20-21th-dotty-milestone-release.md

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---
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layout: blog-page
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title: Announcing Dotty 0.21.0-RC1 – ❄️ Feature Freeze ❄️
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author: Aggelos Biboudis
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authorImg: /images/aggelos.png
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date: 2019-12-21
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---
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Greetings and we wish you Merry Christmas 🎄!
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We are excited to announce 0.21.0-RC1 of Scala 3. In this version we add support
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for non-nullable reference types and a flow-sensitive analysis that work
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complementary. We present new syntax for given extensions, matches and pattern
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matching over quotes. We are also happy to announce that SemanticDB extraction
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is now supported!
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# ❄️ Feature Freeze ❄️
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This release is a HUGE milestone for us, for Dotty, for Scala 3, for our community. Since that
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[initial commit](https://github.com/lampepfl/dotty/commit/90962407e72d88f8f3249ade0f6bd60ff15af5ce)
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on the 6th December of 2012 when the only feature was the basic structure of a
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compiler based on the DOT calculus, we have come a long way.
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7 years and 20k commits later we are happy to announce that we are entering the
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*feature freeze* phase of Scala 3 ❄️❄️☃️. This means that with this release we
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stop adding new features ([Overview of Features](https://dotty.epfl.ch/docs/reference/overview.html)) and we focus on:
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- bug fixing and general quality assurance towards the final release 🐛
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- performance engineering 🏎️
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- documentation improvements 📕
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- education 👨‍🏫
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### Community-build
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Feature Freeze (FF) doesn't mean that development slows down. On the contrary!
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FF means that we can now put the Scala 3 compiler under heavy load, getting it
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ready for industrial strength applications. At the moment we have 23
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projects on our community projects and we expect this number to go up!
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> https://github.com/lampepfl/dotty/tree/master/community-build/community-projects
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This project contains tests to build and test a corpus of open sources Scala 2.x
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projects against Scala 3.
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To run the community-build on a local machine, first fetch all the git
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submodules with `git submodule update --init` and run `sbt community-build/test`
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from the root of the dotty repo.
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### New Issues
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Firstly thank you for all the hard work in issue reporting! FF means that our
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issue tracker will now be more important than ever. We encourage you to stress
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the compiler and report self contained test-cases! Bug minimization is hard and
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a form of art! Help us unearth those nasty bugs! ✊
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Last but not least we restate the mission of Scala 3. Scala has pioneered the
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fusion of object-oriented and functional programming in a typed setting and Scala 3
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will be a big step towards realising the full potential of these ideas. Its main
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objectives are to:
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- become more opinionated by promoting programming idioms we found to work well,
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- simplify where possible,
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- eliminate inconsistencies and surprising behaviours,
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- build on strong foundations to ensure the design hangs together well,
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- consolidate language constructs to improve the language’s consistency, safety, ergonomics, and
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performance.
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You can learn more about Dotty on our [website](https://dotty.epfl.ch).
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<!--more-->
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# What’s new in the 0.21.0-RC1 technology preview?
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## Dotty with explicit nulls and flow typing
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We add support for non-nullable reference types under the compiler option
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`-Yexplicit-nulls`. Nullability needs then to be expressed explicitly via unions
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(e.g. `String|Null`).
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This means the following code will no longer typecheck:
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```scala
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val x: String = null // error: found `Null`, but required `String`
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```
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Instead, to mark a type as nullable we use a type union:
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```scala
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val x: String|Null = null // ok
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```
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This change affects two parts of the compiler. Firstly we have a new type
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hierarchy for `Null` and a _translation layer_ from Java types to Scala types,
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which balances soundness and usability.
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With this release we also introduce a flow-sensitive analysis that refines the
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type of an expression based on control-flow. In the example below `s` is
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`String|Null`. The `if` branch validates the value of `s` against `Null` so `s`
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can be safely considered `String` in that scope.
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```scala
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val s: String|Null = ???
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if (s != null) {
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// s: String
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}
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else {
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// s: String|Null
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}
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```
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Note, that more complex tests are also supported like:
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```scala
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val s: String|Null = ???
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val s2: String|Null = ???
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if (s != null && s2 != null) // s: String and s2: String
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```
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but also in a short-circuting manner:
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```scala
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val s: String|Null = ???
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if (s != null && s.length > 0) // s: String in `s.length > 0`
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```
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For more info refer to the docs on [Explicit Nulls](https://dotty.epfl.ch/docs/reference/other-new-features/explicit-nulls.html).
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## New syntax for given instances defining extension methods
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To make code navigation easier in the case of `given` extension methods we
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change the syntax in the following manner. Hereafter, we write:
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```scala
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given listOps: extension [T](xs: List[T]) { ... }
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given extension (s: String) { ... }
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```
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instead of:
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```scala
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given listOps: [T](xs: List[T]) { ... }
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given (s: String) { ... }
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```
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The rationale is to communicate in a clean way that the parameters go on the
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extension method and not the wrapper (e.g., `listOps`) .
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To learn more about extension methods and given instances for extension methods in particular follow the docs on [Given Instances for Extension Methods](https://dotty.epfl.ch/docs/reference/contextual/extension-methods.html#given-instances-for-extension-methods)
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## New match syntax
