scala / scala-collection-compat

Scala 2.13 Collection Compatibility Library


Build Status

Scala 2.13 Collection Compatibility Library And Migration Tool

Compatibility Library

This library provides some of the new APIs from Scala 2.13 to Scala 2.11 and 2.12. It can be used to cross-build projects. To use this library, add the following to your build.sbt:

libraryDependencies += "org.scala-lang.modules" %% "scala-collection-compat" % "2.0.0"

Version 2.0.0 is compatible with Scala 2.13.0-RC2, 2.12 and 2.11.

Note that there are multiple ways to cross-build projects, see

Note: Please do not release any artifacts against version 1.0.0. Due to #195 we are planning on releasing 2.0.0 that is neither source nor binary compatible with 1.0.0.

Starting with version 1.0.0 backwards binary compatibility will be enforced within each major version (i.e. anything prior to 2.0.0 will be compatible).

The 2.13 collections are mostly backwards compatible, but there are some exceptions. For example, the to method is used with a type parameter in 2.12:[List]

With this compatibility library you can also use the 2.13 syntax which uses a companion object:

  import scala.collection.compat._

The 2.13 version consists only of an empty scala.collection.compat package object that allows you to write import scala.collection.compat._ in 2.13. The 2.11/2.12 version has the compatibility extensions in this package.

The library also adds backported versions of new collection types, currently scala.collection.compat.immutable.ArraySeq. In 2.11/2.12, this type is a new collection implementation. In 2.13, it is an alias for scala.collection.immutable.ArraySeq.

Migration Tool

The migration rules use scalafix. Please see the official installation instruction and, in particular, check that your full Scala version is supported (ex 2.12.8).

// project/plugins.sbt

addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.0")


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.

// build.sbt
scalafixDependencies in ThisBuild += "org.scala-lang.modules" %% "scala-collection-migrations" % "2.0.0"
scalacOptions ++= List("-Yrangepos", "-P:semanticdb:synthetics:on")
// sbt shell
> ;test:scalafix Collections213Upgrade ;scalafix Collections213Upgrade


The Collections213CrossCompat 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 the syntax 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.

// build.sbt
scalafixDependencies in ThisBuild += "org.scala-lang.modules" %% "scala-collection-migrations" % "2.0.0"
libraryDependencies +=  "org.scala-lang.modules" %% "scala-collection-compat" % "2.0.0"
scalacOptions ++= List("-Yrangepos", "-P:semanticdb:synthetics:on")
// sbt shell
> ;test:scalafix Collection213CrossCompat ;scalafix Collection213CrossCompat


// If you are using project/Build.scala add the following imports:
import scalafix.sbt.ScalafixPlugin.autoImport.{scalafixDependencies, scalafixSemanticdb}


The migration tool is not exhaustive and we will continue to improve it over time. If you encounter a use case that’s not supported, please report it as described in the contributing documentation.