Update code examples to show both Scala 2 and Scala 3 syntax

[julienrf]

We would like to have a single website documenting both Scala 2 and Scala 3 since, in most of the cases, their syntax is very similar, and their features are almost all the same. However, there can still be some syntactic differences between idiomatic Scala 2 code and idiomatic Scala 3 code. Therefore, we want to display both styles in our code examples by using tabs:

For code examples that are idiomatic in both Scala 2 and Scala 3, we show a single tab labelled “Scala 2 and 3”:

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Unfortunately, there are lots of code examples in the documentation, and updating all of them is a huge (and repetitive) task for one person. But if a few dozen members of the community were each updating a couple of pages, the job would be done very quickly. So, everyone is welcome to help!

The policy for updating code examples is the following: show the most idiomatic style for all the code examples, for Scala 2 and Scala 3. For Scala 3 code examples, this means using the new control syntax, the indentation-significant syntax, using and given, @main, .* wildcard import, ? wildcard type argument, tuple indexed access, etc.

You can find the instructions to write tabbed code examples in the documentation.

Here is the list of pages that need to be updated. If you are interested in contributing, please add a comment to announce that you start working on a page, and I will mark it “in progress” to prevent overlapping contributions:

• Tour of Scala (source files are in the _tour/ directory)
  • Basics Compare Scala 2 vs Scala 3 with tabs in Tour of Scala / Basics #2458
  • Unified Types Translate tour/unified-types to Scala 3 #2492
  • Classes Enable tabbed Scala 2/3 code in tour/classes. #2474
  • Default Parameter Values Translate tour/default-parameter-values to Scala 3 #2493
  • Named Arguments Add tabs tour named args #2521
  • Traits Tabs for Scala 2/3 in some pages #2455
  • Tuples Enable tabbed Scala 2/3 code in tour/tuples. #2476
  • Class Composition with Mixins Add code tabs for _tour/mixin-class-composition #2524
  • Higher-order Functions Add code tabs for _tour/higher-order-functions #2525
  • Nested Methods Add code tabs for _tour/nested-functions #2528
  • Multiple Parameter Lists (Currying) Add code tabs for _tour/multiple-parameter-lists.md #2530
  • Case Classes Add code tabs for _tour/case-classes #2533
  • Pattern Matching Add code tabs for _tour/pattern-matching and scala3-book/control-structures #2461
  • Singleton Objects Update singleton-objects.md to use scala 2/3 tabs #2497
  • Regular Expression Patterns Add code tabs for _tour/regular-expression-patterns #2534
  • Extractor Objects Add code tabs for _tour/extractor-objects #2535
  • For Comprehensions For-comprehensions article updated with tabs #2494
  • Generic Classes Add code tabs for _tour/ generic-classes #2537
  • Variances Add code tabs for _tour/variances #2547
  • Upper Type Bounds Add code tabs for _tour/upper-type-bounds #2548
  • Lower Type Bounds Add code tabs for _tour/lower-type-bounds #2549
  • Inner Classes Add code tabs for _tour/inner-classes #2550
  • Abstract Type Members Add code tabs for _tour/abstract-types #2551
  • Compound Types Add code tabs for _tour/compound-types #2552
  • Self-type Add code tabs for _tour/self-types #2553
  • Implicit Parameters Tabbed code snippets for the implicit params tour page. #2496
  • Implicit Conversions update tour/implicit-conversions for Scala 3 #2630
  • Polymorphic Methods Add code tabs for _tour/polymorphic-methods #2554
  • Type Inference Add code tabs for _tour/type-inference #2557
  • Operators Add code tabs for _tour/operators #2561
  • By-name Parameters Add code tabs for _tour/by-name-parameters #2564
  • Annotations Add code tabs for _tour/annotations #2566
  • Packages and Imports Add code tabs for _tour/packages-and-imports #2570
  • Package Objects Talk about top-level-definitions in the tour of Scala #2579
• Guides and Overviews (source files are in the _overviews/ directory)
  • Scala Collections
    • Introduction Add code tabs for collections-2.13/introduction, overview and trait-iterable #2571
    • Mutable and Immutable Collections Add code tabs for collections-2.13/introduction, overview and trait-iterable #2571
