scala/scala-collection-compat
{ "createdAt": "2018-03-16T13:39:25Z", "defaultBranch": "main", "description": "makes some Scala 2.13 APIs (primarily collections, also some others) available on 2.11 and 2.12, to aid cross-building", "fullName": "scala/scala-collection-compat", "homepage": "", "language": "Scala", "name": "scala-collection-compat", "pushedAt": "2025-11-24T05:38:15Z", "stargazersCount": 209, "topics": [], "updatedAt": "2025-11-24T05:38:12Z", "url": "https://github.com/scala/scala-collection-compat"}Purpose and scope
Section titled “Purpose and scope”This library makes some Scala 2.13 APIs available on Scala 2.11 and 2.12.
The idea is to facilitate cross-building Scala 2.13 code on the older versions.
Although the name of the library is scala-”collection”-compat, we have now widened the scope to include other parts of the Scala 2.13 standard library besides just collections.
Only the most commonly used APIs are supported; many are missing. Contributions are welcome.
This library is available on the JVM, Scala.js, and Scala Native.
| tool | coordinates |
|---|---|
| sbt | "org.scala-lang.modules" %% "scala-collection-compat" % "<version>" |
| scala-cli, mill | org.scala-lang.modules::scala-collection-compat:<version> |
All future versions will remain backwards binary compatible with 2.0.0. (The 1.0.0 release was withdrawn and should not be used.)
How it works
Section titled “How it works”The Scala 2.13 and Scala 3 versions consist only of an empty scala.collection.compat package object, so import scala.collection.compat._ won’t cause an error in cross-compiled code.
The 2.11 and 2.12 versions have the needed compatibility code in this package.
Changed methods
Section titled “Changed methods”The 2.13 collections redesign did not break source compatibility for most ordinary code, but there are some exceptions.
For example, the to method is used with a type parameter in 2.12:
xs.to[List]With this compatibility library you can instead use the 2.13 syntax, which takes a value parameter:
import scala.collection.compat._xs.to(List)New collections
Section titled “New collections”The library also adds backported versions of new collection types, such as immutable.ArraySeq and immutable.LazyList. (On 2.13, these types are just aliases to standard library types.)
New collection methods
Section titled “New collection methods”Support is included for some 2.13 collections methods such as maxOption.
Other new classes
Section titled “Other new classes”Support is included for some non-collections classes, such as:
@nowarnannotation, added in 2.13.2 and 2.12.13. (The 2.11@nowarndoesn’t do anything, but its presence facilitates cross-compilation.)
Other new methods
Section titled “Other new methods”Support is included for some other methods, such as:
toIntOption(andLong, et al) onString
Migration rules
Section titled “Migration rules”The migration rules use scalafix. Please see the official installation instructions if you are using an old version of sbt (<1.3) or in case the commands below do not work.
addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.10.1")Collection213Upgrade
Section titled “Collection213Upgrade”The Collection213Upgrade rewrite upgrades to the 2.13 collections without the ability to compile the code-base with 2.12 or 2.11. This rewrite is suitable for applications that don’t need to cross-compile against multiple Scala versions.
scalacOptions += "-P:semanticdb:synthetics:on"// sbt shell> scalafixEnable> scalafixAll dependency:Collection213Upgrade@org.scala-lang.modules:scala-collection-migrations:<version>Collection213CrossCompat
Section titled “Collection213CrossCompat”The Collection213CrossCompat rewrite upgrades to the 2.13 collections with the ability to compile the code-base with 2.12 or later. This rewrite is suitable for libraries that are cross-published for multiple Scala versions.
To cross-build for 2.12 and 2.11, the rewrite rule introduces a dependency on the scala-collection-compat module, which provides some APIs of 2.13 on 2.12 and 2.11. This enables you to write your library using the latest 2.13 collections API while still supporting users on an older Scala version.
libraryDependencies += "org.scala-lang.modules" %% "scala-collection-compat" % "<version>"scalacOptions += "-P:semanticdb:synthetics:on"// sbt shell> scalafixEnable> scalafixAll dependency:Collection213CrossCompat@org.scala-lang.modules:scala-collection-migrations:<version>Fixing unused import warnings
Section titled “Fixing unused import warnings”In Scala 2.13 the import scala.collection.compat._ sometimes is not needed (e.g. .to(SeqType) is natively available).
This leads to a unused import warning under Scala 2.13 even though the import is required for Scala 2.12.
In order to work around this, you can pass a compiler option to ignore this specific issue, e.g. in sbt:
scalacOptions += "-Wconf:origin=scala.collection.compat.*:s"Contributing
Section titled “Contributing”The migration tool is not exhaustive. Contributions of additional rewrites are welcome. If you encounter a use case that’s not supported, please report it as described in the [contributing documentation]!(CONTRIBUTING.md#migration-tool).