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This is the official repository for the Scala Programming Language.

To contribute to the Scala Standard Library, Scala Compiler and Scala Language Specification, please send us a pull request from your fork of this repository! We do have to ask you to sign the Scala CLA before we can merge any of your work into our code base, to protect its open source nature.

For more information on building and developing the core of Scala, read on! Please also check out our guidelines for contributing.

We're still using Jira for issue reporting, so please report any issues over there. (We would love to start using GitHub Issues, but we're too resource-constrained to take on this migration right now.)

Get in touch!

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

                                                                                              | username                                                       | talk to me about...                               |

--------------------------------------------------------------------------------------------------|----------------------------------------------------------------|---------------------------------------------------| | @adriaanm | anything (type checker, pattern matcher, CI,...) | | @gkossakowski | infrastructure, incremental compilation, back-end | | @retronym | Java 8 lambdas, tricky bug detective work | | @Ichoran | the collections library, performance | | @lrytz | optimizer, named & default arguments | | @dickwall | process & documentation | | @VladUreche | specialization & the scaladoc tool | | @densh | quasiquotes, parser, string interpolators, macros in standard library | | @xeno-by | macros and reflection |

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

Handy Links

Repository structure

+--build.xml                 The main Ant build script, see also under src/build.       Pulls binary artifacts from remote repository.
+--lib/                      Pre-compiled libraries for the build.
+--src/                      All sources.
   +---/library              Scala Standard Library.
   +---/reflect              Scala Reflection.
   +---/compiler             Scala Compiler.
   +---/eclipse              Eclipse project files.
   +---/intellij-14          IntelliJ project templates.
+--scripts/                  Scripts for the CI jobs (including building releases)
+--test/                     The Scala test suite.
+--build/                    [Generated] Build products output directory for ant.
+--dist/                     [Generated] The destination folder for Scala distributions.

How we roll


You'll need a Java SDK (6 or newer), Apache Ant (version 1.8.0 or above), and curl (for ./

Git Hygiene

As git history is forever, we take great pride in the quality of the commits we merge into the repository. The title of your commit will be read hundreds (of thousands? :-)) of times, so it pays off to spend just a little bit more time to polish it, making it descriptive and concise. Please take a minute to read the advice most projects agree on, and stick to 50-60 characters for the first line, wrapping subsequent ones at 80 (at most).

When not sure how to formulate your commit message, imagine you're writing a bullet item for the next release notes, or describing what the commit does to the code base (use active verbs in the present tense). When your commit title is featured in the next release notes, it will be read by a lot of curious Scala users, looking for the latest improvements. Satisfy their thirst for information with as few words as possible! Also, a commit should convey clearly to your (future) fellow contributors what it does to the code base.

Writing the commit message is a great sanity check that the commit is of the right size. If it does too many things, the description will be unwieldy and tedious to write. Chop it up (git add -u --patch and git rebase are your friends) and simplify!

To pinpoint bugs, we often use git bisect, which is only effective when we can count on each commit building (and passing the test suite). Thus, the CI bot enforces this. Please rebase your development history into a sensible list of self-contained commits that tell the story of your bug fix or improvement. Carve them up so that the riskier bits can be reverted independently. Keep changes focussed by splitting out cleanups from refactorings from actual changes to the logic.

This facilitates reviewing: a commit that reformats code can be judged quickly not to affect anything, so we can focus on the meat of the PR. It also helps when merging between long-running branches, reducing conflicts (or providing at least a limited scope for each one).

Please do not @mention anyone in the commit message -- that's what the PR description and comments are for. Every time a commit is shuffled through github (in a merge in some fork, say), every @mention results in an email to that person (the core team treats them as personal email, straight to their inbox, so please don't flood us :-)).


Please consider nominating a reviewer for your PR in the PR's description or a comment. If unsure, not to worry -- the core team will assign one for you.

Your reviewer is also your mentor, who will help you rework your PR so that it meets our requirements. We strive to give timely feedback, and apologize for those times when we are overwhelmed by the volume of contributions. Please feel free to ping us. You are entitled to regular progress updates and at least a quick assessment of feasibility of a bigger PR.

To help you plan your contributions, we communicate our plans on a regular basis on scala-internals, and deadlines are tracked as due dates for GitHub milestones.


