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Scala 2.13.0

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@SethTisue SethTisue released this 07 Jun 12:15
· 6646 commits to 2.13.x since this release

We are delighted to announce the availability of Scala 2.13.0!

Release summary

2.13 improves Scala in the following areas:

  • Collections: Standard library collections have been overhauled for simplicity, performance, and safety. This is the centerpiece of the release.
  • Standard library: Future is faster and more robust. Elsewhere, useful classes and methods have been added.
  • Language: Literal types, partial unification, by-name implicits, more.
  • Compiler: 5-10% faster, deterministic output, improved optimizer.

To learn more, read on.

Collections redesign

Standard library collections have been overhauled for simplicity, performance, and safety.

This is the centerpiece of the release.

Most ordinary code that used the old collections will continue to work as-is, except as detailed below.

The most important collections changes are:

  • Simpler method signatures
    • No more CanBuildFrom. Transformation methods no longer take an implicit CanBuildFrom parameter.
    • The resulting library is easier to understand (in code, Scaladoc, and IDE code completion).
    • It also makes user code compile faster.
    • A new BuildFrom implicit is used in a handful of places that need it.
  • Simpler type hierarchy
    • No more Traversable and TraversableOnce.
      • They remain only as deprecated aliases for Iterable and IterableOnce.
    • Parallel collections are now a separate module.
      • As a result, GenSeq, GenTraversableOnce, et al are gone.
  • Immutable scala.Seq
    • Seq is now an alias for collection.immutable.Seq
      • Before, it was an alias for the possibly-mutable collection.Seq.
    • This also changes the type of varargs in methods and pattern matches.
    • Arrays passed as varargs are defensively copied. (#6970)
  • Simplified views that work
    • collection.Views have been vastly simplified and should now work reliably.
  • Alphanumeric method names
    • All symbolic operators are now just aliases for descriptive alphanumeric method names.
    • Examples: ++ (alias for concat), +: (alias for prepended), :+ (alias for appended), and so on.
  • New, faster HashMap/Set implementations
    • Both immutable (d5ae93e) and mutable (#7348) versions were completely replaced.
    • They substantially outperform the old implementations in most scenarios.
    • The mutable versions now perform on par with the Java standard library's implementations.
  • New concrete collections
  • New abstract collection type SeqMap

Additional collections changes:

  • New to(Collection) method
    • Replaces old to[Collection] method.
    • The argument is the companion object of the desired collection type, for example .to(Vector).
    • The API change enables support for all collection types (including Map, BitSet, et al).
  • No more collection.breakOut
    • It required CanBuildFrom, which no longer exists.
    • To avoid constructing intermediate collections, use .view and .to(Collection) instead.
  • List and Vector are safer
    • They now offer safe publication under the Java Memory Model, using releaseFence (#6425)
  • Added in-place operations on mutable collections
    • filterInPlace, mapInPlace, flatMapInPlace, and others.
    • These methods update the original collection in-place, rather than creating a new collection to hold the results.
  • Java interop has moved
    • Extension methods for Scala are now in scala.jdk
    • Explicit converters for Java are in scala.jdk.javaapi
    • The reorganization centralizes all to-and-from-Java converters, including both collection and non-collection types, in a single package.
    • Added Stepper, which is like Iterator but supports efficient unboxed iteration, Java interop. (#7458)
  • Collection serialization has changed
    • Collections now use the serialization proxy pattern uniformly whenever possible. (#6676, #7624, scala-dev#562, sbt/sbt#89)
    • In some classloading environments, notably sbt's non-forked test execution, code changes may be needed.
  • Added .unfold
    • This allows constructing a collection or iterator from an initial element and a repeated Option-returning operation, terminating on None.
    • This was added collection companion objects and to Iterator (#6851)
  • Added .lengthIs/.sizeIs and .sizeCompare
    • These allow fluent size comparisons without traversing the whole collection (#6950, #6758)
    • Examples: xs.sizeIs < 10, xs.sizeIs == 2
  • Error-prone Map methods deprecated
    • Deprecated .filterKeys and .mapValues (#7014)
    • Instead, use the new methods of the same names on MapView (e.g. .view.filterKeys)
  • Added .lazyZip
  • Added .tapEach method
    • This allows inserting side-effects in a chain of method calls on a collection or view. (#7124)
  • Added .updatedWith and updateWith methods to Maps
    • updatedWith is on immutable.Map, updateWith is on mutable.Map. (#7320, #7517)
    • These methods allow modifying a map entry based on its current value.
  • Added .maxOption, .minOption, .maxByOption, .minByOption
    • These methods safely handle the empty case. (#6574)
  • Deprecated symbolic methods with multiple arguments
    • Such methods may be disallowed entirely in a future Scala. (#6719)
  • Adding custom collections and operations works very differently
    • See documentation links below.

