scala – scala

This is Scala 2! Welcome!

This is the home of the Scala 2
standard library, compiler, and language spec.

For Scala 3, visit scala/scala3.

How to contribute

Issues and bug reports for Scala 2 are located in scala/bug. That tracker is also where new contributors may find issues to work on: good first issues, help wanted.

For coordinating broader efforts, we also use the scala/scala-dev tracker.

To contribute here, please open a pull request from your fork of this repository.

If your change affects the standard library, see CONTRIBUTING.md for remarks about what sort of changes might be accepted in Scala 2 and/or Scala 3.

We require that you sign the Scala CLA before we can merge any of your work, to protect Scala's future as open source software.

The general workflow is as follows.

1. Find/file an issue in scala/bug (or submit a well-documented PR right away!).
2. Fork the scala/scala repo.
3. Push your changes to a branch in your forked repo. For coding guidelines, go here.
4. Submit a pull request to scala/scala from your forked repo.

For more information on building and developing the core of Scala, read the rest of this README, especially for setting up your machine!

Get in touch!

In order to get in touch with other Scala contributors, join the
#scala-contributors channel on the Scala Discord chat, or post on
contributors.scala-lang.org (Discourse).

If you need some help with your PR at any time, please feel free to @-mention anyone from the list below, and we will do our best to help you out:

--------------------------------------------------------------------------------------------------|----------------------------------------------------------------|---------------------------------------------------|
<img src="https://avatars.githubusercontent.com/lrytz" height="50px" title="Lukas Rytz"/> | <code class="ra0-md-code">@lrytz</code> | back end, optimizer, named & default arguments, reporters |
<img src="https://avatars.githubusercontent.com/retronym" height="50px" title="Jason Zaugg"/> | <code class="ra0-md-code">@retronym</code> | compiler performance, weird compiler bugs, lambdas |
<img src="https://avatars.githubusercontent.com/SethTisue" height="50px" title="Seth Tisue"/> | <code class="ra0-md-code">@SethTisue</code> | getting started, build, CI, community build, Jenkins, docs, library, REPL |
<img src="https://avatars.githubusercontent.com/dwijnand" height="50px" title="Dale Wijnand"/> | <code class="ra0-md-code">@dwijnand</code> | pattern matcher, MiMa, partest |
<img src="https://avatars.githubusercontent.com/som-snytt" height="50px" title="Som Snytt"/> | <code class="ra0-md-code">@som-snytt</code> | warnings/lints/errors, REPL, compiler options, compiler internals, partest |
<img src="https://avatars.githubusercontent.com/Ichoran" height="50px" title="Rex Kerr"/> | <code class="ra0-md-code">@Ichoran</code> | collections library, performance |
<img src="https://avatars.githubusercontent.com/viktorklang" height="50px" title="Viktor Klang"/> | <code class="ra0-md-code">@viktorklang</code> | concurrency, futures |
<img src="https://avatars.githubusercontent.com/sjrd" height="50px" title="Sébastien Doeraene"/> | <code class="ra0-md-code">@sjrd</code> | interactions with Scala.js |
<img src="https://avatars.githubusercontent.com/bishabosha" height="50px" title="Jamie Thompson"/> | <code class="ra0-md-code">@bishabosha</code> | TASTy reader |
<img src="https://avatars.githubusercontent.com/joroKr21" height="50px" title="Georgi Krastev"/> | <code class="ra0-md-code">@joroKr21</code> | higher-kinded types, implicits, variance |

P.S.: If you have some spare time to help out around here, we would be delighted to add your name to this list!

Branches

Target the oldest branch you would like your changes to end up in. We periodically merge forward from 2.12.x to 2.13.x. Most changes should target 2.13.x, as 2.12.x is now under minimal maintenance.

If your change is difficult to merge forward, you may be asked to also submit a separate PR targeting the newer branch.

If your change is version-specific and shouldn't be merged forward, put [nomerge] in the PR name.

If your change is a backport from a newer branch and thus doesn't need to be merged forward, put [backport] in the PR name.

Choosing a branch

Most changes should target 2.13.x. We are increasingly reluctant to target 2.12.x unless there is a special reason (e.g. if an especially bad bug is found, or if there is commercial sponsorship). See Scala 2 maintenance.