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We introduce an improved treatment of `match`. We reintroduce `match` as an
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alphanumeric, left-associative, infix operator that can support chain matches:
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```scala
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xs match {
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case Nil => "empty"
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case x :: xs1 => "nonempty"
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} match {
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case "empty" => 0
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case "nonempty" => 1
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}
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```
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By using the new treatment we can now offer `match` as a method:
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```scala
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xs.match {
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case Nil => false
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case _ => true
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}
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```
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You can read more on our docs [Match Expressions](https://dotty.epfl.ch/docs/reference/changed-features/match-syntax.html) and on the interesting discussions on [contributors](https://contributors.scala-lang.org/t/pre-sip-demote-match-keyword-to-a-method/2137/2).
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## New quoted pattern matching
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We introduce a high-level API to deconstruct or extract values out of `Expr`
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using pattern matching. It consists of quoted patterns that allows to
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deconstruct complex code that contains a precise structure, types or methods.
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Patterns `'{ ... }` can be placed in any location where Scala expects a pattern.
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For example, the following snippet implements a simple, 1-level, non-recursive
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rewriter macro for exponents. `rewrite` is a an inline method definition
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designating a macro. To inspect an `expr` value we can now use the quoted syntax
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as patterns inside a match expression. Notice that quotes designate patterns and
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`$`, the familiar syntax for splices is used to _extract_ (capture) information
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out of a pattern.
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```scala
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inline def rewrite(expr: => Double): Double = ${rewrite('expr)}
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def rewrite(expr: Expr[Double])(given QuoteContext): Expr[Double] = {
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val res = expr match {
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// product rule
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case '{ power2($a, $x) * power2($b, $y)} if a.matches(b) => '{ power2($a, $x + $y) }
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// rules of 1
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case '{ power2($a, 1)} => a
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case '{ power2(1, $a)} => '{ 1.0 }
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// rule of 0
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case '{ power2($a, 0)} => '{ 1.0 }
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// power rule
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case '{ power2(power2($a, $x), $y)} => '{ power2($a, $x * $y ) }
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case _ => expr
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}
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res
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}
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```
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Additionally to the quoted patterns we provide extractors in
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`scala.quoted.matching` that extract static information from `Expr`s. Namely
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`Const`, `ExprSeq` and `ConstSeq`.
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To learn more read our docs on [pattern matching over quotes](https://dotty.epfl.ch/docs/reference/metaprogramming/macros.html#pattern-matching-on-quoted-expressions).
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## Added support for SemanticDB extraction
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Scala 3 now offers support for SemanticDB extraction producing `.semanticdb` files.
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Users can use the `-Ysemanticdb` compiler flag.
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# Let us know what you think!
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If you have questions or any sort of feedback, feel free to send us a message on our
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[Gitter channel](https://gitter.im/lampepfl/dotty). If you encounter a bug, please
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[open an issue on GitHub](https://github.com/lampepfl/dotty/issues/new).
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## Contributing
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Thank you to all the contributors who made this release possible!
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According to `git shortlog -sn --no-merges 0.20.0-RC1..0.21.0-RC1` these are:
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```
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<todo>
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```
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If you want to get your hands dirty and contribute to Dotty, now is a good time to get involved!
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Head to our [Getting Started page for new contributors](https://dotty.epfl.ch/docs/contributing/getting-started.html),
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and have a look at some of the [good first issues](https://github.com/lampepfl/dotty/issues?q=is%3Aissue+is%3Aopen+label%3Aexp%3Anovice).
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They make perfect entry points into hacking on the compiler.
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We are looking forward to having you join the team of contributors.
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## Library authors: Join our community build
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Dotty now has a set of widely-used community libraries that are built against every nightly Dotty
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snapshot. Currently, this includes ScalaPB, algebra, scalatest, scopt and squants.
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Join our [community build](https://github.com/lampepfl/dotty-community-build)
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to make sure that our regression suite includes your library.
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[Scastie]: https://scastie.scala-lang.org/?target=dotty
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[@odersky]: https://github.com/odersky
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[@DarkDimius]: https://github.com/DarkDimius
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[@smarter]: https://github.com/smarter
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[@felixmulder]: https://github.com/felixmulder
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[@nicolasstucki]: https://github.com/nicolasstucki
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[@liufengyun]: https://github.com/liufengyun
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[@OlivierBlanvillain]: https://github.com/OlivierBlanvillain
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[@biboudis]: https://github.com/biboudis
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[@allanrenucci]: https://github.com/allanrenucci
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[@Blaisorblade]: https://github.com/Blaisorblade
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[@Duhemm]: https://github.com/Duhemm
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[@AleksanderBG]: https://github.com/AleksanderBG
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[@milessabin]: https://github.com/milessabin
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[@anatoliykmetyuk]: https://github.com/anatoliykmetyuk

‎docs/docs/reference/features-classification.md

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- [Union types](new-types/union-types.md),
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- [Type lambdas](new-types/type-lambdas.md),
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replacing encodings using structural types and type projection.
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- [Context Queries](contextual/implicit-function-types.md)
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(_aka_ implicit function types) offering abstraction over given parameters.
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- [Implicit Function Types](contextual/implicit-function-types.md) offering abstraction over given parameters.
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**Status: essential**
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‎docs/docs/reference/overview.md

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- [Intersection types](new-types/intersection-types.md), replacing compound types,
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- [Union types](new-types/union-types.md),
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- [Type lambdas](new-types/type-lambdas.md),
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replacing encodings using structural types and type projection.
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- [Context Queries](contextual/implicit-function-types.md)
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(_aka_ implicit function types) offering abstraction over implicit parameters.
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- [Type lambdas](new-types/type-lambdas.md), replacing encodings using structural types and type projection.
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- [Implicit Function Types](contextual/implicit-function-types.md), offering abstraction over given parameters.
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## Simplifications
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