To learn more about the new APIs and how to adapt your code, consult:

We welcome assistance in continuing to expand and improve these documents.


Futures were internally redesigned, with these goals:

  • provide expected behavior under a broader set of failure conditions
  • provide a foundation for increased performance
  • support more robust applications


  • Updated and revised our Future and Promise implementation. (#6610, #7663)
    • Among other changes, handling of InterruptedException and RejectedExecutionException is improved.
  • Made the global ExecutionContext “batched”. (#7470)
    • (This change was reverted in Scala 2.13.4. The batched executor is available for opt-in, but will no longer be the default. See the 2.13.4 release notes.)
  • Added synchronous ("parasitic") ExecutionContext. (#7784)

Standard library: additions

  • Integrated Java interop (#7987)
    • The old scala-java8-compat module is now part of the standard library. (#7458)
    • This provides converters for options, function types and Java streams.
    • (As mentioned above, collection converters such as JavaConverters were moved to fit into the new scheme.)
  • new: scala.util.Using
    • This uses the loan pattern to provide a simple form of automatic resource management. (#6907, #7468)
  • new: use s interpolator in pattern matches
    • Provides a simple string matcher as the dual of the simple string interpolator. (#7387)
    • Example: val s"$day-$month-$year" = "11-June-2019"
  • new: pipe and tap
    • These chaining operations are available via import scala.util.chaining._. (#7007)
    • Example: 3.pipe(_ * 5) evaluates to 15
    • Example: 9.tap(println) first prints 9, then returns it
  • new: .toIntOption, et al
    • These new extension methods on String are provided by StringOps.
    • They parse literals and efficiently reject invalid input without throwing exceptions. (#6538)
    • Examples: "12".toIntOption => Some(12), "12.3".toIntOption => None, "12.3".toDoubleOption => Some(12.3)
  • new: named Product elements
  • new: .withRight, .withLeft
    • These methods on Left and Right allow upcasting the other type parameter (#7011, #7080)
    • Example: Left(3).withRight[String] has type Either[Int, String] without having to write Int
  • new: Ordering.Double.TotalOrdering, Ordering.Float.TotalOrdering
  • new: converters between functions and extractors
    • New methods provide conversions among optional Functions, PartialFunctions and extractor objects. (#7111)
  • new: @unused annotation
    • This annotation is useful for suppressing warnings under -Xlint. (#7623)

Standard library: changes

  • Library fits in compact1 profile
  • Option extends IterableOnce
    • This improves type inference when calling an overloaded flatMap. (#8038)
  • Undeprecate linesIterator to avoid conflict with JDK 11's String.lines (#7269)
  • PartialFunction now overloads andThen. (#7263)
  • Replaced Cloneable/Serializable traits with type aliases (#6729)
  • ControlThrowable never suppresses (#7413)
  • BigDecimal's handling of MathContexts has changed (#6884)

Standard library: deprecations and removals

Not a complete list, only the deprecations users are likeliest to encounter.

