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‎docs/blog/_posts/2020-12-18-scala3-m3.md

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---
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layout: blog-page
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title: Scala 3.0.0-M3: developer's preview before RC1
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author: Anatolii Kmetiuk
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authorImg: /images/anatolii.png
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date: 2020-12-18
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---
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We are happy to announce the release of Scala 3.0.0-M3. This release is the Developer's Preview release intended to contain all the features meant for RC1 which is tentatively planned for January 2021. The purpose of M3 is to give the community a chance to try out all the features and give us feedback before sealing them in RC1.
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To collect the feedback, we have designed a [Scala 3 Developer’s Preview satisfaction survey (3-5 min)](https://docs.google.com/forms/d/e/1FAIpQLSflVmTu9lhrtnSTh2tKAjUGrt3WvEgwlDqZg66O3EVSXd1aJg/viewform?usp=sf_link) – please fill it in to express how you feel about Scala 3.
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For the weeks between M2 and M3, we were tying up all the loose ends so that to end 2020 having done everything intended for the release. Also, we have used this time as the last chance to review and fine-tune the language syntax. A large number of consultations with the community took place during this time so that to ensure everyone is on the same page.
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In this article, you will find the summary of the most important changes that took place.
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<!--more-->
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# SBT plugin bump
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SBT plugin was bumped to 0.5.0. Because of the changes in PR [#10607](https://github.com/lampepfl/dotty/pull/10607) this release of Scala 3 will not work with earlier versions of the SBT plugin. So if you are an early adopter of Scala 3 who has some projects in it, you will need to bump your SBT plugin version to migrate to M3.
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# Final syntactic tweaks
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## `as` dropped from the `given` syntax
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The following syntax is obsolete:
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```scala
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given intOrd as Ordering[Int]:
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...
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given listOrd[T: Ordering] as Ordering[List[T]]:
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...
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given Ordering[Int]:
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...
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given [T: Ordering] as Ordering[List[T]]:
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...
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given global as ExecutionContext = ForkJoinContext()
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given Context = ctx
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```
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Here is how the above is expressed now:
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```scala
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given intOrd: Ordering[Int] with
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...
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given listOrd[T: Ordering]: Ordering[List[T]] with
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...
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given Ordering[Int] with
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...
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given [T: Ordering]: Ordering[List[T]] with
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...
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given global: ExecutionContext = ForkJoinContext()
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given Context = ctx
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```
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You can find an extensive discussion on the above change in its [PR #10538](https://github.com/lampepfl/dotty/pull/10538).
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## Drop `as` in patterns
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Since we dropped `as` from `given`s, we lost a strong reason for having `as` at all. For this reason, we dropped them also from patterns. If previously the correct binding syntax is patterns was:
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```scala
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case opt as Some(foo)
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```
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Now it got reverted to the Scala 2 style:
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```scala
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case opt @ Some(foo)
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```
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## Switch back to the old context function closure syntax
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Previously, context function closures were written as follows:
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```scala
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(using s: Show[String]) => s.show("foobar")
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```
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Now, we have reverted to the following syntax:
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```scala
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(s: Show[String]) ?=> s.show("foobar")
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```
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# `Matchable` trait – a new top-level type
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The motivation for this change is the fact that it is possible to access the underlying type referenced by an opaque type using pattern matching:
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```scala
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val imm = IArray(1,2,3) // supposedly immutable...
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imm match
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case a: Array[Int] => a(0) = 0 // but that's shown to be lie
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```
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To address this change, a new trait, `Matchable`, is introduced at the top of the type hierarchy:
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```scala
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abstract class Any:
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def asInstanceOf
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def ==
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def !=
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def ##
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def equals
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def hashCode
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def toString
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trait Matchable extends Any:
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def isInstanceOf
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def getClass
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class AnyVal extends Any, Matchable
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class Object extends Any, Matchable
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```
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With the addition of this type, we now make the compiler emit warnings when trying to do a trick like the one discussed above:
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```scala
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-- Warning: i7314.scala:6:12 ---------------------------------------------------
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6 | case a: Array[Int] =>
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| ^^^^^^^^^^
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| pattern selector should be an instance of Matchable,
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| but it has unmatchable type opaques.IArray[Int] instead
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```
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The change involved an extensive discussion which you can read in the [PR #10670](https://github.com/lampepfl/dotty/pull/10670).
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# Tooling improvements
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As the work on Scala 3 gets closer to the stable release, the focus increasingly shifts on the tooling available to get started with Scala 3.
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As part of the tooling effort, Scala 3 documentation tool is rapidly improved. [PR #10522](https://github.com/lampepfl/dotty/pull/10522) proves that the doctool can generate documentation for the community build projects.
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[PR #10491](https://github.com/lampepfl/dotty/pull/10491) introduced the scripting capability to Scala 3. Consider the following source named e.g. `Main.scala`:
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```scala
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@main def Test(name: String): Unit =
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println(s"Hello ${name}!")
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```
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If you have Scala 3 binaries on your path (which you can get by following the steps at the [Dotty website](https://dotty.epfl.ch/)'s section "Try DOtty"), you can run the `scala Main.scala World` command. This command will compile the source in question to a temporary directory and run the discovered main method with the argument `World`.
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Note the difference from the Scala 2 scripting implementation. In Scala 2, we do not require the users to have a `main` method in their scripts due to it being too cumbersome to write. In Scala 3, thanks to the top-level definitions and the `@main` annotations, `main` methods are one-liners and hence are more suited for scripts.
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The documentation for this feature is available [here](https://dotty.epfl.ch/docs/usage/getting-started.html#scala-3-for-scripting).
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# Metaprogramming changes
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Metaprogramming-wise, the majority of work is focused on polishing the API and making it more uniform. The following notable metaprogramming API changes occurred between M2 and M3:
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- Add reflect `MatchCase` `TypeRepr` (#10735)[https://github.com/lampepfl/dotty/pull/10735]
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- Rework reflect Symbol fields API (#10705)[https://github.com/lampepfl/dotty/pull/10705]
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- Remove `Expr.StringContext.unapply` (#10675)[https://github.com/lampepfl/dotty/pull/10675]
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- Rename `Liftable` to `ToExpr` and `Unliftable` to `FromExpr` (#10618)[https://github.com/lampepfl/dotty/pull/10618]
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- Remove Unliftable[Unit] (#10570)[https://github.com/lampepfl/dotty/pull/10570]
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- Remove reflect.LambdaType (#10548)[https://github.com/lampepfl/dotty/pull/10548]
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- Add `scala.quoted.Expr.unapply` as dual of `Expr.apply` (#10580)[https://github.com/lampepfl/dotty/pull/10580]
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- Move `Quotes` as last parameter in `ExprMap.transform` (#10519)[https://github.com/lampepfl/dotty/pull/10519]
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- Unify quoted.report and reflect.Reporting (#10474)[https://github.com/lampepfl/dotty/pull/10474]
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- Fix #10359: Add GivenSelector to reflection API (#10469)[https://github.com/lampepfl/dotty/pull/10469]
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- Rework reflect show API (#10661)[https://github.com/lampepfl/dotty/pull/10661]
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- Fix #6542: Pickle line sizes in TASTy (#10363)[https://github.com/lampepfl/dotty/pull/10363]
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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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## Contributors
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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 3.0.0-M2..3.0.0-M3` 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 shapeless, ScalaPB, algebra, scalatest, scopt and squants.
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Join our [community build](https://github.com/lampepfl/dotty/tree/master/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

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