Dotty Documentation



Announcing Dotty 0.13.0-RC1 with Spark support, top level definitions and redesigned implicits

Hello hello! This is the second release for 2019. Spark, top level definitions and redesigned implicits ✨🎊🎉 are the most important inclusions in this release and you will understand why we are super excited, in a bit!

Without further ado, today we release the version 0.13.0-RC1 of the Dotty compiler. This release serves as a technology preview that demonstrates new language features and the compiler supporting them.

Dotty is the project name for technologies that are being considered for inclusion 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 realising 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 behaviours,
  • build on strong foundations to ensure the design hangs together well,
  • consolidate language constructs to improve the language’s consistency, safety, ergonomics, and performance.

You can learn more about Dotty on our website.

This is our 13th scheduled release according to our 6-week release schedule.

What’s new in the 0.13.0-RC1 technology preview?

Experimental support for Spark

Dotty projects have always been able to depend on Scala 2 libraries, and this usually works fine (as long as the Dotty code does not call a Scala 2 macro directly). However, Spark was known to not work correctly as it heavily relies on Java serialization which we were not fully supporting.

Meanwhile, at EPFL, we've started updating our Scala courses to use Dotty instead of Scala 2, the Functional Programming course given last semester went smoothly, but the Parallelism and Concurrency course given in the Spring semester teaches Spark, which means we needed to support it in Dotty!

Luckily, this turned out to be mostly straightforward: we adopted the object serialization scheme and lambda serialization scheme pioneered by Scala 2, and that was enough to make our Spark assignments run correctly! This doesn't mean that our support is perfect however, so don't hesitate to open an issue if something is amiss.

Introducing top level definitions

Top level definitions are now supported. This means that package objects are now redundant, and will be phased out. This means that all kinds of definitions can be written at the top level.

package p

type Labelled[T] = (String, T)

val a: Labelled[Int] = ("count", 1)
def b = a._2

You can read about dropping package objects at the documentation linked or at the relevant PR #5754.

All things impl... implied

Scala's implicits are its most distinguished feature. They are the fundamental way to abstract over context. They represent a single concept with an extremely varied number of use cases, among them: implementing type classes, establishing context, dependency injection, expressing capabilities, computing new types and proving relationships between them.

However, with great power comes great responsibility. The current design of implicits has shown some limitations, which we have been trying to identify and address to make Scala a clearer and more pleasant language. First of all, we found that the syntactic similarity was too great between implicit conversions and implicit values that depend on other implicit values. Both of them appear in the snippet below:

implicit def i1(implicit x: T): C[T] = ... // 1: conditional implicit value
implicit def i2(x: T): C[T] = ...          // 2: implicit conversion

Some users used implicit conversions, in an unprincipled matter. This overuse of implicit conversions decluttered code. However, while implicit conversions can be useful to remove clutter, their abuse makes it harder for people to reason about the code.

The implicit keyword is used for both implicit conversions and conditional implicit values and we identified that their semantic differences must be communicated more clearly syntactically. Furthermore, the implicit keyword is ascribed too many overloaded meanings in the language (implicit vals, defs, objects, parameters). For instance, a newcomer can easily confuse the two examples above, although they demonstrate completely different things, a typeclass instance is an implicit object or val if unconditional and an implicit def with implicit parameters if conditional; arguably all of them are surprisingly similar (syntactically). Another consideration is that the implicit keyword annotates a whole parameter section instead of a single parameter, and passing an argument to an implicit parameter looks like a regular application. This is problematic because it can create confusion regarding what parameter gets passed in a call. Last but not least, sometimes implicit parameters are merely propagated in nested function calls and not used at all, so giving names to implicit parameters is often redundant and only adds noise to a function signature.

Consequently, we introduce two new language features:

  1. implied instance definitions designated syntactically by the scheme implied ... for and,
  2. inferable parameters designated by the keyword given.

In the code below we demonstrate both of them. This code defines a trait Ord and two implied instance definitions. IntOrd defines an implied instance for the type Ord[Int] whereas ListOrd[T] defines implied instances of Ord[List[T]] for all types T that come with an implied Ord[T] instance themselves. The given clause in ListOrd defines an inferable parameter.

trait Ord[T] {
  def compare(x: T, y: T): Int
  def (x: T) < (y: T) = compare(x, y) < 0
  def (x: T) > (y: T) = compare(x, y) > 0

implied IntOrd for Ord[Int] {
  def compare(x: Int, y: Int) =
    if (x < y) -1 else if (x > y) +1 else 0

implied ListOrd[T] given (ord: Ord[T]) for Ord[List[T]] {
  def compare(xs: List[T], ys: List[T]): Int = (xs, ys) match {
    case (Nil, Nil) => 0
    case (Nil, _) => -1
    case (_, Nil) => +1
    case (x :: xs1, y :: ys1) =>
      val fst =, y)
      if (fst != 0) fst else xs1.compareTo(ys1)

A given clause can also designate an inferable parameter for functions:

def max[T](x: T, y: T) given (ord: Ord[T]): T =
  if (, y) < 1) y else x

With this scheme all invocations of the max function below are equally valid:

max(2, 3) given IntOrd
max(List(1, 2, 3), Nil)
max(2, 3)

We introduce Anonymous Implied Instances which are used when we do not need a name for an implied instance:

implied for Ord[Int] { ... }

For convenience, we also introduce Implied Alias Instances. They offer aliases for implied instances. For example, the line below offers an alias with the name ctx (could also be anonymous if name can be omitted). Each time an implied instance of ExecutionContext is demanded the right-hand side is returned.

implied ctx for ExecutionContext = currentThreadPool().context

We have also added a synonym to implicitly, which is often more natural to spell out in user code. Functions like the that have only inferable parameters are also called context queries from now on. Consequently, to summon an implied instance of Ord[List[Int]] we write:


How do we import implied instances in scope? From now on, normal import clauses fetch all definitions except implied instance into scope whereas Implied Imports bring only implied instances in scope.

object A {
  class TC
  implied tc for TC
  def f given TC = ???
object B {
  import A._         // normal import clause
  import implied A._ // implied import clause

You can read more about implied imports from the docs or the relevant PR #5868.

