What’s new in the 0.19.0-RC1 technology preview?

Given syntax reworked

the method (a better version of implicitly in Scala 3) was renamed to summon.

given definitions now closely resemble ordinary definitions:

given Int = 10  // Anonymous
given x: String = "foo"  // Named
given f(given x: Int): Option[Int] = Some(x * x)  // With given parameters
given [T](given opt: Option[T]): List[T] = opt.toList  // Anonymous with type parameters

@main def Test = println(summon[List[Int]])

Note that as was dropped and given must now go inside the parentheses as opposed of being used in an infix style.

All of the experimental syntax related to givens – such as delegate for, given as, infix-style given – is now dropped.

Colons dropped from class or object definitions

Now you can define an object as follows:

object Bar
  val x = 10

@main def Test = println(Bar.x)  // 10

In 0.18.1-RC1, you would have needed to put a colon after Bar. The colon was also dropped for traits, classes and enums.

Allow given in pattern bindings

Consider you have the following monadic flow:

for
  x   <- bar
  res <- foo(given x)
yield res

Writing entire programs in a monadic flow is not uncommon in functional programming. When working in this style, a situation may arise as shown above: one statement of the monadic flow implicitly depends on the result of another one. It was impossible to declare a pattern variable as a given, which necessitated passing it around explicitly. Not anymore! Now, you can write the above code as follows:

for
  given x: Int <- bar  // Int, or whatever type you are extracting
  res <- foo
yield res

Note that the type of the given variable must be specified explicitly.

Full example:

def foo(given x: Int): Option[Int] = Some(x * x)
def bar = Some(10)

@main def Test =
  for
    given x: Int <- bar
    res <- foo
  yield println(res)

This syntax is allowed anywhere where a pattern is allowed. So you can write:

user match
  case User(_, Some(given email: Email)) => sendEmail

Full example:

opaque type Email = String
object Email
  def apply(value: String): Email = value

def sendEmail(given m: Email): Unit =
  println(s"Sent an email to $m")

case class User(name: String, email: Option[Email])

@main def Test =
  val user = User("Tom", Some(Email("tom@gmail.com")))
  user match
    case User(_, Some(given email: Email)) => sendEmail

Replace given matches by a library method

Given matches was a feature that allowed to query the implicit scope and execute different logic based on what was found there. We have replaced this feature with a library method called summonFrom. You can use it as follows:

import compiletime.summonFrom

given Int = 10

@main inline def Test = summonFrom {
  case str: String => println(s"String $str")
  case int: Int => println(s"Int $int")  // Int 10
}

The above code will print "Int 10" since an integer with the value 10 was present in the implicit scope but no String was present.

Notice that we had to define the Test method as inline since summonFrom can only be used from an inline method.

Wildcard types written with ?

You can now use both _ and ? to express wildcard types. For example:

@main def Test =
  val xs: List[Int] = (1 to 10).toList
  xs match
    case xss: List[?] => println(s"It is a list")

This is the first step in a multi-step process to disallow _ as wildcards so that we can use underscores for both terms and type parameters instead. This will make the language more regular.

Lambda parameters must be enclosed in parentheses

Lambda parameters with type ascriptions are now required to be enclosed in parentheses. E.g. x: Int => x * x is no longer legal, it must be written as (x: Int) => x * x. However, you can still write x => x * x, that is, if x does not have an explicit type ascription.

Dottydoc redesign

The output of Dottydoc has been redesigned. It is now fully responsive: every element, including API docs and search, is adapted to both small and big screens.

The most visible changes are the toolbar and the sidebar. They now have a common darker background, which makes them more readable and helps separating navigation from content. Also, the sidebar is collapsible and has been optimized so that it doesn't glitch when the page loads.

The toolbar's logo can now be set with the -project-logo option. For instance, -project-logo dotty-logo.svg will make /images/dotty-logo.svg appear in the toolbar.

The front page has been redesigned too, with a new responsive menu and improved contrast.

Overall, every page has been updated with consistent settings of fonts and colors. A more detailed comparison between the new and the old design can be found here.

Metaprogramming Progress

We're making steady progress on the Dotty metaprogramming capability. In our previous work, we've implemented a bunch of functions for working with expressions. For example, we have a capability to convert a list of expressions into an expression of list, or a tuple of expressions into an expression of tuple.

In this release, we have collected this family of functions in one place – the companion object scala.quoted.Expr. Currently, the following methods are available in that object for working with expressions:

• nullExpr – an expression of null.
• unitExpr – an expression of unit.
• block – given a list of statements and a final expression, concatenates them in a block.
• ofSeq, ofList – constructs an expression of collection from a collection of expressions
• ofTuple – constructs an expression of tuple from either a tuple of expressions or a sequence of expressions.

Also, x.toExpr syntax which lifts x into an expression is now deprecated. It is replaced with Expr(x).

Let us know what you think!

If you have questions or any sort of feedback, feel free to send us a message on our Gitter channel. If you encounter a bug, please open an issue on GitHub.

Contributing

Thank you to all the contributors who made this release possible!

According to git shortlog -sn --no-merges 0.18.1-RC1..0.19.0-RC1 these are:

    87  Martin Odersky
    50  Nicolas Stucki
    42  Guillaume R
    33  Nikita Eshkeev
    20  Guillaume Martres
     9  Liu Fengyun
     8  Anatolii
     5  Robert Stoll
     3  Miles Sabin
     1  Sam Desborough
     1  Anatolii Kmetiuk
     1  Jon Pretty
     1  Oron Port
     1  Aggelos Biboudis

If you want to get your hands dirty and contribute to Dotty, now is a good time to get involved! Head to our Getting Started page for new contributors, and have a look at some of the good first issues. They make perfect entry points into hacking on the compiler.

We are looking forward to having you join the team of contributors.

Library authors: Join our community build

Dotty now has a set of widely-used community libraries that are built against every nightly Dotty snapshot. Currently, this includes ScalaPB, algebra, scalatest, scopt and squants. Join our community build to make sure that our regression suite includes your library.