Scala 3.0.0-M3: developer's preview before RC1
We are happy to announce the release of Scala 3.0.0-M3. This release is the Developer's Preview release. It is intended to contain all the features meant for RC1, which is tentatively planned for January 2021. The purpose of M3 is to give the larger community, beyond early adopters, a chance to try out all the features and give us feedback before sealing them in RC1.
To collect the feedback, we have designed a Scala 3 Developer’s Preview satisfaction survey (3-5 min) – please fill it in to express how you feel about Scala 3.
During the weeks between M2 and M3, we have been tying up all the loose ends, so that everything intended for the release could be finished before ending 2020. In addition, we have used this time to review and fine-tune the language syntax. Finally, a large number of consultations with the community took place during this time.
In this article, you will find the most important changes of this release compared to Scala 3.0.0-M2.
To read more about all things that surround the Scala 3 release in the next months, check out the "Scala 3 - Crossing the finish line" blog post on scala-lang.org
You can try out the M3 version online via Scastie.
sbt plugin update
We published a new version of the sbt plugin sbt-dotty
, v0.5.1. Because of the changes in PR #10607, this release of Scala 3 will not work with earlier versions of sbt-dotty. You will need to upgrade sbt-dotty to 0.5.1 to be able to use Scala 3.0.0-M3.
Final syntactic tweaks
as
dropped from the given
syntax
The following syntax is obsolete:
given intOrd as Ordering[Int]:
...
given listOrd[T: Ordering] as Ordering[List[T]]:
...
given Ordering[Int]:
...
given [T: Ordering] as Ordering[List[T]]:
...
given global as ExecutionContext = ForkJoinContext()
given Context = ctx
Here is how the above is expressed now:
given intOrd: Ordering[Int] with
...
given listOrd[T: Ordering]: Ordering[List[T]] with
...
given Ordering[Int] with
...
given [T: Ordering]: Ordering[List[T]] with
...
given global: ExecutionContext = ForkJoinContext()
given Context = ctx
You can find a discussion of the above change in the PR #10538.
Drop as
in patterns
Since we dropped as
from given
s, we lost a strong reason for having as
at all. Therefore, we dropped as
from patterns as well. The following syntax, valid in Scala 3.0.0-M2, is not accepted anymore:
case opt as Some(foo)
Instead, code that used it should be reverted to use the existing syntax of Scala 2, using @
:
case opt @ Some(foo)
Switch back to the old context function closure syntax
Previously, context function closures were written as follows:
(using s: Show[String]) => s.show("foobar")
Now, we have reverted to the following syntax:
(s: Show[String]) ?=> s.show("foobar")
Matchable
trait – a new top-level type
Although opaque types are supposed to provide an opaque abstraction of their underlying types, it is currently too easy to break the abstraction using pattern matching:
val imm = IArray(1,2,3) // supposedly immutable...
imm match
case a: Array[Int] => a(0) = 0 // but that's shown to be lie
To address this change, we introduce a new trait Matchable
near the top of the type hierarchy:
abstract class Any:
def asInstanceOf
def ==
def !=
def ##
def equals
def hashCode
def toString
trait Matchable extends Any:
def isInstanceOf
def getClass
class AnyVal extends Any, Matchable
class Object extends Any, Matchable
Matchable
is currently a marker trait without any methods. Over time, we intend to migrate the methods getClass
and isInstanceOf
to it, since these are closely related to pattern-matching.
In the meantime, the compiler will emit warnings when trying to call those methods, or when performing a match
, on an value that is not a Matchable
. For example, the above example is now flagged:
-- Warning: i7314.scala:6:12 ---------------------------------------------------
6 | case a: Array[Int] =>
| ^^^^^^^^^^
| pattern selector should be an instance of Matchable,
| but it has unmatchable type opaques.IArray[Int] instead
Note that the warnings are only active with language mode 3.1-migration
or higher - see the documentation on the Language Versions to learn how to enable it.
You can read the discussion of this change in the PR #10670. You can also read more about it in the documentation.
Tooling improvements
As we are getting closer to a stable release of Scala 3, the focus increasingly shifts on the tooling available to get started with Scala 3.
For a while now, we are not using the old dottydoc documentation tool for building the documentation. We are developing an entirely new tool, scala3doc, from scratch. This new documentation tool is more robust and faster than the old one.
As part of the tooling effort, this new Scala 3 documentation tool is rapidly improved. PR #10522 proves that the doctool can generate documentation for the community build projects. You can access this documentation via the following link.
PR #10491 introduced scripting support in Scala 3. Consider the following source named Main.scala
:
@main def Test(name: String): Unit =
println(s"Hello ${name}!")
If you have Scala 3 binaries on your path (which you can get by following the steps on the Dotty website, in the section "Try Dotty"), you can run the following command:
$ scala Main.scala World
This will compile the source in question to a temporary directory and run the discovered main method with the argument World
.
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.
The documentation for this feature is available here.
Metaprogramming changes
We have been polishing the metaprogramming API and making it more uniform. The following notable changes occurred between M2 and M3:
- Add
Expr.asTerm
#10694 - Add reflect
MatchCase
TypeRepr
#10735 - Rework reflect Symbol fields API #10705
- Remove
Expr.StringContext.unapply
#10675 - Rename
Liftable
toToExpr
andUnliftable
toFromExpr
#10618 - Remove Unliftable[Unit] #10570
- Remove reflect.LambdaType #10548
- Add
scala.quoted.Expr.unapply
as dual ofExpr.apply
#10580 - Move
Quotes
as last parameter inExprMap.transform
#10519 - Rework reflect Constant API #10753
- Unify quoted.report and reflect.Reporting #10474
- Fix #10359: Add GivenSelector to reflection API #10469
- Rework reflect show API #10661
- Fix #10709: Add missing level check before inlining #10781
Let us know what you think!
If you have questions or any sort of feedback, feel free to send us a message on our Gitter channel. If you encounter a bug, please open an issue on GitHub.
Contributors
Thank you to all the contributors who made this release possible 🎉
According to git shortlog -sn --no-merges 3.0.0-M2..3.0.0-M3
these are:
80 Nicolas Stucki
73 Martin Odersky
64 Krzysztof Romanowski
32 Liu Fengyun
28 Aleksander Boruch-Gruszecki
22 Anatolii Kmetiuk
17 Guillaume Martres
17 Sébastien Doeraene
14 Andrzej Ratajczak
13 Tom Grigg
8 Filip Zybała
7 Lan, Jian
5 Olivier Blanvillain
5 Som Snytt
5 Jamie Thompson
4 Stéphane Micheloud
4 Lionel Parreaux
3 Adrien Piquerez
3 Artur Opala
3 Hanns Holger Rutz
3 Michael Pilquist
3 Michał Pałka
3 bishabosha
2 Jonathan Brachthäuser
2 Camila Andrea Gonzalez Williamson
1 Mikael Blomstrand
1 Francois GORET
1 Felix Mulder
1 Raphael Jolly
1 Robert Stoll
1 Ruslan Shevchenko
1 Seth Tisue
1 Eugene Yokota
1 Amadou CISSE
1 Akhtiam Sakaev
1 Martin Duhem
1 Tomasz Godzik
1 Matthew Pickering
1 odersky
If you want to get your hands dirty and contribute to Scala 3, head to our Getting Started page for new contributors and read the docs on contributing. You are also very welcome to contact someone from our team to help you get started and see what currently we need help with.
We are looking forward to having you join the team of contributors.