This is the official repository for the Scala Programming Language standard library, compiler, and language spec.
How to contribute
To contribute in this repo, please open a pull request from your fork of this repository.
For coordinating bigger work items, we use the scala/scala-dev tracker.
We do have to ask you to sign the Scala CLA before we can merge any of your work, to protect its open source nature.
The general workflow is as follows.
- Find/file an issue in scala/bug (or submit a well-documented PR right away!).
- Fork the scala/scala repo.
- Push your changes to a branch in your forked repo. For coding guidelines, go here.
- Submit a pull request to scala/scala from your forked repo.
For more information on building and developing the core of Scala, make sure to read the rest of this README, especially for setting up your machine!
Get in touch!
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:
|username||talk to me about...|
||type checker, pattern matcher, infrastructure, language spec|
||getting started, build, developer docs, community build, Jenkins, library|
||compiler performance, weird compiler bugs, Java 8 lambdas, REPL|
||back end, optimizer, named & default arguments, reporters|
||collections library, performance|
||concurrency, parallel collections, specialization|
||specialization, back end|
||interactions with Scala.js|
P.S.: If you have some spare time to help out around here, we would be delighted to add your name to this list!
scala/ +--build.sbt The main sbt build script +--lib/ Pre-compiled libraries for the build +--src/ All sources +---/library Scala Standard Library +---/library-aux Scala Auxiliary Library, for bootstrapping and documentation purposes +---/reflect Scala Reflection +---/compiler Scala Compiler +---/interactive Scala Interactive Compiler, for clients such as an IDE (aka Presentation Compiler) +---/intellij IntelliJ project templates +---/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 +--spec/ The Scala language specification +--scripts/ Scripts for the CI jobs (including building releases) +--test/ The Scala test suite +---/files Partest tests +---/junit JUnit tests +---/scalacheck ScalaCheck tests +--build/ [Generated] Build output directory
Get ready to contribute
You need the following tools:
- Java SDK. The baseline version is 8 for both 2.12.x and 2.13.x. It may be possible to use a later SDK for local development, but the CI will verify against the baseline version.
- sbt (sbt 0.13 on the 2.12.x branch, sbt 1 on the 2.13.x branch)
MacOS and Linux work. Windows may work if you use Cygwin. Community help with keeping the build working on Windows is appreciated.
Tools we use
We are grateful for the following OSS licenses:
During ordinary development, a new Scala build is built by the previously released version. For short, we call the previous release "starr": the stable reference release. Building with starr is sufficient for most kinds of changes.
However, a full build of Scala (a bootstrap, as performed by our CI) requires two layers. 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. See below for how to do a bootstrap build locally.
For history on how the current scheme was arrived at, see https://groups.google.com/d/topic/scala-internals/gp5JsM1E0Fo/discussion.
Using the sbt build
Once you've started an
sbt session you can run one of the core commands:
compilecompiles all sub-projects (library, reflect, compiler, scaladoc, etc)
scalacrun the REPL / compiler directly from sbt (accept options / arguments)
enableOptimizerreloads 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.
enableOptimizerand also 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/mkBingenerates runner scripts (
scalac, etc) in
dist/mkPackcreates a build in the Scala distribution format in
testruns the JUnit test,
testOnly *immutable.ListTestruns a subset
partestruns partest tests (accepts options, try
scalacheck/testruns scalacheck tests,
scalacheck/testOnly *FloatFormatTestruns a subset
publishLocalpublishes a distribution locally (can be used as
scalaVersionin other sbt projects)
set baseVersionSuffix := "bin-abcd123-SNAPSHOT"where
abcd123is the git hash of the revision being published. You can also use something custom like
"bin-mypatch". This changes the version number from
2.12.2-SNAPSHOTto something more stable (
- Note that the
-binstring marks the version binary compatible. Using it in sbt will cause the
2.12. If the version is not binary compatible, we recommend using
set publishArtifact in (Compile, packageDoc) in ThisBuild := falseto 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
resulting in a cyclical dependency. Try disabling global sbt plugins (perhaps by
temporarily commenting them out in
We recommend to keep local test files in the
sandbox directory which is listed in
.gitignore of the Scala repo.
Note that sbt's incremental compilation is often too coarse for the Scala compiler codebase and re-compiles too many files, resulting in long build times (check sbt#1104 for progress on that front). In the meantime you can:
- Use IntelliJ IDEA for incremental compiles (see IDE Setup below) - its incremental compiler is a bit less conservative, but usually correct.
To perform a bootstrap using sbt
- first a build is published either locally or on a temporary repository,
- then a separate invocation of sbt (using the previously built version as
starr) is used to build / publish the actual build.
publishLocal(you may want to specify a custom version suffix and skip generating API docs, see above).
- Quit sbt and start a new sbt instance using
<version>is the version number you published locally.
We suggest using IntelliJ IDEA (see src/intellij/README.md).
(Metals should also work, but we don't yet have instructions or sample configuration for that. A pull request in this area would be exceedingly welcome.)
In order to use IntelliJ's incremental compiler:
dist/mkBinin sbt to get a build and the runner scripts in
- run "Build" - "Make Project" in IntelliJ
Now you can edit and build in IntelliJ and use the scripts (compiler, REPL) to
directly test your changes. You can also run the
commands in sbt. Enable "Ant mode" (explained above) to prevent sbt's incremental
compiler from re-compiling (too many) files before each
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.
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
Once you submit a PR your commits will be automatically tested by the Scala CI.
If you see a spurious build failure, 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,
feel free to submit a PR and add the
WIP label. In case your WIP branch contains
a large number of commits (that you didn't clean up / squash yet for review),
[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 WIP PRs
will be closed eventually, which does not mean the change is being rejected.
CI performs a full bootstrap. The first task,
a build of your commit to the temporary repository
Note that this build is not yet bootstrapped, its bytecode is built using the
starr. The version number is
is the commit hash. For binary incompatible builds, the version number is
You can use Scala builds in the validation repository locally by adding a resolver
and specifying the corresponding
$ sbt > set resolvers += "pr" at "https://scala-ci.typesafe.com/artifactory/scala-pr-validation-snapshots/" > set scalaVersion := "2.12.2-bin-abcd123-SNAPSHOT" > console
The Scala CI builds nightly download releases and publishes them to the following locations:
The CI also publishes nightly API docs:
Using a nightly build in sbt is explained in this Stack Overflow answer
Scala CI internals
The build bot that watches PRs, triggers testing builds and applies the "reviewed" label after an LGTM comment is in the scala/scabot repo.
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
..-integrate-community-build. See the