Example mill project that compiles using Scala 3

NOTE: Scala 3 support in mill is experimental. For now, sbt remains the preferred option. See https://github.com/scala/scala3-example-project for an example sbt project.

Usage

This is a normal mill project. You can compile code with mill root.compile and run it with mill root.run, mill -i root.console will start a Scala 3 REPL.

IDE support

Scala 3 comes built-in with IDE support, unfortunately this support is only available using sbt and not mill for now, see http://dotty.epfl.ch/docs/usage/ide-support.html

Using Scala 3 in an existing project

build.sc

Any version number that starts with 3. is automatically recognized as Scala 3; you don't need to set up anything:

def scalaVersion = "3.1.1"

Nightly builds

If the latest release of Scala 3 is missing a bugfix or feature you need, you may wish to use a nightly build. Look at the bottom of the list of releases to find the version number for the latest nightly build.

Getting your project to compile with Scala 3

When porting an existing project, it's a good idea to start out with the Scala 2 compatibility mode (note that this mode affects typechecking and thus may prevent some valid Scala 3 code from compiling) by adding to your build.sc:

def scalacOptions = T {
  if (Dep.isDotty(scalaVersion()))
    Seq("-language:Scala2")
  else
    Seq()
}

Using the Dep.isDotty method ensures that this option will only be set when compiling with Scala 3.

A tool to port code from Scala 2.x to Scala 3 is available; see https://scalacenter.github.io/scala-3-migration-guide/docs/tooling/scala-3-migrate-plugin.html

If your build contains dependencies that have only been published for Scala 2.x, you may be able to get them to work on Scala 3 by replacing:

ivy"a::b:c"

by:

ivy"a::b:c".withDottyCompat(scalaVersion())