This document describes the main changes for collection users that migrate to Scala 2.13 and shows how to cross-build projects with Scala 2.11 / 2.12 and 2.13.

For an in-depth overview of the Scala 2.13 collections library, see the collections guide. The implementation details of the 2.13 collections are explained in the document the architecture of Scala collections.

The most important changes in the Scala 2.13 collections library are:

• scala.Seq[+A] is now an alias for scala.collection.immutable.Seq[A] (instead of scala.collection.Seq[A]). Note that this also changes the type of Scala varargs methods.
• scala.IndexedSeq[+A] is now an alias for scala.collection.immutable.IndexedSeq[A] (instead of scala.collection.IndexedSeq[A]).
• Transformation methods no longer have an implicit CanBuildFrom parameter. This makes the library easier to understand (in source code, Scaladoc, and IDE code completion). It also makes compiling user code more efficient.
• The type hierarchy is simplified. Traversable no longer exists, only Iterable.
• The to[Collection] method was replaced by the to(Collection) method.
• The toC methods are strict by convention and yield the default collection type where applicable. For example, Iterator.continually(42).take(10).toSeq produces a List[Int] and without the limit would not.
• toIterable is deprecated wherever defined. For Iterator, in particular, prefer to(LazyList).
• Views have been vastly simplified and work reliably now. They no longer extend their corresponding collection type, for example, an IndexedSeqView no longer extends IndexedSeq.
• collection.breakOut no longer exists, use .view and .to(Collection) instead.
• Immutable hash sets and hash maps have a new implementation (ChampHashSet and ChampHashMap, based on the “CHAMP” encoding).
• New collection types:
  • immutable.ArraySeq is an effectively immutable sequence that wraps an array
  • immutable.LazyList is a linked list that is lazy in its state, i.e., whether it’s empty or non-empty. This allows creating a LazyList without evaluating the head element. immutable.Stream, which has a strict head and a lazy tail, is deprecated.
• Deprecated collections were removed (MutableList, immutable.Stack, others)
• Parallel collections are now in a separate hierarchy in a separate module.
• The scala.jdk.StreamConverters object provides extension methods to create (sequential or parallel) Java 8 streams for Scala collections.

Tools for migrating and cross-building

The scala-collection-compat is a library released for 2.11, 2.12 and 2.13 that provides some new APIs from Scala 2.13 for the older versions. This simplifies cross-building projects.

The module also provides migration rules for scalafix that can update a project’s source code to work with the 2.13 collections library.

scala.Seq, varargs and scala.IndexedSeq migration

In Scala 2.13 scala.Seq[+A] is an alias for scala.collection.immutable.Seq[A], instead of scala.collection.Seq[A], and scala.IndexedSeq[+A] is an alias for scala.collection.immutable.IndexedSeq[A]. These changes require some planning depending on how your code is going to be used.

The change in definition of scala.Seq also has the effect of making the type of varargs parameters immutable sequences, due to SLS 6.6, so in a method such as orderFood(xs: _*) the varargs parameter xs must be an immutable sequence.

Therefore, any method signature in Scala 2.13 which includes scala.Seq, varargs, or scala.IndexedSeq is going to have a breaking change in API semantics (as the immutable sequence types require more — immutability — than the not-immutable types). For example, users of a method like def orderFood(order: Seq[Order]): Seq[Food] would previously have been able to pass in an ArrayBuffer of Order, but cannot in 2.13.

Migrating varargs

The change for varargs is unavoidable, as you cannot change the type used at definition site. The options available for migrating the usage sites are the following:

• change the value to already be an immutable sequence, which allows for direct varargs usage: xs: _*,
• change the value to be an immutable sequence on the fly by calling .toSeq: xs.toSeq: _*, which will only copy data if the sequence wasn’t already immutable
• use scala.collection.immutable.ArraySeq.unsafeWrapArray to wrap your array and avoid copying, but see its scaladoc

Option 1: migrate back to scala.collection.Seq

The first, in some ways simplest, migration strategy for all non-varargs usages of scala.Seq is to replace them with scala.collection.Seq (and require users to call .toSeq or unsafeWrapArray when passing such sequences to varargs methods).

We recommend using import scala.collection/import scala.collection.immutable and collection.Seq/immutable.Seq.

We recommend against using import scala.collection.Seq, which shadows the automatically imported scala.Seq, because even if it’s a one-line change it causes name confusion. For code generation or macros the safest option is using the fully-qualified _root_.scala.collection.Seq.