Repository structure

Most importantly:

scala/
+--build.sbt                 The main sbt build definition
+--project/                  The rest of the sbt build
+--src/                      All sources
   +---/library              Scala Standard Library
   +---/reflect              Scala Reflection
   +---/compiler             Scala Compiler
+--test/                     The Scala test suite
   +---/files                Partest tests
   +---/junit                JUnit tests
   +---/scalacheck           ScalaCheck tests
+--spec/                     The Scala language specification

but also:

scala/
   +---/library-aux          Scala Auxiliary Library, for bootstrapping and documentation purposes
   +---/interactive          Scala Interactive Compiler, for clients such as an IDE (aka Presentation Compiler)
   +---/manual               Scala's runner scripts "man" (manual) pages
   +---/partest              Scala's internal parallel testing framework
   +---/partest-javaagent    Partest's helper java agent
   +---/repl                 Scala REPL core
   +---/repl-frontend        Scala REPL frontend
   +---/scaladoc             Scala's documentation tool
   +---/scalap               Scala's class file decompiler
   +---/testkit              Scala's unit-testing kit
+--admin/                    Scripts for the CI jobs and releasing
+--doc/                      Additional licenses and copyrights
+--scripts/                  Scripts for the CI jobs and releasing
+--tools/                    Scripts useful for local development
+--build/                    Build products
+--dist/                     Build products
+--target/                   Build products

Get ready to contribute

Requirements

You need the following tools:

• Java SDK. The baseline version is 8 for both 2.12.x and 2.13.x. It is almost always fine

to use a later SDK (such as 17 or 21) for local development. CI will verify against the
baseline version.

• sbt

MacOS and Linux work. Windows may work if you use Cygwin. Community help with keeping
the build working on Windows and documenting any needed setup is appreciated.

Tools we use

We are grateful for the following OSS licenses:

• JProfiler Java profiler
• YourKit Java Profiler
• IntelliJ IDEA

Build setup

Basics

During ordinary development, a new Scala build is built by the
previously released version, known as the "reference compiler" or,
slangily, as "STARR" (stable reference release). Building with STARR
is sufficient for most kinds of changes.

However, a full build of Scala is _bootstrapped_. Bootstrapping has
two steps: first, build with STARR; then, build again using the
freshly built compiler, leaving STARR behind. This guarantees that
every Scala version can build itself.

If you change the code generation part of the Scala compiler, your
changes will only show up in the bytecode of the library and compiler
after a bootstrap. Our CI does a bootstrapped build.

Bootstrapping locally*: To perform a bootstrap, run restarrFull
within an sbt session. This will build and publish the Scala
distribution to your local artifact repository and then switch sbt to
use that version as its new scalaVersion. You may then revert back
with reload. Note restarrFull will also write the STARR version
to buildcharacter.properties so you can switch back to it with
restarr without republishing. This will switch the sbt session to
use the build-restarr and target-restarr directories instead of
build and target, which avoids wiping out classfiles and
incremental metadata. IntelliJ will continue to be configured to
compile and run tests using the starr version in
versions.properties.

For history on how the current scheme was arrived at, see
https://groups.google.com/d/topic/scala-internals/gp5JsM1E0Fo/discussion.

Building with fatal warnings*: To make warnings in the project fatal (i.e. turn them into errors), run set Global / fatalWarnings := true in sbt (replace Global with the name of a module—such as reflect—to only make warnings fatal for that module). To disable fatal warnings again, either reload sbt, or run set Global / fatalWarnings := false (again, replace Global with the name of a module if you only enabled fatal warnings for that module). CI always has fatal warnings enabled.

Using the sbt build

Once you've started an sbt session you can run one of the core commands:

• compile compiles all sub-projects (library, reflect, compiler, scaladoc, etc)
• scala / scalac run the REPL / compiler directly from sbt (accept options /

arguments)

• enableOptimizer reloads the build with the Scala optimizer enabled. Our releases are built this way. Enable this when working on compiler performance improvements. When the optimizer is enabled the build will be slower and incremental builds can be incorrect.
• setupPublishCore runs enableOptimizer and configures a version number based on the current Git SHA. Often used as part of bootstrapping: sbt setupPublishCore publishLocal && sbt -Dstarr.version=<VERSION> testAll
• dist/mkBin generates runner scripts (scala, scalac, etc) in build/quick/bin
• dist/mkPack creates a build in the Scala distribution format in build/pack
• junit/test runs the JUnit tests; junit/testOnly *Foo runs a subset
• scalacheck/test runs scalacheck tests, use testOnly to run a subset
• partest runs partest tests (accepts options, try partest --help)
• publishLocal publishes a distribution locally (can be used as scalaVersion in

other sbt projects)

• Optionally set baseVersionSuffix := "bin-abcd123-SNAPSHOT"

where abcd123 is the git hash of the revision being published. You can also
use something custom like "bin-mypatch". This changes the version number from
2.13.2-SNAPSHOT to something more stable (2.13.2-bin-abcd123-SNAPSHOT).

