Scala 2.11.0-RC1 is now available!

Thursday 6 March 2014


We are very pleased to announce the first release candidate of Scala 2.11.0! Download it now from or via Maven Central.

Please do try out this release candidate to help us find any serious regressions before the final release. The next release candidate will be cut on Monday March 17, if there are no unresolved blocker bugs at noon (PST). Subsequent RCs will be released on a weekly schedule, with Monday at noon (PST) being the cut-off for blocker bug reports. Our goal is to have no more than three RCs for this release – please help us achieve this by testing your project soon!

Code that compiled on 2.10.x without deprecation warnings should compile on 2.11.x (we do no guarantee this for experimental APIs, such as reflection). If not, please file a regression. We are working with the community to ensure availability of the core projects of the Scala 2.11.x eco-system, please see below for a list. This release is not binary compatible with the 2.10.x series, to allow us to keep improving the Scala standard library.

For production use, we recommend the latest stable release, 2.10.3 (2.10.4 final coming soon).

The Scala 2.11.x series targets Java 6, with (evolving) experimental support for Java 8. In 2.11.0, Java 8 support is mostly limited to reading Java 8 bytecode and parsing Java 8 source. Stay tuned for more complete (experimental) Java 8 support.

The Scala team and contributors fixed 544 bugs that are exclusive to Scala 2.11.0-RC1! We also backported as many as possible. With the release of 2.11, 2.10 backports will be dialed back.

Since the last milestone, we fixed 133 issues via 154 merged pull requests.

A big thank you to everyone who’s helped improve Scala by reporting bugs, improving our documentation, participating in mailing lists and other public fora, and – of course – submitting and reviewing pull requests! You are all awesome.

Concretely, between Jan 2013 and today, 69 contributors have helped improve Scala!

Our gratitude goes to @paulp, @som-snytt, @soc, @JamesIry, @Ichoran, @vigdorchik, @kzys, @Blaisorblade, @rjolly, @gourlaysama, @xuwei-k, @sschaef, @rklaehn, @lexspoon, @folone, @qerub, @etaty, @ViniciusMiana, @ScrapCodes, @pavelpavlov, @jedesah, @ihji, @harrah, @aztek, @clhodapp, @vy, @eed3si9n, @mergeconflict, @greenrd, @rjfwhite, @danielhopkins, @khernyo, @u-abramchuk, @mt2309, @ivmaykov, @yllan, @jrudolph, @jannic, @non, @dcsobral, @chuvoks, @rtyley.

With special thanks to the team at EPFL: @xeno-by, @densh, @magarciaEPFL, @VladimirNik, @lrytz, @VladUreche, @heathermiller, @sjrd, @hubertp, @OlivierBlanvillain, @namin, @cvogt, @vjovanov.

If you find any errors or omissions in these relates notes, please submit a PR!

Reporting Bugs / Known Issues

Please file any bugs you encounter. If you’re unsure whether something is a bug, please contact the scala-user mailing list.

Before reporting a bug, please have a look at these known issues.

Scala IDE for Eclipse

The Scala IDE with this release built in is available from this update site for Eclipse 4.2/4.3 (Juno/Kepler). Please have a look at the getting started guide for more info.

Available projects

The following Scala projects have already been released against 2.11.0-RC1! We’d love to include yours in this list as soon as it’s available – please submit a PR to update these release notes.

"org.scalacheck"    %% "scalacheck"         % "1.11.3"
"org.scalafx"       %% "scalafx"            % "1.0.0-R8"
"org.scalatest"     %% "scalatest"          % "2.1.0"
"org.specs2"        %% "specs2"             % "2.3.9"
"com.typesafe.akka" %% "akka-actor"         % "2.3.0-RC4"
"org.scalaz"        %% "scalaz-core"        % "7.0.6"
"com.nocandysw"     %% "platform-executing" % "0.5.0"

NOTE: RC1 ships with akka-actor 2.3.0-RC4 (the final is out now, but wasn’t yet available when RC1 was cut). The next Scala 2.11 RC will ship with akka-actor 2.3.0 final.

Cross-building with sbt 0.13

When cross-building between Scala versions, you often need to vary the versions of your dependencies. In particular, the new scala modules (such as scala-xml) are no longer included in scala-library, so you’ll have to add an explicit dependency on it to use Scala’s xml support.

Here’s how we recommend handling this in sbt 0.13. For the full build, see [@gkossakowski’s](’s) example.

scalaVersion        := "2.11.0-RC1"

crossScalaVersions  := Seq("2.11.0-RC1", "2.10.3")

// add scala-xml dependency when needed (for Scala 2.11 and newer)
// this mechanism supports cross-version publishing
libraryDependencies := {
  CrossVersion.partialVersion(scalaVersion.value) match {
    case Some((2, scalaMajor)) if scalaMajor >= 11 =>
      libraryDependencies.value :+ "org.scala-lang.modules" %% "scala-xml" % "1.0.0"
    case _ =>

Important changes

For most cases, code that compiled under 2.10.x without deprecation warnings should not be affected. We’ve verified this by compiling a sizeable number of open source projects in the Scala community build.