    • Trait Iterable Add code tabs for collections-2.13/introduction, overview and trait-iterable #2571
    • The sequence traits Seq, IndexedSeq, and LinearSeq Add code tabs for collections-2.13/introduction, overview and trait-iterable #2571
    • Sets Add code tabs for collections-2.13/sets, immutabes and muttabes #2572
    • Maps Tabs for Scala 2/3 in some pages #2455
    • Concrete Immutable Collection Classes Add code tabs for collections-2.13/sets, immutabes and muttabes #2572
    • Concrete Mutable Collection Classes Add code tabs for collections-2.13/sets, immutabes and muttabes #2572
    • Arrays Add code tabs for collections-2.13/arrays, strings, equality and views #2574
    • Strings Add code tabs for collections-2.13/arrays, strings, equality and views #2574
    • Performance Characteristics (no code examples)
    • Equality Add code tabs for collections-2.13/arrays, strings, equality and views #2574
    • Views Add code tabs for collections-2.13/arrays, strings, equality and views #2574
    • Iterators Add code tabs for collections-2.13/iterators, creating..., conversions... and options #2575
    • Creating Collections From Scratch Add code tabs for collections-2.13/iterators, creating..., conversions... and options #2575
    • Conversions Between Java and Scala Collections Add code tabs for collections-2.13/iterators, creating..., conversions... and options #2575
  • The Architecture of Scala Collections Add code tabs for overviews/architecture_collections-2.13 #2576 (review)
  • Implementing Custom Collections Add code tabs for overviews/custom-collections #2585
  • Adding Custom Collection Operations Custom operations #2589
  • Migration from Scala 2 to Scala 3
    • Migration Tutorial Add code tabs to the Scala 3 migration tutorial #2597
    • Scala 3 Syntax Rewriting Add code tabs to the Scala 3 syntax rewriting #2643
    • Incompatibility Table
      • Syntactic Changes Add code tabs to Syntactic Changes #2722
      • Dropped Features Add code tabs to the Scala 3 Dropped Features #2723
      • Contextual Abstractions Add code tabs to contextual abstractions page #2724
      • Other Changed Features Add code tabs to Other changes page #2725
      • Type Checker Add code tabs to type checker page #2726
      • Type Inference
    • Compiler Plugins
      • Kind Projector Migration
  • Scala 3 Macros (marked as scala 3 only)
  • Value Classes and Universal Traits mark value classes as scala 2 only #2863
  • An Overview of TASTy Add more guidelines for labelling code examples for Scala 2/3 #2768
  • String Interpolation Tabs for Scala 2/3 in some pages #2455
  • Implicit Classes Add code tabs to Contextual Abstractions #2787
  • Library Author Guide port library authors guide to Scala 3 #2873
  • Futures and Promises Port futures article to use Scala 3 tabs #2467
  • Optimizer (marked as scala 2 only)
• Scala 3 Book (source files are in the _overviews/scala3-book directory)
  • Scala 3 Features Add Scala 2 tabs in scala-3-book/scala-features #2495
  • Why Scala 3? Why scala 3 tabs code #2510
  • A Taste of Scala
    • Hello, World! Add Scala version picker to Scala Book Hello World Page, remember preference #2450
    • The REPL Add code tabs for scala3-book/taste-repl #2487
    • Variables and Data Types add scala 2&3 tables arround code in num 7 #2568
    • Control Structures add code tabs in num8 #2580
    • Domain Modeling add code tabs in num9 #2581
    • Methods add code tabs in num10. #2582
    • First-Class Functions add code tabs in num11. #2583
    • Singleton Objects add code tabs in num12. #2584
    • Collections add code tabs in num13. #2586
    • Contextual Abstractions add code tabs in num14. #2587
    • Top-level Definitions add code tabs in num15. #2588
  • A First Look at Types A first look at types tabs code #2511
  • Control Structures Add code tabs for _tour/pattern-matching and scala3-book/control-structures #2461
  • Domain Modeling (no code)
    • Tools add code tabs in num20. #2590
    • OOP Modeling add code tabs in num21. #2591
    • FP Modeling add code tabs in num22. #2592
  • Methods (no code)
    • Method Features add code tabs in num24. #2594
    • main Methods add code tabs. #2609
  • Functions (no code)
    • Anonymous Functions Add Scala 2 code tabs in scala3/book/fun-anonymous-functions #2602
