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At Scala Days Lausanne 2019 in July, Martin Odersky, the lead designer of Scala, gave a tour of the upcoming major version, Scala 3.0. He talked about the roadmap to Scala 3.0, its new features, how its situation is different from Python 2 vs 3, and much more.

Roadmap to Scala 3.0

Odersky announced that “Scala 3.0 has almost arrived” since all the features are fleshed out, with implementations in the latest releases of Dotty, the next generation compiler for Scala. The team plans to go into feature freeze and release Scala 3.0 M1 in fall this year.

Following that the team will focus on stabilization, complete SIP process, and write specs and user docs. They will also work on community build, compatibility, and migration tasks. All these tasks will take about a year, so we can expect Scala 3.0 release in fall 2020.

Scala 2.13 was released in June this year. It was shipped with redesigned collections, updated futures implementation, language changes including literal types, partial unification on by default, by-name implicits, macro annotations, among others. The team is also working simultaneously on its next release, Scala 2.14. Its main focus will be to ease out the migration process from Scala 2 to 3 by defining migration tools, shim libraries, targeted deprecations, and more.


What’s new in this major release

There is a whole lot of improvements coming in Scala 3.0, some of which Odersky discussed in his talk:

  • Scala will drop the ‘new’ keyword: Starting with Scala 3.0, you will be able to omit ‘new’ from almost all instance creations. With this change, developers will no longer have to define a case class just to get nice constructor calls. Also, this will prevent accidental infinite loops in the cases when ‘apply’ has the same arguments as the constructor.
  • Top-level definitions: In Scala 3.0, top-level definitions will be added as a replacement for package objects. This is because only one package object definition is allowed per package. Also, a trait or class defined in the package object is different from the one defined in the package, which can lead to unexpected behavior.
  • Redesigned enumeration support: Previously, Scala did not provide a very straightforward way to define enums. With this update, developers will have a simple way to define new types with a finite number of values or constructions. They will also be able to add parameters and define fields and methods.
  • Union types: In previous versions, union types can be defined with the help of constructs such as Either or subtyping hierarchies, but these constructs are bulkier. Adding union types to the language will fix Scala’s least upper bounds problem and provide added modelling power.
  • Extension methods: With extension methods, you can define methods that can be used infix without any boilerplate. These will essentially replace implicit classes.
  • Delegates: Implicit is a “bedrock of programming” in Scala. However, they suffer from several limitations. Odersky calls implicit conversions “recipe for disaster” because they tend to interact very badly with each other and add too much implicitness. Delegates will be their simpler and safer alternative.
  • Functions everywhere: In Scala, functions and methods are two different things. While methods are members of classes and objects, functions are objects themselves. Until now, methods were quite powerful as compared to functions. They are defined by properties like dependent, polymorphic, and implicit. With Scala 3.0, these properties will be associated with functions as well.

Recent discussions regarding the updates in Scala 3.0

A minimal alternative for scala-reflect and TypeTag

Scala 3.0 will drop support for ‘scala-reflect’ and ‘TypeTag’. Also, there hasn’t been much discussion about its alternative. However, some developers believe that it is an important feature and is currently in use by many projects including Spark and doobie.

Explaining the reason behind dropping the support, a SIP committee member, wrote on the discussion forum, “The goal in Scala 3 is what we had before scala-reflect. For use-cases where you only need an 80% solution, you should be able to accomplish that with straight-up Java reflection. If you need more, TASTY can provide you the basics. However, we don’t think a 100% solution is something folks need, and it’s unclear if there should be a “core” implementation that is not 100%.

Odersky shared that Scala 3.0 has quoted.Type as an alternative to TypeTag. He commented, “Scala 3 has the quoted package, with quoted.Expr as a representation of expressions and quoted.Type as a representation of types. quoted.Type essentially replaces TypeTag. It does not have the same API but has similar functionality. It should be easier to use since it integrates well with quoted terms and pattern matching.

Follow the discussion on Scala Contributors.

Python-style indentation (for now experimental)

Last month, Odersky proposed to bring indentation based syntax in Scala while also supporting the brace-based. This is because when it was first created, most of the languages used braces. However, with time indentation-based syntax has actually become the conventional syntax.

Listing the reasons behind this change, Odersky wrote,

  • The most widely taught language is now (or will be soon, in any case) Python, which is indentation based.
  • Other popular functional languages are also indentation based (e..g Haskell, F#, Elm, Agda, Idris).
  • Documentation and configuration files have shifted from HTML and XML to markdown and yaml, which are both indentation based.
  • So by now indentation is very natural, even obvious, to developers. There’s a chance that anything else will increasingly be considered “crufty”

Odersky on whether Scala 3.0 is a new language

Odersky answers this with both yes and no. Yes, because Scala 3.0 will include several language changes including feature removals. The introduced new constructs will improve user experience and on-boarding dramatically. There will also be a need to rewrite current Scala books to reflect the recent developments. No, because it will still be Scala and all core constructs will still be the same.

He concludes, “Between yes and no, I think the fairest answer is to say it is really a process. Scala 3 keeps most constructs of Scala 2.13, alongside the new ones. Some constructs like old implicits and so on will be phased out in the 3.x release train. So, that requires some temporary duplication in the language, but the end result should be a more compact and regular language.

Comparing with Python 2 and 3, Odersky believes that Scala’s situation is better because of static typing and binary compatibility. The current version of Dotty can be linked with Scala 2.12 or 2.13 files. He shared that in the future, it will be possible to have a Dotty library module that can then be used by both Scala 2 and 3 modules.

Read also: Core Python team confirms sunsetting Python 2 on January 1, 2020

Watch Odersky’s talk to know more in detail.

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