  • String-building using + with a non-String type on the left (aka any2stringadd) is deprecated. (#6315, #6755)
  • PartialFunction.fromFunction replaces PartialFunction.apply (#6703)
  • Right projections on Either are deprecated. (#6682, #8012)
  • Deprecated @usecase Scaladoc tag. (#7462)
  • Deprecated universal Equiv. (#7414)
  • The following modules are no longer included in the distribution: scala-xml, scala-parser-combinators, scala-swing.
    • They are community-maintained and published to Maven Central.
  • Assorted deprecated methods and classes throughout the standard library have been removed entirely.

Language changes

2.13 is primarily a library release, not a language/compiler release. Regardless, some language changes are included:


  • Literal types
    • Literals (for strings, integers, and so on) now have associated literal types. (#5310)
    • See the original proposal, SIP-23, for motivation and details.
    • The compiler will provide instances of a new typeclass scala.ValueOf[T] for all singleton types T.
    • A Singleton upper bound prevents widening (e.g. T <: Int with Singleton).
    • The value of a singleton type can be accessed by calling method valueOf[T]. Example: val one: 1 = valueOf[1]
  • Partial unification on by default
  • By-name implicits with recursive dictionaries
    • This extends by-name method arguments to support implicit (not just explicit) parameters.
    • This enables implicit search to construct recursive values.
    • The flagship use-case is typeclass derivation.
    • Details: see the by-name implicits SIP, #6050, #7368
  • Underscores in numeric literals
    • Underscores can now be used as a spacer. (#6989)
    • Example: 1_000_000

Experimental features:


  • Procedure syntax deprecated
    • Deprecated: def m() { ... } (#6325)
    • Use instead: def m(): Unit = { ... }
  • View bounds deprecated
    • Deprecated: A <% B (#6500)
    • Use instead: (implicit ev: A => B)
  • Symbol literals deprecated
    • Symbols themselves remain supported, only the single-quote syntax is deprecated. (#7395)
    • Library designers may wish to change their APIs to use String instead.
    • Deprecated: 'foo
    • Use instead: Symbol("foo")
  • Unicode arrows deprecated
    • In particular, the single arrow operators had the wrong precedence. (#7540)
    • Deprecated: , ,
    • Use instead: =>, ->, <-
  • postfixOps syntax disabled by default
    • The syntax, already deprecated in 2.12, causes an error in 2.13 unless the feature is explicitly enabled. (#6831)
    • Error: xs size
    • Use instead: xs.size


  • Imports, including wildcard imports, now shadow locally defined identifiers. (#6589)
  • Underscore is no longer a legal identifier unless backquoted (bug#10384)
    • val _ = is now a pattern match (and discards the value without incurring a warning)
    • implicit val _ = is also now a pattern match (and is useless, because it no longer adds to implicit scope)
  • Don't assume unsound type for ident/literal patterns. (#6502)
    • Matches of the form case x@N involve calling equals, so it was unsound to type x as N.type.
    • Consider rewriting as case x:N.type.
  • Make extractor patterns null safe. (#6485)
    • null is treated as no match.
  • Better typing for overloaded higher-order methods (#6871, #7631)
    • This change was a key enabler for the new collections design.
  • Rework unification of Object and Any in Java/Scala interop (#7966)
  • Name-based pattern matching has changed to enable immutable Seq matches (#7068)
  • Automatic eta-expansion of zero-argument methods is no longer deprecated (#7660)
  • Improve binary stability of extension methods (#7896)
  • Macros must now have explicit return types (#6942)
  • Mixin fields with trait setters are no longer JVM final (#7028)
    • In addition, object fields are now static (#7270)
  • Support implicitNotFound on parameters (#6340)
  • Disallow repeated parameters except in method signatures (#7399)
  • Value-discard warnings can be suppressed via type ascription to Unit. (#7563)
  • x op () now parses as x.op(()) not x.op() (#7684)