As an example, the migration would look something like this:

import scala.collection

object FoodToGo {
  def orderFood(order: collection.Seq[Order]): collection.Seq[Food]
}

However, users of this code in Scala 2.13 would also have to migrate, as the result type is source-incompatible with any scala.Seq (or just Seq) usage in their code:

val food: Seq[Food] = FoodToGo.orderFood(order) // won't compile

The simplest workaround is to ask your users to call .toSeq on the result which will return an immutable Seq, and only copy data if the sequence wasn’t immutable:

val food: Seq[Food] = FoodToGo.orderFood(order).toSeq // add .toSeq

Option 2: use scala.collection.Seq for parameters and scala.collection.immutable.Seq for result types

The second, intermediate, migration strategy would be to change all methods to accept not-immutable Seq but return immutable Seq, following the robustness principle (also known as “Postel’s law”):

import scala.collection
import scala.collection.immutable

object FoodToGo {
  def orderFood(order: collection.Seq[Order]): immutable.Seq[Food]
}

Option 3: use immutable sequences

The third migration strategy is to change your API to use immutable sequences for both parameter and result types. When cross-building your library for Scala 2.12 and 2.13 this could either mean:

• continuing to use scala.Seq which means it stays source and binary-compatible in 2.12, but would have to have immutable sequence semantics (but that might already be the case).
• switch to explicitly using immutable Seq in both Scala 2.12 and 2.13, which means breaking source, binary and (possibly) semantic compatibility in 2.12:

import scala.collection.immutable

object FoodToGo {
  def orderFood(order: immutable.Seq[Order]): immutable.Seq[Food]
}

Shadowing scala.Seq and scala.IndexedSeq

You maybe be interested in entirely banning plain Seq usage. You can use the compiler to do so by declaring your own package-level (and package private) Seq type which will mask scala.Seq.

package example

import scala.annotation.compileTimeOnly

/**
  * In Scala 2.13, `scala.Seq` changed from aliasing `scala.collection.Seq` to aliasing
  * `scala.collection.immutable.Seq`.  In this code base usage of unqualified `Seq` is banned: use
  * `immutable.Seq` or `collection.Seq` instead.
  *
  * import scala.collection
  * import scala.collection.immutable
  *
  * This `Seq` trait is a dummy type to prevent the use of `Seq`.
  */
@compileTimeOnly("Use immutable.Seq or collection.Seq")
private[example] trait Seq[A1]

/**
  * In Scala 2.13, `scala.IndexedSeq` changed from aliasing `scala.collection.IndexedSeq` to aliasing
  * `scala.collection.immutable.IndexedSeq`.  In this code base usage of unqualified `IndexedSeq` is
  * banned: use `immutable.IndexedSeq` or `collection.IndexedSeq`.
  *
  * import scala.collection
  * import scala.collection.immutable
  *
  * This `IndexedSeq` trait is a dummy type to prevent the use of `IndexedSeq`.
  */
@compileTimeOnly("Use immutable.IndexedSeq or collection.IndexedSeq")
private[example] trait IndexedSeq[A1]

This might be useful during the migration to catch usages of unqualified Seq and IndexedSeq.

What are the breaking changes?

The following table summarizes the breaking changes. The “Automatic Migration Rule” column gives the name of the migration rule that can be used to automatically update old code to the new expected form.

Some classes have been removed, made private or have no equivalent in the new design:

• ArrayStack
• mutable.FlatHashTable
• mutable.HashTable
• History
• Immutable
• IndexedSeqOptimized
• LazyBuilder
• mutable.LinearSeq
• LinkedEntry
• MapBuilder
• Mutable
• MutableList
• Publisher
• ResizableArray
• RevertibleHistory
• SeqForwarder
• SetBuilder
• Sizing
• SliceInterval
• StackBuilder
• StreamView
• Subscriber
• Undoable
• WrappedArrayBuilder

Other notable changes are:

• Iterable.partition invokes iterator twice on non-strict collections and assumes it gets two iterators over the same elements. Strict subclasses override partition do perform only a single traversal
• Equality between collections is not anymore defined at the level of Iterable. It is defined separately in the Set, Seq and Map branches. Another consequence is that Iterable does not anymore have a canEqual method.

• The new collections makes more use of overloading. You can find more information about the motivation behind this choice here. For instance, Map.map is overloaded:

  scala> Map(1 -> "a").map
    def map[B](f: ((Int, String)) => B): scala.collection.immutable.Iterable[B]
    def map[K2, V2](f: ((Int, String)) => (K2, V2)): scala.collection.immutable.Map[K2,V2]
  

  Type inference has been improved so that Map(1 -> "a").map(x => (x._1 + 1, x._2)) works, the compiler can infer the parameter type for the function literal. However, using a method reference in 2.13.0-M4 (improvement are on the way for 2.13.0) does not work, and an explicit eta-expansion is necessary:

  scala> def f(t: (Int, String)) = (t._1 + 1, t._2)
  scala> Map(1 -> "a").map(f)
                          ^
        error: missing argument list for method f
        Unapplied methods are only converted to functions when a function type is expected.
        You can make this conversion explicit by writing `f _` or `f(_)` instead of `f`.
  scala> Map(1 -> "a").map(f _)
  res10: scala.collection.immutable.Map[Int,String] = ChampHashMap(2 -> a)
  

• Views have been completely redesigned, and we expect their usage to have a more predictable evaluation model. You can read more about the new design here.
• mutable.ArraySeq (which wraps an Array[AnyRef] in 2.12, meaning that primitives were boxed in the array) can now wrap boxed and unboxed arrays. mutable.ArraySeq in 2.13 is in fact equivalent to WrappedArray in 2.12, there are specialized subclasses for primitive arrays. Note that a mutable.ArraySeq can be used either way for primitive arrays (TODO: document how). WrappedArray is deprecated.
• There is no “default” Factory (previously known as [A, C] => CanBuildFrom[Nothing, A, C]): use Factory[A, Vector[A]] explicitly instead.
• Array.deep has been removed.