Changes to the reflection API may cause breakages, but these breakages can be easily fixed in a manner that is source-compatible with Scala 2.10.x. Follow our reflection/macro changelog for detailed instructions.

We’ve decided to fix the following more obscure deviations from specified behavior without deprecating them first.

  • SI-4577 Compile x match { case _ : Foo.type => } to Foo eq x, as specified. It used to be Foo == x (without warning). If that’s what you meant, write case Foo =>.

The following changes were made after a deprecation cycle (Thank you, @soc, for leading the deprecation effort!)

  • SI-6809 Case classes without a parameter list are no longer allowed.
  • SI-7618 Octal number literals no longer supported.

Finally, some notable improvements and bug fixes:

  • SI-7296 Case classes with > 22 parameters are now allowed.
  • SI-3346 Implicit arguments of implicit conversions now guide type inference.
  • SI-6240 Thread safety of reflection API.
  • #3037 Experimental support for SAM synthesis.
  • #2848 Name-based pattern-matching.
  • SI-7475 Improvements to access checks, aligned with the spec (see also the linked issues).
  • SI-6169 Infer bounds of Java-defined existential types.
  • SI-6566 Right-hand sides of type aliases are now considered invariant for variance checking.
  • SI-5917 Improve public AST creation facilities.
  • SI-8063 Expose much needed methods in public reflection/macro API.
  • SI-8126 Add -Xsource option (make 2.11 type checker behave like 2.10 where possible).

To catch future changes like this early, you can run the compiler under -Xfuture, which makes it behave like the next major version, where possible, to alert you to upcoming breaking changes.


Deprecation is essential to two of the 2.11.x series’ three themes (faster/smaller/stabler). They make the language and the libraries smaller, and thus easier to use and maintain, which ultimately improves stability. We are very proud of Scala’s first decade, which brought us to where we are, and we are actively working on minimizing the downsides of this legacy, as exemplified by 2.11.x’s focus on deprecation, modularization and infrastructure work.

The following language “warts” have been deprecated:

  • SI-7605 Procedure syntax (only under -Xfuture).
  • SI-5479 DelayedInit. We will continue support for the important extends App idiom. We won’t drop DelayedInit until there’s a replacement for important use cases. (More details and a proposed alternative.)
  • SI-6455 Rewrite of .withFilter to .filter: you must implement withFilter to be compatible with for-comprehensions.
  • SI-8035 Automatic insertion of () on missing argument lists.
  • SI-6675 Auto-tupling in patterns.
  • SI-7247 NotNull, which was never fully implemented – slated for removal in 2.12.
  • SI-1503 Unsound type assumption for stable identifier and literal patterns.
  • SI-7629 View bounds (under -Xfuture).

We’d like to emphasize the following library deprecations:

  • #3103, #3191, #3582 Collection classes and methods that are (very) difficult to extend safely have been slated for being marked final. Proxies and wrappers that were not adequately implemented or kept up-to-date have been deprecated, along with other minor inconsistencies.
  • scala-actors is now deprecated and will be removed in 2.12.
  • SI-7958 Deprecate scala.concurrent.future and scala.concurrent.promise
  • SI-3235 Deprecate round on Int and Long (#3581).
  • We are looking for maintainers to take over the following modules: scala-swing, scala-continuations. 2.12 will not include them if no new maintainer is found. We will likely keep maintaining the other modules (scala-xml, scala-parser-combinators), but help is still greatly appreciated.

Deprecation is closely linked to source and binary compatibility. We say two versions are source compatible when they compile the same programs with the same results. Deprecation requires qualifying this statement: “assuming there are no deprecation warnings”. This is what allows us to evolve the Scala platform and keep it healthy. We move slowly to guarantee smooth upgrades, but we want to keep improving as well!

Binary Compatibility

When two versions of Scala are binary compatible, it is safe to compile your project on one Scala version and link against another Scala version at run time. Safe run-time linkage (only!) means that the JVM does not throw a (subclass of) LinkageError when executing your program in the mixed scenario, assuming that none arise when compiling and running on the same version of Scala. Concretely, this means you may have external dependencies on your run-time classpath that use a different version of Scala than the one you’re compiling with, as long as they’re binary compatibile. In other words, separate compilation on different binary compatible versions does not introduce problems compared to compiling and running everything on the same version of Scala.

We check binary compatibility automatically with MiMa. We strive to maintain a similar invariant for the behavior (as opposed to just linkage) of the standard library, but this is not checked mechanically (Scala is not a proof assistant so this is out of reach for its type system).

Forwards and Back

We distinguish forwards and backwards compatibility (think of these as properties of a sequence of versions, not of an individual version). Maintaining backwards compatibility means code compiled on an older version will link with code compiled with newer ones. Forwards compatibility allows you to compile on new versions and run on older ones.

