-
Notifications
You must be signed in to change notification settings - Fork 1k
Add guidance on how to migrate scala.Seq #1326
New issue
Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.
By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.
Already on GitHub? Sign in to your account
Changes from 4 commits
2d8f8e5
8178166
54b38f7
6c56623
2b4de2d
cc9b455
File filter
Filter by extension
Conversations
Jump to
Diff view
Diff view
There are no files selected for viewing
Original file line number | Diff line number | Diff line change |
---|---|---|
|
@@ -10,6 +10,7 @@ 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]({{ site.baseurl }}/overviews/collections-2.13/introduction.html). The implementation details of the 2.13 collections are explained in the document [the architecture of Scala collections]({{ site.baseurl }}/overviews/core/architecture-of-scala-213-collections.html). | ||
|
||
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. | ||
- 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. | ||
|
@@ -22,14 +23,38 @@ The most important changes in the Scala 2.13 collections library are: | |
- Deprecated collections were removed (`MutableList`, `immutable.Stack`, others) | ||
- Parallel collections are now in a separate hierarchy in a [separate module](https://github.com/scala/scala-parallel-collections). | ||
- The `scala.jdk.StreamConverters` object provides extension methods to create (sequential or parallel) Java 8 streams for Scala collections. | ||
- `scala.Seq` is now an alias for `scala.collection.immutable.Seq` (no longer `scala.collection.Seq`). Note that this also changes the type of Scala varargs methods. | ||
|
||
## Tools for migrating and cross-building | ||
|
||
The [scala-collection-compat](https://github.com/scala/scala-collection-compat) is a library released for 2.11, 2.12 and 2.13 that provides some of the new APIs from Scala 2.13 for the older versions. This simplifies cross-building projects. | ||
|
||
The module also provides [migratrion rules](https://github.com/scala/scala-collection-compat#migration-tool) for [scalafix](https://scalacenter.github.io/scalafix/docs/users/installation.html) that can update a project's source code to work with the 2.13 collections library. | ||
|
||
## scala.Seq migration | ||
|
||
In Scala 2.13 `scala.Seq[+A]` is an alias for `scala.collection.immutable.Seq[A]`, instead of `scala.collection.Seq[A]`. This change requires some planning depending on how your code is going to be used. | ||
|
||
If you're making an application, and simply migrating a Scala 2.12 code base to 2.13, it might be ok to keep using `scala.Seq` in your code. | ||
|
||
If you're making a library intended to be used by other programmers, then using `scala.Seq` or varargs is going to be a breaking change in the API semantics. For example, if there was a function `def orderFood(order: Seq[Order]): Seq[Food]`, previously the library user would have been able to pass in an array of `Order`, but it won't work for 2.13. | ||
|
||
- if you cross build with Scala 2.12 and want to maintain the API semantics for 2.13 version of your library, or | ||
- if your library users frequently uses mutable collections such as `Array` | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. If code has been optimized to the point that you build an array instead of a regular collection, you're probably okay calling There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Arrays might come up simply as Java API that you provide as a subset of API. |
||
|
||
you can import `scala.collection.Seq` ("CSeq") explicitly in your code. | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. It might be worth mentioning that this is a potentially source-breaking change. If a user in 2.12 has:
Then that will no longer compile in 2.13 There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Also, it might be a good strategy for APIs to only move to There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I've added this in 2b4de2d. |
||
|
||
~~~ scala | ||
import scala.collection.Seq | ||
~~~ | ||
|
||
In the future when your API is able to break the source compatibility, it might also make sense to migrate towards the `scala.collection.immutable.Seq` ("ISeq") for both Scala 2.12 and Scala 2.13. | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. it doesn't seem like you refer to "ISeq" or "CSeq" anywhere, so the parentheticals can probably be removed There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. ok. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Added commits that ended up adding references to ISeq and CSeq. |
||
|
||
~~~ scala | ||
import scala.collection.immutable.Seq | ||
~~~ | ||
|
||
Note that in Scala 2.13 the sequence passed into as a varargs as `orderFood(xs: _*)` must also be immutable. This is because the sequence passed in as a varargs must conform to `scala.Seq` according to [SLS 6.6](https://www.scala-lang.org/files/archive/spec/2.12/06-expressions.html#function-applications). Thus, if your API exposes varargs it will be an unavoidable breaking change. This might affect Java interoperability. | ||
|
||
## 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. | ||
|
@@ -77,7 +102,6 @@ Some classes have been removed, made private or have no equivalent in the new de | |
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 | ||
- `scala.Seq[+A]` is now `scala.collection.immutable.Seq[A]` (this also affects varargs methods). | ||
- 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](http://scala-lang.org/blog/2017/05/30/tribulations-canbuildfrom.html). For instance, `Map.map` is overloaded: | ||
|
@@ -236,4 +260,4 @@ Examples of libraries that cross-compile with separate source directories: | |
To learn about differences when implementing custom collection types or operations, see the following documents: | ||
- [The architecture of Scala collections]({{ site.baseurl }}/overviews/core/architecture-of-scala-213-collections.html) | ||
- [Implementing custom collections]({{ site.baseurl }}/overviews/core/custom-collections.html) | ||
- [Adding custom collection operations]({{ site.baseurl }}/overviews/core/custom-collection-operations.html) | ||
- [Adding custom collection operations]({{ site.baseurl }}/overviews/core/custom-collection-operations.html) |
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
This sounds like "keep using
scala.Seq
" should be the safe and simple option. I'd argue it's the opposite (unless your code happens to just work out of the box). In a complex codebase the simplest way forward is to change all references fromscala.Seq
toscala.collection.Seq
and possibly inserttoSeq
calls where necessary. This is easy to do and will cross-compile on 2.12 without any changes in semantics.Switching your own codebase from
scala.collection.Seq
toscala.immutable.Seq
is the more advanced refactoring. In many cases it may have been the intention all along, so it makes sense to do it. It also avoids unnecessary copying when varargs are involved, but it's much harder to get right consistently in a large codebase.There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
Interesting. I'll mention recommendation to use scala.collection.Seq first then.
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
I've added this in 2b4de2d.