Breaking changes with old syntax still supported

The following table lists the changes that continue to work with a deprecation warning.

Deprecated things in 2.12 that have been removed in 2.13

• collection.JavaConversions. Use scala.jdk.CollectionConverters instead. Previous advice was to use collection.JavaConverters which is now deprecated ;
• collection.mutable.MutableList (was not deprecated in 2.12 but was considered to be an implementation detail for implementing other collections). Use an ArrayDeque or mutable.ListBuffer instead, or a List and a var ;
• collection.immutable.Stack. Use a List instead ;
• StackProxy, MapProxy, SetProxy, SeqProxy, etc. No replacement ;
• SynchronizedMap, SynchronizedBuffer, etc. Use java.util.concurrent instead ;

Are there new collection types?

scala.collection.immutable.ArraySeq is an immutable sequence backed by an array. It is used to pass varargs parameters.

The scala-collection-contrib module provides decorators enriching the collections with new operations. You can think of this artifact as an incubator: if we get evidence that these operations should be part of the core, we might eventually move them.

The following collections are provided:

• MultiSet (both mutable and immutable)
• SortedMultiSet (both mutable and immutable)
• MultiDict (both mutable and immutable)
• SortedMultiDict (both mutable and immutable)

Are there new operations on collections?

The following new partitioning operations are available:

def groupMap[K, B](key: A => K)(f: A => B): Map[K, CC[B]] // (Where `CC` can be `List`, for instance)
def groupMapReduce[K, B](key: A => K)(f: A => B)(g: (B, B) => B): Map[K, B]

groupMap is equivalent to groupBy(key).mapValues(_.map(f)).

groupMapReduce is equivalent to groupBy(key).mapValues(_.map(f).reduce(g)).

Mutable collections now have transformation operations that modify the collection in place:

def mapInPlace(f: A => A): this.type
def flatMapInPlace(f: A => IterableOnce[A]): this.type
def filterInPlace(p: A => Boolean): this.type
def patchInPlace(from: Int, patch: scala.collection.Seq[A], replaced: Int): this.type

Other new operations are distinctBy and partitionMap

def distinctBy[B](f: A => B): C // `C` can be `List[Int]`, for instance
def partitionMap[A1, A2](f: A => Either[A1, A2]): (CC[A1], CC[A2]) // `CC` can be `List`, for instance

Last, additional operations are provided by the scala-collection-contrib module. You can think of this artifact as an incubator: if we get evidence that these operations should be part of the core, we might eventually move them.

The new operations are provided via an implicit enrichment. You need to add the following import to make them available:

import strawman.collection.decorators._

The following operations are provided:

• Seq
  • intersperse
• Map
  • zipByKey / join / zipByKeyWith
  • mergeByKey / fullOuterJoin / mergeByKeyWith / leftOuterJoin / rightOuterJoin

Are there new implementations of existing collection types (changes in performance characteristics)?

The default Set and Map are backed by a ChampHashSet and a ChampHashMap, respectively. The performance characteristics are the same but the operation implementations are faster. These data structures also have a lower memory footprint.

mutable.Queue and mutable.Stack now use mutable.ArrayDeque. This data structure supports constant time index access, and amortized constant time insert and remove operations.

How do I cross-build my project against Scala 2.12 and Scala 2.13?

Most usages of collections are compatible and can cross-compile 2.12 and 2.13 (at the cost of some warnings, sometimes).

If you cannot get your code to cross-compile, there are various solutions:

• You can use the scala-collection-compat library, which makes some of 2.13’s APIs available to 2.11 and 2.12. This solution does not always work, for example if your library implements custom collection types.
• You can maintain a separate branch with the changes for 2.13 and publish releases for 2.13 from this branch.

• You can put source files that don’t cross-compile in separate directories and configure sbt to assemble the sources according to the Scala version (see also the examples below):

  // Adds a `src/main/scala-2.13+` source directory for Scala 2.13 and newer
  // and a `src/main/scala-2.13-` source directory for Scala version older than 2.13
  unmanagedSourceDirectories in Compile += {
    val sourceDir = (sourceDirectory in Compile).value
    CrossVersion.partialVersion(scalaVersion.value) match {
      case Some((2, n)) if n >= 13 => sourceDir / "scala-2.13+"
      case _                       => sourceDir / "scala-2.13-"
    }
  }
  

Examples of libraries that cross-compile with separate source directories:

• https://github.com/scala/scala-parser-combinators/pull/152
• https://github.com/scala/scala-xml/pull/222
• Some other examples are listed here: https://github.com/scala/community-builds/issues/710

Collection Implementers

To learn about differences when implementing custom collection types or operations, see the following documents:

• The architecture of Scala collections
• Implementing custom collections
• Adding custom collection operations