Workshop Scala 3

The workshop in this project aims for you to discover some specific features of Scala 3 language.

You can the Github repo with

$ git clone https://github.com/univalence/workshop-scala3

In this workshop, we will focus on:

• basic scala concepts for beginners: variables, constants, functions, control structure, class, trait, pattern-matching...
• Toplevel main function
• Block delimitation
• Enum type
• Extension method
• Contextual abstraction and typeclass
• Selectable (structural types)
• Metaprogramming with inline
• Opaque types

Exercises

The exercises in this workshop are based on Scala 3 macros. You just have to read the comments and launch main functions to see what you have to do.

Once run, the output indicates you the section and exercise title, the expressions to check inside each exercise and their result (error, failure, or success). If the expression to check fails, the output shows the line number in the file. If it has an error (it has thrown an exception), you will see the line number of the wrong expression and the exception (without the stacktrace).

Do not hesitate to use a debugger or to print intermediate results.

The macros defined are:

• section blocks (a group of exercises) with a title
• exercise blocks with a title and an activation flag
• check that contains a boolean expression and check it
• !? explicit placeholders for a type definition to complete
• ?? or ??? or |>? explicit placeholders for a missing implementation

If an exercise is not activated, you can turn its flag to true, once you want to solve it.

How to run

Intellij

• Open the file you want to run (e.g 00-introduction.scala)

• Near to the method with @main annotation, click on the green arrow to run the main method

• The output will be displayed in the console

Visual Studio Code

When using Scala with VS Code, please consider installing the metals extension for VS Code which add IDE functionnality to VS Code (for Scala projects). For this section, we will assume you have installed the metals extension. If not please go to the SBT section.

• When opening the project, a prompt suggestion to import the build will appear, click on Import build button

• Open the file you want to run (e.g 00-introduction.scala)

• Above the @main annotation, click on the run button to run the main method

• The output will be displayed in the TERMINAL tab into the bottom panel

• Inside the temrinal tab, press any key to close the run prompt

For more details about metals, please refer to the official documentation.

SBT

• launch the terminal: $> sbt

• execute the project: $> run

• The following message Multiple main classes detected. Select one to run: will be printed. Choose the number of the main method you want to run (e.g: 1).

• The output will be displayed in the console

Topics not seen in this workshop

We will not see this other topics:

• Union type: ability to express that an element can be of one type or another (eg. String | Null)
• Intersection type: (eg. A & B)
• Trait parameter
• Export
• Parameter untupling: now, you can write l.foldLeft(0)((sum, v) => sum + v)
• and many more...

Dropped/deprecated features in Scala 3

Some features are deprecated and will be removed, or are already removed in Scala 3:

• Scala 2 macros -> inline and Scala 3 macros
• Do-while loop (no replacement)
• Procedure syntax (def f() { ... }) -> Use full function declaration syntax (def f(): Unit = { ... })
• Package object -> Use top-level declaration
• Limit to 22, for number of parameters in functions and case classes, and for number of elements in tuples
• XML literals -> there is an XML string interpolation (xml"<hello />")
• Symbol (no replacement)
• private[this] -> private
• protected[this] (no replacement)
• Wildcard Initializer (var a: A = _) -> var a: A = scala.compiletime.uninitialized
• and some more...

Have fun!