  • Deterministic, reproducible compilation
    • The compiler generates identical output for identical input in more cases, for reproducible builds. (scala-dev#405)
  • Optimizer improvements
    • Operations on collections and arrays are now optimized more, including improved inlining. (#7133)


  • Scaladoc supports setting canonical URLs (#7834)
    • This helps search engines identify the most relevant/recent version of a page when several versions are available.
  • Compiler suggests possible corrections for unrecognized identifiers (#6711)
    • Example: List(1).sizzle => value sizzle is not a member of List[Int], did you mean size?
  • Added -Yimports for custom preamble imports. (#6764)
    • Example: -Yimports:x,y,z means x, y, and z are root imports of the form: import x._ { import y._ { import z._ { ... } } }
  • The scala-compiler JAR no longer depends on scala-xml (#6436)

Plus, changes to compiler options:

  • Partition options by function: -V for verbose, -W for warnings
    • In general, the old flags still exist as aliases. (#7908)
    • Exceptions (breaking changes) include:
      • Replaced -warn-option with -Woption.
      • Replaced -Xprint:all with -Vprint:_
    • -Werror is now recommended over -Xfatal-warnings.
  • Promoted -deprecation to -Xlint:deprecation (#7714)
  • Deprecated -Xfuture (#7328)
    • Instead, use e.g. -Xsource:2.14
  • Removed -Yno-adapted-args
  • Removed -Xmax-classfile-length
    • It's hard-coded to 240 now (#7497)

Scripting, environment, and integrations

  • The script runner (scala Script.scala) no longer uses the fsc compilation daemon by default. (#6747)
    • The daemon was not reliable and will likely be removed entirely from a future release.
  • JEP 293 style long command-line options are now supported (#6499)
  • The REPL has undergone an internal refactoring to enable future improvements. (#7384)
  • Ant support is no longer bundled. (#6255)

Compiler performance

We invested heavily in compiler speedups during the 2.13 cycle, but most of that work actually already landed in the 2.12.x series, with more to come in 2.12.9.

In addition, compiler performance in 2.13 is 5-10% better compared to 2.12, thanks mainly to the new collections. See performance graph.

Also, certain kinds of code now compile much faster because the compiler aggressively prunes polymorphic implicits during search (#6580).


Upgrading from 2.12? Enable -Xmigration while upgrading to request migration advice from the compiler.


Like Scala 2.12, the 2.13 series targets Java 8, minimum. Both 2.12 and 2.13 also work on later JDKs such as JDK 11; see our JDK Compatibility Guide.

Although Scala 2.11, 2.12, and 2.13 are mostly source compatible to facilitate cross-building, they are not binary compatible. This allows us to keep improving the Scala compiler and standard library.

All 2.13.x releases will be fully binary compatible with 2.13.0, in according with the policy we have followed since 2.10.

Projects built with sbt must use at least sbt 1.2.8 (or 0.13.18) to use Scala 2.13. To move to 2.13, bump the scalaVersion setting in your existing project.

Scala also works with Maven, Gradle, and other build tools.

Obtaining Scala

Scala releases are available through a variety of channels, including (but not limited to):

  • Bump the scalaVersion setting in your sbt-based project
  • Download a distribution from
  • Obtain JARs via Maven Central
  • Certain package managers (such as homebrew and SDKMAN) offer Scala.

Reporting bugs

Please file any bugs you encounter on our issue tracker. If you aren't yet sure something is a bug, ask on


A big thank you to everyone who's helped improve Scala by reporting bugs, improving our documentation, kindly helping others on forums and at meetups, and submitting and reviewing pull requests! You are all magnificent.

Scala 2.13.0 is the result of merging over 1500 pull requests.

The pull request queue is managed by the Scala team at Lightbend: Adriaan Moors, Lukas Rytz, Jason Zaugg, Seth Tisue, Stefan Zeiger, Eugene Yokota.