    • Function Variables Add code tabs in fun-function-variables.md #2623
    • Eta Expansion add code tabs to num30 #2644
    • Higher-Order Functions add code tabs to scala3 book fun-hofs #2598
    • Write Your Own map Method Add code tabs in scala3/book/fun-write-map-function #2625
    • Creating a Method That Returns a Function Add code tabs to fun-write-method-returns-function #2664
  • Packaging and Imports add code tab in packaging-imports.md #2745
  • Scala Collections Add snippet tabs for collections in scala3book #2614
    • Collections Types Add snippet tabs for collections in scala3book #2614
    • Collections Methods Add snippet tabs for collections in scala3book #2614
  • Functional Programming Add snippet tabs for functional programming in scala3book #2617
    • What is Functional Programming (no code)
    • Immutable Values Add snippet tabs for functional programming in scala3book #2617
    • Pure Functions Add snippet tabs for functional programming in scala3book #2617
    • Functions Are Values Add snippet tabs for functional programming in scala3book #2617
    • Functional Error Handling Add snippet tabs for functional programming in scala3book #2617
  • Types and the Type System Add code tabs for _overviews/scala3-book/types-{introduction,generics}.md #2599
    • Inferred Types Add code tabs for _overviews/scala3-book/types-{introduction,generics}.md #2599
    • Generics Add code tabs for _overviews/scala3-book/types-{introduction,generics}.md #2599
    • Intersection Types Add code tabs for _overviews/scala3-book/types-intersection.md #2653
    • Union Types Add more guidelines for labelling code examples for Scala 2/3 #2768
    • Algebraic Data Types Add more guidelines for labelling code examples for Scala 2/3 #2768
    • Variance add code tab in types-variance.md #2756
    • Opaque Types mark value classes as scala 2 only #2863
    • Structural Types mark structural types as Scala 3 only #2864
    • Dependent Function Types mark dependent function types as Scala 3 only #2865
    • Other Types mark dependent function types as Scala 3 only #2865
  • Contextual Abstractions
    • Given Instances and Using Clauses add code tabs in _overviews/scala3-book/num59 #2640
    • Type Classes Add code tabs to Contextual Abstractions #2787
    • Context Bounds Updated pages to be “Scala 3 Only” #2689
    • Given Imports Updated pages to be “Scala 3 Only” #2689
    • Extension Methods Add code tabs to Contextual Abstractions #2787
    • Multiversal Equality Updated pages to be “Scala 3 Only” #2689
    • Implicit Conversions Add code tabs to Contextual Abstractions #2787
  • Concurrency
  • Scala Tools
    • Building and Testing Scala Projects with sbt
    • Worksheets
  • Interacting with Java port java interaction to scala 2 #2867
  • Scala for Java Developers
  • Scala for JavaScript Developers
  • Scala for Python Developers
• Tutorials (source files are in the _overviews/tutorials directory or _getting-started/)
  • Getting started with Scala and sbt on the command line feature: Add Scala 3 section for getting started with sbt #2475
  • Getting started with Scala in IntelliJ Add Scala 2 / Scala 3 syntax to the Getting Started with Intellij tutorial #2454
  • Scala for Java Programmers Add scala 3 code demos for Scala for Java Programmers #2499
  • Scala on Android
  • Scala with Maven

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Note: in some cases, not only the code examples need to be updated, but also the text in the documentation (e.g. @main methods, extension methods, contextual abstractions, enums). In that case, the task is more complicated. Feel free to ask for help on the #scala-contributors channel of the Scala discord server, or consider switching to a simpler page.

[flomebul]

I tried to update "Named Arguments". I'm waiting for feedback before trying update on other page.

As my update is now merged via #2521 (thanks bishabosha), I propose to update a next page: "Toor / Class Composition with Mixins"

[bishabosha]

I leave a note here to say that https://docs.scala-lang.org/contribute/add-guides.html#scala-2-vs-scala-3 should be updated to explain the following points

• single tab for Scala 3 should read "Scala 3 Only"
• single tab for Scala 2 should read "Scala 2 Only"
• Scala 3 code tabs should not have ```scala mdoc in the code fence