• Note that the -bin string marks the version binary compatible. Using it in

sbt will cause the scalaBinaryVersion to be 2.13. If the version is not
binary compatible, we recommend using -pre, e.g., 2.14.0-pre-abcd123-SNAPSHOT.

• Optionally set ThisBuild / Compile / packageDoc / publishArtifact := false

to skip generating / publishing API docs (speeds up the process).

If a command results in an error message like `a module is not authorized to depend on
itself`, it may be that a global sbt plugin is causing
a cyclical dependency. Try disabling global sbt plugins (perhaps by
temporarily commenting them out in ~/.sbt/1.0/plugins/plugins.sbt).

Sandbox

We recommend keeping local test files in the sandbox directory which is listed in
the .gitignore of the Scala repo.

IDE setup

In IntelliJ IDEA, use "File - Open...", select the project folder and use
"Open as: sbt project".

• JUnit tests can be launched / debugged from the IDE, including tests that

run the compiler (e.g., QuickfixTest).

• Building in IntelliJ interoperates with the sbt build; the compiler script in

build/quick/bin (generated by running sbt dist/mkBin) runs the classfiles
built via the IDE.

In VSCode / Metals, open the project directory and import the project as ususal.

Coding guidelines

Our guidelines for contributing are explained in CONTRIBUTING.md.
It contains useful information on our coding standards, testing, documentation, how
we use git and GitHub and how to get your code reviewed.

Refer to our LLM policy for rules and guidelines regarding the use
of LLM-based tools in your contributions.

You may also want to check out the following resources:

• The "Scala Hacker Guide"

covers some of the same ground as this README, but in greater detail and in a more
tutorial style, using a running example.

• Scala documentation site

Scala CI

<img src="https://travis-ci.com/scala/scala.svg?branch=2.13.x" alt="Build Status" class="ra0-md-img" loading="lazy" />

Once you submit a PR your commits will be automatically tested by the Scala CI.

Our CI setup is always evolving. See
scala/scala-dev#751
for more details on how things currently work and how we expect they
might change.

If you see a spurious failure on Jenkins, you can post /rebuild as a PR comment.
The scabot README lists all available commands.

If you'd like to test your patch before having everything polished for review,
you can have Travis CI build your branch (make sure you have a fork and have Travis CI
enabled for branch builds on it first, and then push your branch). Also
feel free to submit a draft PR. In case your draft branch contains
a large number of commits (that you didn't clean up / squash yet for review),
consider adding [ci: last-only] to the PR title. That way only the last commit
will be tested, saving some energy and CI-resources. Note that inactive draft PRs
will be closed eventually, which does not mean the change is being rejected.

CI performs a compiler bootstrap. The first task, validatePublishCore, publishes
a build of your commit to the temporary repository
https://scala-ci.typesafe.com/artifactory/scala-pr-validation-snapshots.
Note that this build is not yet bootstrapped, its bytecode is built using the
current STARR. The version number is 2.13.2-bin-abcd123-SNAPSHOT where abcd123
is the commit hash. For binary incompatible builds, the version number is
2.14.0-pre-abcd123-SNAPSHOT.

You can use Scala builds in the validation repository locally by adding a resolver
and specifying the corresponding scalaVersion:

$ sbt
> set resolvers += "pr" at "https://scala-ci.typesafe.com/artifactory/scala-pr-validation-snapshots/"
> set scalaVersion := "2.13.17-bin-abcd123-SNAPSHOT"
> console

"Nightly" builds

The Scala CI publishes these to
https://scala-ci.typesafe.com/artifactory/scala-integration/ .

Using a nightly build in sbt and other tools is explained on this
doc page.

Although we casually refer to these as "nightly" builds, they aren't
actually built nightly, but "mergely". That is to say, a build is
published for every merged PR.

Scala CI internals

The Scala CI runs as a Jenkins instance on scala-ci.typesafe.com,
configured by a chef cookbook at scala/scala-jenkins-infra.

The build bot that watches PRs, triggers testing builds and applies the "reviewed" label
after an LGTM comment is in the scala/scabot repo.

Community build

The Scala community build is an important method for testing Scala
releases. A community build can be launched for any Scala commit, even
before the commit's PR has been merged. That commit is then used to
build a large number of open-source projects from source and run their
test suites.

To request a community build run on your PR, just ask in a comment on
the PR and a Scala team member (probably @SethTisue) will take care of
it. (details)

Community builds run on the Scala Jenkins instance. The jobs are
named ..-integrate-community-build. See the
scala/community-builds
repo.