Once you've gained some experience with the code base and the process, the logical next step is to offers reviews for others's contributions. The main goal of this whole process, in the end, is to ensure the health of the Scala project by improving the quality of the code base, the documentation, as well as this process itself. Thank you for doing your part!

Tips & Tricks

Once the publish-core task has completed on a commit, you can try it out in sbt as follows:

$ sbt

> set resolvers += "pr" at ""
> set scalaVersion := "<milestone>-<sha7>-SNAPSHOT"
> console

Here, <milestone> is the milestone targeted by the PR (e.g., 2.11.6), and <sha7> is the 7-character sha (the format used by GitHub on the web).

IDE Setup


Download the Scala IDE bundle. It comes preconfigured for optimal performance.

  • Run ant init to download some necessary jars.
  • Import the project (in src/eclipse) via FileImport Existing Projects into Workspace. Check all projects and click ok.

For important details on building, debugging and file encodings, please see the excellent tutorial on and the included in src/eclipse.

IntelliJ 14

Use the latest IntelliJ IDEA release and install the Scala plugin from within the IDE.

The following steps are required to use IntelliJ IDEA on Scala trunk

  • Run ant init. This will download some JARs to ./build/deps, which are included in IntelliJ's classpath.
  • Run src/intellij-14/
  • Open ./src/intellij-14/scala.ipr in IntelliJ
  • File, Project Settings, Project, SDK. Create an SDK entry named "1.6" containing the Java 1.6 SDK. (You may use a later SDK for local development, but the CI will verify against Java 6.)

Compilation within IDEA is performed in "-Dlocker.skip=1" mode: the sources are built directly using the STARR compiler (which is downloaded from maven, according to starr.version in

Building with Ant

NOTE: we are working on migrating the build to sbt.

Run ant build-opt to build an optimized version of the compiler. Verify your build using ant test-opt.

The Scala build system is based on Apache Ant. Most required pre-compiled libraries are part of the repository (in 'lib/'). The following however is assumed to be installed on the build machine:

Tips and tricks

Here are some common commands. Most ant targets offer a -opt variant that runs under -optimise (CI runs the -optimize variant).

  • ./ downloads all binary artifacts associated with this commit.
  • ant -p prints out information about the commonly used ant targets.
  • ant or ant build: A quick compilation (to build/quick) of your changes using the locker compiler.

A typical debug cycle incrementally builds quick, then uses it to compile and run the file sandbox/test.scala as follows:

  • ant && build/quick/bin/scalac -d sandbox sandbox/test.scala && build/quick/bin/scala -cp sandbox Test

We typically alias build/quick/bin/scalac -d sandbox to qsc and build/quick/bin/scala -cp sandbox to qs in our shell.

ant test-opt tests that your code is working and fit to be committed:

  • Runs the test suite and bootstrapping test on quick.
  • You can run the suite only (skipping strap) with 'ant test.suite'.

ant docs generates the HTML documentation for the library from the sources using the scaladoc tool in quick. Note: on most machines this requires more heap than is allocate by default. You can adjust the parameters with ANT_OPTS. Example command line:

ANT_OPTS = "-Xms512M -Xmx2048M -Xss1M -XX:MaxPermSize=128M" ant docs
  • ant dist builds a distribution in 'dists/latest'.
  • ant all.clean Removes all build files and all distributions.

Bootstrapping concepts

NOTE: This is somewhat outdated, but the ideas still hold.

In order to guarantee the bootstrapping of the Scala compiler, the ant build compiles Scala in layers. Each layer is a complete compiled Scala compiler and library. A superior layer is always compiled by the layer just below it. Here is a short description of the four layers that the build uses, from bottom to top:

  • starr: the stable reference Scala release. We use an official version of Scala (specified by starr.version in, downloaded from maven central.
  • locker: the local reference which is compiled by starr and is the work compiler in a typical development cycle. Add locker.skip=true to to skip this step and speed up development when you're not changing code generation. In any case, after it has been built once, it is “frozen” in this state. Updating it to fit the current source code must be explicitly requested (ant locker.unlock).
  • quick: the layer which is incrementally built when testing changes in the compiler or library. This is considered an actual new version when locker is up-to-date in relation to the source code.
  • strap: a test layer used to check stability of the build.

For each layer, the Scala library is compiled first and the compiler next. That means that any changes in the library can immediately be used in the compiler without an intermediate build. On the other hand, if building the library requires changes in the compiler, a new locker must be built if bootstrapping is still possible, or a new starr if it is not.