Example sbt project that compiles using Dotty

Usage

This is a normal sbt project, you can compile code with sbt compile and run it with sbt run, sbt console will start a Dotty REPL.

If compiling this example project fails, you probably have a global sbt plugin that does not work with dotty, try to disable all plugins in ~/.sbt/1.0/plugins and ~/.sbt/1.0.

IDE support

Dotty comes built-in with IDE support, to try it out see http://dotty.epfl.ch/docs/usage/ide-support.html

Making a new Dotty project

The fastest way to start a new Dotty project is to use one of the following templates:

Using Dotty in an existing project

You will need to make the following adjustments to your build:

project/plugins.sbt

addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.5.1")

project/build.properties

sbt.version=1.4.6

You must use sbt 1.4 or newer; older versions of sbt are not supported.

build.sbt

Any version number that starts with 0. is automatically recognized as Dotty by the sbt-dotty plugin, you don't need to set up anything:

scalaVersion := "3.0.0-M3"

Nightly builds

If the latest release of Dotty is missing a bugfix or feature you need, you may wish to use a nightly build. Look at the bottom of https://repo1.maven.org/maven2/org/scala-lang/scala3-compiler_3.0.0-RC1/ to find the version number for the latest nightly build. Alternatively, you can set scalaVersion := dottyLatestNightlyBuild.get to always use the latest nightly build of dotty.

Getting your project to compile with Dotty

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 Dotty code from compiling) by adding to your build.sbt:

scalacOptions ++= { if (isDotty.value) Seq("-source:3.0-migration") else Nil }

Using the isDotty setting ensures that this option will only be set when compiling with Dotty. For more information on the -source flag, see http://dotty.epfl.ch/docs/usage/language-versions.html, for more information on migrating to Scala 3 see the migration guide.

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

    libraryDependencies += "a" %% "b" % "c"

by:

    libraryDependencies += ("a" %% "b" % "c").withDottyCompat(scalaVersion.value)

This will have no effect when compiling with Scala 2.x, but when compiling with Dotty this will change the cross-version to a Scala 2.x one. This works because Dotty is currently retro-compatible with Scala 2.x.

Alternatively, to set this setting on all your dependencies, you can use:

    libraryDependencies := libraryDependencies.value.map(_.withDottyCompat(scalaVersion.value))

Discuss

Feel free to come chat with us on the Dotty gitter!

bracevac/scala3-example-project