Merge pull request #872 from odersky/add-scala-3

Blog post on Scala 3

Author: odersky

blog/_posts/2018-04-19-scala-3.md

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+---
+layout: blog-detail
+post-type: blog
+by: Martin Odersky
+title: "Towards Scala 3"
+---
+
+Now that Scala 2.13 is only a few months away, it's time to consider the
+roadmap beyond it. It's been no secret that the work on [Dotty](https://github.com/lampepfl/dotty) over the
+last 5 years was intended to explore what a new Scala could look
+like. We are now at a stage where we can commit: Dotty will become
+Scala 3.0.
+
+Of course, this statement invites many follow-up questions. Here are
+some answers we can already give today. We expect there will be more
+questions and answers as things shape up.
+
+_When will it come out?_
+
+The intent is to publish the final Scala 3.0 soon after Scala 2.14. At the
+current release schedule (which might still change), that means early 2020.
+
+_What is Scala 2.14 for?_
+
+Scala 2.14's main focus will be on smoothing the migration to Scala 3.
+It will do this by defining migration tools, shim libraries, and
+targeted deprecations, among others.
+
+_What's new in Scala 3?_
+
+Scala has pioneered the fusion of object-oriented and functional
+programming in a typed setting. Scala 3 will be a big step towards
+realizing the full potential of these ideas. Its main objectives are
+to
+
+ - become more opinionated by promoting programming idioms we found
+to work well,
+ - simplify where possible,
+ - eliminate inconsistencies and surprising behavior,
+ - build on strong foundations to ensure the design hangs well together,
+ - consolidate language constructs to improve the language’s consistency, safety, ergonomics, and performance.
+
+The main language changes, either implemented or projected, are listed
+in the [Reference section on the Dotty website](http://dotty.epfl.ch/docs/reference/overview.html).
+Many of the new features will be submitted to the [SIP](https://docs.scala-lang.org/sips) process, subject to approval.
+
+It's worth emphasizing that Scala 2 and Scala 3 are fundamentally the
+same language. The compiler is new, but nearly everything Scala
+programmers already know about Scala 2 applies to Scala 3 as well, and
+most ordinary Scala 2 code will also work on Scala 3 with only minor
+changes.
+
+_What about migration?_
+
+As with previous Scala upgrades, Scala 3 is not binary compatible with Scala 2.
+They are mostly source compatible, but differences exist. However:
+
+ - Scala 3 code can use Scala 2 artifacts because the Scala 3 compiler
+   understands the classfile format for sources compiled with Scala 2.12 and upwards.
+ - Scala 3 and Scala 2 share the same standard library.
+ - With some small tweaks it is possible to cross-build code for both Scala 2 and 3.
+   We will provide a guide defining the shared language subset that can be compiled under both versions.
+ - The Scala 3 compiler has a `-language:Scala2` option that lets it compile most Scala 2 code
+   and at the same time highlights necessary rewritings as migration warnings.
+ - The compiler can perform many of the rewritings automatically using a `-rewrite` option.
+ - Migration through automatic rewriting will also be offered through the [scalafix](https://github.com/scalacenter/scalafix) tool, which can convert sources to the cross-buildable language subset without requiring Scala 3 to be installed.
+
+_What’s the expected state of tool support?_
+
+ - Compiler: The Scala 3 compiler `dotc` has been used to compile itself and a growing set of libraries for a number of years now.
+ - IDEs: IDE support is provided by having `dotc` implement LSP, the [Language Server Protocol](https://langserver.org),
+   including standard operations such as completion and hyperlinking and more advanced ones
+    such as find references or rename. There’s a [VS Code plugin](http://dotty.epfl.ch/docs/usage/ide-support.html) incorporating these operations.
+   JetBrains has also released a first version of Scala 3 support in their [Scala IntelliJ plugin](https://blog.jetbrains.com/scala/2017/03/23/scala-plugin-for-intellij-idea-2017-1-cleaner-ui-sbt-shell-repl-worksheet-akka-support-and-more), and we intend to work with them on further improvements.
+ - REPL: A friendly REPL is supported by the compiler
+ - Docs: A revamped Scaladoc tool generates docs for viewing in a browser and (in the future) also in the IDE..
+ - Build tools: There is a Dotty/Scala 3 plugin for [sbt](https://www.scala-sbt.org), and we will also work on Scala 3 integration in other build tools.
+
+_What about stability?_
+
+ - A [community build](https://github.com/lampepfl/dotty-community-build) contains some [initial open source projects](https://github.com/lampepfl/dotty-community-build/blob/master/src/test/scala/dotty/communitybuild/CommunityBuildTest.scala) that are compiled nightly using Scala 3. We plan to add a lot more projects to the build between now and the final release.
+ - We plan to use the period of developer previews to ensure that core projects are published for Scala 3.
+ - We have incorporated most of the Scala 2 regression tests in the Scala 3 test suite and will keep including new tests.
+ - In the near future we plan to build all Scala 3 tools using a previous version of the `dotc` compiler itself.
+   So far all tools are built first with the current Scala compiler and then again with `dotc`.
+   Basing the build exclusively on Scala 3 has the advantage that it lets us “eat our own dog food”
+   and try out the usability of Scala 3’s new language feature on a larger scale.
+
+_When can I try it out?_
+
+You can start working with Dotty now. See the [getting
+started](http://dotty.epfl.ch/docs/contributing/getting-started.html)
+guide. Dotty releases are published every 6 weeks. We expect to be in
+feature-freeze and to release developer previews for Scala 3.0 in the
+first half of 2019.
+
+_What about macros?_
+
+Stay tuned! We are about to release another blog post specifically
+about that issue.
+
+_How can I help?_
+
+Scala 3 is developed completely in the open at
+[https://github.com/lampepfl/dotty](https://github.com/lampepfl/dotty).
+Get involved there, by fixing and
+opening issues, making pull requests, and participating in the
+discussions.