Thus, backwards compatibility precludes the removal of (non-private) methods, as older versions could call them, not knowing they would be removed, whereas forwards compatibility disallows adding new (non-private) methods, because newer programs may come to depend on them, which would prevent them from running on older versions (private methods are exempted here as well, as their definition and call sites must be in the same compilation unit).

These are strict constraints, but they have worked well for us in the Scala 2.10.x series. They didn’t stop us from fixing 372 issues in the 2.10.x series post 2.10.0. The advantages are clear, so we will maintain this policy in the 2.11.x series, and are looking (but not yet commiting!) to extend it to include major versions in the future.


Just like the 2.10.x series, we guarantee forwards and backwards compatibility of the "org.scala-lang" % "scala-library" % "2.11.x" and "org.scala-lang" % "scala-reflect" % "2.11.x" artifacts, except for anything under the scala.reflect.internal package, as scala-reflect is still experimental. We also strongly discourage relying on the stability of scala.concurrent.impl and scala.reflect.runtime, though we will only break compatibility for severe bugs here.

Note that we will only enforce backwards binary compatibility for the new modules (artifacts under the groupId org.scala-lang.modules). As they are opt-in, it’s less of a burden to require having the latest version on the classpath. (Without forward compatibility, the latest version of the artifact must be on the run-time classpath to avoid linkage errors.)

Finally, Scala 2.11.0 introduces scala-library-all to aggregate the modules that constitute a Scala release. Note that this means it does not provide forward binary compatibility, whereas the core scala-library artifact does. We consider the versions of the modules that "scala-library-all" % "2.11.x" depends on to be the canonical ones, that are part of the official Scala distribution. (The distribution itself is defined by the new scala-dist maven artifact.)

New features in the 2.11 series

This release contains all of the bug fixes and improvements made in the 2.10 series, as well as:

  • Collections
    • Immutable HashMaps and HashSets perform faster filters, unions, and the like, with improved structural sharing (lower memory usage or churn).
    • Mutable LongMap and AnyRefMap have been added to provide improved performance when keys are Long or AnyRef (performance enhancement of up to 4x or 2x respectively).
    • BigDecimal is more explicit about rounding and numeric representations, and better handles very large values without exhausting memory (by avoiding unnecessary conversions to BigInt).
    • List has improved performance on map, flatMap, and collect.
    • See also Deprecation above: we have slated many classes and methods to become final, to clarify which classes are not meant to be subclassed and to facilitate future maintenance and performance improvements.
  • Modularization
    • The core Scala standard library jar has shed 20% of its bytecode. The modules for xml, parsing, swing as well as the (unsupported) continuations plugin and library are available individually or via scala-library-all. Note that this artifact has weaker binary compatibility guarantees than scala-library – as explained above.
    • The compiler has been modularized internally, to separate the presentation compiler, scaladoc and the REPL. We hope this will make it easier to contribute. In this release, all of these modules are still packaged in scala-compiler.jar. We plan to ship them in separate JARs in 2.12.x.
  • Reflection, macros and quasiquotes
    • Please see this detailed changelog that lists all significant changes and provides advice on forward and backward compatibility.
    • See also this summary of the experimental side of the 2.11 development cycle.
    • #3321 introduced Sprinter, a new AST pretty-printing library! Very useful for tools that deal with source code.
  • Back-end
    • The GenBCode back-end (experimental in 2.11). See @magarciaepfl’s extensive documentation.
    • A new experimental way of compiling closures, implemented by @JamesIry. With -Ydelambdafy:method anonymous functions are compiled faster, with a smaller bytecode footprint. This works by keeping the function body as a private (static, if no this reference is needed) method of the enclosing class, and at the last moment during compilation emitting a small anonymous class that extends FunctionN and delegates to it. This sets the scene for a smooth migration to Java 8-style lambdas (not yet implemented).
    • Branch elimination through constant analysis #2214
  • Compiler Performance
    • Incremental compilation has been improved significantly. To try it out, upgrade to sbt 0.13.2-M2 and add incOptions := incOptions.value.withNameHashing(true) to your build! Other build tools are also supported. More info at this sbt issue – that’s where most of the work happened. More features are planned, e.g. class-based tracking.
    • We’ve been optimizing the batch compiler’s performance as well, and will continue to work on this during the 2.11.x cycle.
    • Improve performance of reflection SI-6638
  • IDE
  • REPL
  • Warnings
    • Warn about unused private / local terms and types, and unused imports, under -Xlint. This will even tell you when a local var could be a val.
  • Slimming down the compiler
    • The experimental .NET backend has been removed from the compiler.
    • Scala 2.10 shipped with new implementations of the Pattern Matcher and the Bytecode Emitter. We have removed the old implementations.
    • Search and destroy mission for ~5000 chunks of dead code. #1648

License clarification

Scala is now distributed under the standard 3-clause BSD license. Originally, the same 3-clause BSD license was adopted, but slightly reworded over the years, and the “Scala License” was born. We’re now back to the standard formulation to avoid confusion.