New in Scala 3

The exciting new version of Scala 3 brings many improvements and new features. Here we provide you with a quick overview of the most important changes. If you want to dig deeper, there are a few references at your disposal:

• The Scala 3 Book targets developers new to the Scala language.
• The Syntax Summary provides you with a formal description of the new syntax.
• The Language Reference gives a detailed description of the changes from Scala 2 to Scala 3.
• The Migration Guide provides you with all the information necessary to move from Scala 2 to Scala 3.
• The Scala 3 Contributing Guide dives deeper into the compiler, including a guide to fix issues.

What’s new in Scala 3

Scala 3 is a complete overhaul of the Scala language. At its core, many aspects of the type-system have been changed to be more principled. While this also brings exciting new features along (like union types), first and foremost, it means that the type-system gets (even) less in your way and for instance type-inference and overload resolution are much improved.

New & Shiny: The Syntax

Besides many (minor) cleanups, the Scala 3 syntax offers the following improvements:

• A new “quiet” syntax for control structures like if, while, and for (new control syntax)
• The new keyword is optional (aka creator applications)
• Optional braces that supports a distraction-free, indentation sensitive style of programming
• Change of type-level wildcards from _ to ?.
• Implicits (and their syntax) have been heavily revised.

Opinionated: Contextual Abstractions

One underlying core concept of Scala was (and still is to some degree) to provide users with a small set of powerful features that can be combined to great (and sometimes even unforeseen) expressivity. For example, the feature of implicits has been used to model contextual abstraction, to express type-level computation, model type-classes, perform implicit coercions, encode extension methods, and many more. Learning from these use cases, Scala 3 takes a slightly different approach and focuses on intent rather than mechanism. Instead of offering one very powerful feature, Scala 3 offers multiple tailored language features, allowing programmers to directly express their intent:

• Abstracting over contextual information. Using clauses allow programmers to abstract over information that is available in the calling context and should be passed implicitly. As an improvement over Scala 2 implicits, using clauses can be specified by type, freeing function signatures from term variable names that are never explicitly referred to.

• Providing Type-class instances. Given instances allow programmers to define the canonical value of a certain type. This makes programming with type-classes more straightforward without leaking implementation details.

• Retroactively extending classes. In Scala 2, extension methods had to be encoded using implicit conversions or implicit classes. In contrast, in Scala 3 extension methods are now directly built into the language, leading to better error messages and improved type inference.

• Viewing one type as another. Implicit conversions have been redesigned from the ground up as instances of a type-class Conversion.

• Higher-order contextual abstractions. The all-new feature of context functions makes contextual abstractions a first-class citizen. They are an important tool for library authors and allow to express concise domain specific languages.

• Actionable feedback from the compiler. In case an implicit parameter cannot be resolved by the compiler, it now provides import suggestions that may fix the problem.

Say What You Mean: Type System Improvements

Besides greatly improved type inference, the Scala 3 type system also offers many new features, giving you powerful tools to statically express invariants in the types:

• Enumerations. Enums have been redesigned to blend well with case classes and form the new standard to express algebraic data types.

• Opaque Types. Hide implementation details behind opaque type aliases without paying for it in performance! Opaque types supersede value classes and allow you to set up an abstraction barrier without causing additional boxing overhead.

• Intersection and union types. Basing the type system on new foundations led to the introduction of new type system features: instances of intersection types, like A & B, are instances of both A and of B. Instances of union types, like A | B, are instances of either A or B. Both constructs allow programmers to flexibly express type constraints outside the inheritance hierarchy.

• Dependent function types. Scala 2 already allowed return types to depend on (value) arguments. In Scala 3 it is now possible to abstract over this pattern and express dependent function types. In the type type F = (e: Entry) => e.Key the result type depends on the argument!

• Polymorphic function types. Like with dependent function types, Scala 2 supported methods that allow type parameters, but did not allow programmers to abstract over those methods. In Scala 3, polymorphic function types like [A] => List[A] => List[A] can abstract over functions that take type arguments in addition to their value arguments.

• Type lambdas. What needed to be expressed using a compiler plugin in Scala 2 is now a first-class feature in Scala 3: Type lambdas are type level functions that can be passed as (higher-kinded) type arguments without requiring an auxiliary type definition.

• Match types. Instead of encoding type-level computation using implicit resolution, Scala 3 offers direct support for matching on types. Integrating type-level computation into the type checker enables improved error messages and removes the need for complicated encodings.

Re-envisioned: Object-Oriented Programming

Scala has always been at the frontier between functional programming and object-oriented programming – and Scala 3 pushes boundaries in both directions! The above-mentioned type system changes and the redesign of contextual abstractions make functional programming easier than before. At the same time, the following novel features enable well-structured object-oriented designs and support best practices.

• Pass it on. Traits move closer to classes and now can also take parameters, making them even more powerful as a tool for modular software decomposition.
• Plan for extension. Extending classes that are not intended for extension is a long-standing problem in object-oriented design. To address this issue, open classes require library designers to explicitly mark classes as open.
• Hide implementation details. Utility traits that implement behavior sometimes should not be part of inferred types. In Scala 3, those traits can be marked as transparent hiding the inheritance from the user (in inferred types).
• Composition over inheritance. This phrase is often cited, but tedious to implement. Not so with Scala 3’s export clauses: symmetric to imports, export clauses allow the user to define aliases for selected members of an object.
• No more NPEs (experimental). Scala 3 is safer than ever: explicit null moves null out of the type hierarchy, helping you to catch errors statically; additional checks for safe initialization detect access to uninitialized objects.

Batteries Included: Metaprogramming

While macros in Scala 2 were an experimental feature only, Scala 3 comes with a powerful arsenal of tools for metaprogramming. The macro tutorial contains detailed information on the different facilities. In particular, Scala 3 offers the following features for metaprogramming:

• Inline. As the basic starting point, the inline feature allows values and methods to be reduced at compile time. This simple feature already covers many use-cases and at the same time provides the entry point for more advanced features.
• Compile-time operations. The package scala.compiletime contains additional functionality that can be used to implement inline methods.
• Quoted code blocks. Scala 3 adds the new feature of quasi-quotation for code, providing a convenient high-level interface to construct and analyse code. Constructing code for adding one and one is as easy as '{ 1 + 1 }.
• Reflection API. For more advanced use cases quotes.reflect provides more detailed control to inspect and generate program trees.

If you want to learn more about metaprogramming in Scala 3, we invite you to take our tutorial.