Towards Scala 3

Thursday 19 April 2018

Martin Odersky

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 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. Many of the new features will be submitted to the SIP 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 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, including standard operations such as completion and hyperlinking and more advanced ones such as find references or rename. There’s a VS Code plugin incorporating these operations. JetBrains has also released a first version of Scala 3 support in their Scala IntelliJ plugin, 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, and we will also work on Scala 3 integration in other build tools.

What about stability?

  • A community build contains some initial open source projects 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 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 Get involved there, by fixing and opening issues, making pull requests, and participating in the discussions.