As we mentioned above, context queries are functions with (only) inferable parameters. Here is an example of such a function:

type Contextual[T] = given Context => T

Context queries--previously named implicit function types (IFTs)--are now also expressed with given, providing types for first-class context queries. This is merely an alignment of IFTs into the new scheme.

You can read more about the alternative to implicits through the Contextual Abstractions section of our documentation or for a deep dive from the relevant PR chain that originated from #5458. The syntax changes for new implicits are summarized in #5825.

This release offers the support for typeclass derivation as a language feature. Typeclass derivation is a way to generate instances of certain type classes automatically or with minimal code hints, and is now supported natively with dedicated language support. A type class in this sense is any trait or class with a type parameter that describes the type being operated on. Commonly used examples are Ordering, Show, or Pickling. We introduce a new derives clause that generates implied instances of the Eql, Ordering, and Pickling traits in the companion object Tree. Take a look at the example below:

enum Tree[T] derives Eql, Ordering, Pickling {
  case Branch(left: Tree[T], right: Tree[T])
  case Leaf(elem: T)

where the generated implied instances are the ones below:

implied [T: Eql]      for Eql[Tree[T]]      = Eql.derived
implied [T: Ordering] for Ordering[Tree[T]] = Ordering.derived
implied [T: Pickling] for Pickling[Tree[T]] = Pickling.derived

Note, the new syntax:

A extends B, C { ... }

which replaces:

A extends B with C { ... }

With typeclass derivation we can also derive types. A trait or class can appear in a derives clause if its companion object defines a method named derived. The type and implementation of a derived method are arbitrary, but typically it has a definition like this:

def derived[T] given Generic[T] = ...

You can read more about Typeclass Derivation or have a deep dive at the relevant PRs: #5540 and #5839.

Multiversal equality is now supported through the Eql marker trait (renamed from Eq to differentiate it from Cats' Eq). For example, in order to be able to compare integers with strings now, instead of a custom implicit we can provide a derived implicit instance:

implied for Eql[Int, String] = Eql.derived

You can read more about how we based multiversal equality on typeclass derivation through the relevant PR #5843.

Implicit conversions are now defined by implied instances of the scala.Conversion class. For example:

implied for Conversion[String, Token] {
  def apply(str: String): Token = new KeyWord(str)

Note: that these release notes contain only a brief summary of the new features, for more details please read our documentation page under the new section named Contextual Abstractions. Equally important with the documentation of each feature, please consult the Relationship with Scala 2 Implicits section as well.

Implicit resolution rule changes

PR #5887 applies the following changes to implicit resolution:

  1. nested implicits always take precedence over outer ones
  2. no more shadowing checks
  3. package prefixes are not considered.

SemanticDB generator

SemanticDB is a data model for semantic information such as symbols and types about programs in Scala and other languages. SemanticDB decouples production and consumption of semantic information, establishing documented means for communication between tools. With PR #5761 we add the first prototype for the generation of SemanticDB information from TASTy.

And much more!

Please read our release notes for more details!

Trying out Dotty


You can set up a new sbt project with Dotty as the compiler by running:

sbt new lampepfl/dotty.g8

For more details on using Dotty with sbt, see the example project.

IDE support

Start using the Dotty IDE in any Dotty project by following the IDE guide.

Standalone installation

Releases are available for download on the Releases section of the Dotty repository:

For macOS users, we also provide a homebrew package that can be installed by running:

brew install lampepfl/brew/dotty

In case you have already installed Dotty via brew, you should instead update it:

brew upgrade dotty

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.


Thank you to all the contributors who made this release possible!

According to git shortlog -sn --no-merges 0.12.0-RC1..0.13.0-RC1 these are:

   309  Martin Odersky
   116  Nicolas Stucki
    52  Guillaume Martres
    42  poechsel
    22  Aggelos Biboudis
    20  Paolo G. Giarrusso
    19  Olivier Blanvillain
    11  Liu Fengyun
     5  Allan Renucci
     4  Miles Sabin
     3  Tobias Bordenca
     3  Lionel Parreaux
     3  Abel Nieto
     2  Lukas Rytz
     1  lpwisniewski
     1  Adriaan Moors
     1  Georg Schmid
     1  Jentsch
     1  Marc Karassev
     1  Daniel Murray
     1  Olivier ROLAND
     1  Raphael Jolly
     1  Stéphane Micheloud
     1  Sébastien Doeraene
     1  Umayah Abdennabi

If you want to get your hands dirty and contribute to Dotty, now is a good time to get involved! Head to our Getting Started page for new contributors, and have a look at some of the good first issues. They make perfect entry points into hacking on the compiler.

We are looking forward to having you join the team of contributors.

Library authors: Join our community build

Dotty now has a set of widely-used community libraries that are built against every nightly Dotty snapshot. Currently this includes ScalaPB, algebra, scalatest, scopt and squants. Join our community build to make sure that our regression suite includes your library.

Aggelos Biboudis