Thanks to Lightbend for their continued sponsorship of the Scala core team’s efforts. Lightbend offers commercial support for Scala.

The new collections API was developed in fruitful collaboration with the Scala Center, with major contributions from the community.

This release was brought to you by 162 contributors, according to git shortlog -sn --no-merges 2.13.x ^2.12.x ^e2a211c. Thank you Julien Richard-Foy, Lukas Rytz, Jason Zaugg, A. P. Marki, Stefan Zeiger, Kenji Yoshida, Adriaan Moors, Josh Lemer, NthPortal, Miles Sabin, Diego E. Alonso-Blas, Seth Tisue, Guillaume Martres, Marcelo Cenerino, Dale Wijnand, Odd Möller, Martin Odersky, Allan Renucci, Georgi Krastev, Eugene Yokota, Harrison Houghton, Martijn Hoekstra, Viktor Klang, Aaron S. Hawley, Ólafur Páll Geirsson, Rex Kerr, hepin1989, Philippus Baalman, Matthew de Detrich, Isaac Levy, exoego, Greg Methvin, Jasper Moeys, Sébastien Doeraene, wholock, Aggelos Biboudis, yui-knk, Matthias Sperl, Xavier Fernández Salas, Ethan Pronovost, Janek Bogucki, awibisono, BuildTools, Mike Skells, Jimin Hsieh, Jonathan Frawley, Xavier GUIHOT, Chris Phelps, chanyong.moon, Cong Zhao, Enno Runne, LPTK, Pathikrit Bhowmick, Yuval Dagan, Li Haoyi, Guillaume Massé, Christopher Hunt, Kamil Kloch, Marco Zühlke, Danila Matveev, Juliusz Sompolski,杨博 (Yang Bo), Masaru Nomura, Benjamin Kurczyk, Vince, taku0, Arnout Engelen, Tim Ruhland, Nicolas Stucki, Nicolas Rinaudo, Stephen Nancekivell, ashwin, Kobenko Mikhail, Song Kun, Anthony Tjuatja, k.bigwheel, ke-to, kelebra, mcintdan, mmocentre, psilospore, roman, svatsan, texasbruce, tim-zh, valydia, veera venky,虎鸣, Adianto Wibisono, Alden Torres, Alejandro Sellero, Alessandro Buggin, Alex Glukhovtsev, Alex Levenson, Alexey-NM, Anatolii, Andrei Baidarov, Andriy Plokhotnyuk, Bakhytzhan Karabalin, Benni, Callum Turnbull, Chris Birchall, Chujie Zeng, Cody Allen, Daniel Dietrich, Daniel Moss, Daniel Slapman, David Barri, David Hoepelman, Denis Buzdalov, Denys Shabalin, Dhanesh, Dhanesh Arole, Edin Dudojević, Eugene Platonov, Faiz Halde, Gabriel Claramunt, Heikki Vesalainen, Iaroslav Zeigerman, Jack Koenig, Jean Michel Rouly, Jeff Brower, Jeff Shaw, Josh, Kazuhiro Sera, Kentaro Tokutomi, Lionel Parreaux, Magnolia.K, Martin Duhem, Michael Steindorfer, Nafer Sanabria, Narek Asadorian, Oleksii Tkachuk, Oscar Boykin, PJ Fanning, Paolo Giarrusso, Pap Lőrinc, Pavel Petlinsky, Peter Fraenkel, Philip, Piotr Kukielka, Qiang Sima, Rob Norris, Robin Stephenson, Rui Gonçalves, Ruud Welling, Ryan McDougall, ShankarShastri, Simão Martins, Sperl Matthias, Sujeet Kausallya Gholap, Uttej Guduru, Vincent de Haan, Vladimir Parfinenko, Vlastimil Dort, Yang Bo, Zizheng Tai, ccjoywang, esarbe, howtonotwin, jvican.


We again thank our contributors and the entire Scala community.

May you find Scala 2.13 a joy to code in!