What’s new in the 0.20.0-RC1 technology preview?

Syntax change for type parameters of extension methods

When writing extension methods with type parameters, the type parameters must come first, e.g.:

def [T](xs: List[T]) append (ys: List[T]): List[T] = ...

Previously, the same would have been written as:

def (xs: List[T]) append [T] (ys: List[T]): List[T] = ...

An argument for the old syntax is that it aligns the definition and call syntax. On the other hand, the new syntax maintains the general rule that parameter introductions always come before parameter uses. The decisive argument to switch is to be consistent with the new collective parameter syntax, where append would be written like this:

given [T](xs: List[T])
  def append (ys: List[T]): List[T] = ...

To avoid misalignment of type parameters between definition and call syntax, we considered disallowing explicit type parameters for extension methods altogether, and to require that the method is called as a normal method instead. But that would not work for anonymous givens as in the last example above.

Infer private[this]

We now infer the private[this] modifier for variables if all the accesses to a variable are via this. Explicit private[this] and protected[this] in code are deprecated under the -strict flag.

The main reasons for dropping private[this] are:

• It is syntactically an irregular case. A pair of brackets usually encloses a type, but this is a value.
• Its effect over private is purely local and can be easily inferred.
• It leads to bike shedding: should I use private or private[this]? One is shorter but the other might be more efficient.

protected[this] by now influences compiler decisions in no way at all. Hence it is reasonable to drop it.

with keyword's new role

with keyword can now optionally precede the class body. So that you can write your classes as follows:

trait A with {
  def f: Int
}
class C(x: Int) extends A with {
  def f = x
}
type T = A with {
  def f: Int
}

Or, equivalently:

trait A with
  def f: Int
class C(x: Int) extends A with
  def f = x
type T = A with
  def f: Int

The problem this change solves is that it is very easy to accidentally outdent a class member – and it will end up outside the class. The benefit of the new with is that starts an indentation block. Since the compiler knows for sure an indentation block must follow, it will emit an error if you forget to indent your statement.

Inline given specialization

It is now possible to specialize inline givens with the help of <: as follows:

trait A
class B extends A

inline given tc <: A = B()

val x: B = summon[A]

This change brings givens even with the ordinary inline defs.

Normal parameters can follow given parameters

Previously normal parameters after given parameter was disallowed mainly because they looked awkward with the old syntax. With the syntax being improved, this restriction is now lifted and you can write, e.g., the following program:

class C(val x: Int)
def f(x: Int)(given c: C)(y: Int) = x + c.x + y

then is optional at line end

So the following is now legal:

val y1 =
  if x > 0
    1
  else
    2

It is easy to forget to put then at the end of the line if nothing else follows it, but also easy to infer that it must be inserted there.

Metaprogramming Progress

We are making a steady progress developing and improving the metaprogramming features of Dotty. Here are metaprogramming highlights of this release:

• Fix #7189: Do not try to load contents if file does not exist #7476
• Add customizable names for definitions in quotes #7346
• Rename scala.quoted.matching.{Bind => Sym} #7332
• Replace AsFunction implicit class with Expr.reduce #7299

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 0.19.0-RC1..0.20.0-RC1 these are:

    99  Martin Odersky
    64  Nicolas Stucki
    16  Nikita Eshkeev
    15  Guillaume Martres
     9  Robert Stoll
     8  Anatolii
     5  Liu Fengyun
     5  Olivier Blanvillain
     3  Miles Sabin
     2  Aggelos Biboudis
     2  Jamie Thompson
     2  Antoine Brunner
     2  Ben Elliott
     2  Guillaume R
     1  noti0na1
     1  Ashwin Bhaskar
     1  Batanick
     1  Bojan Dunaj
     1  Harpreet Singh
     1  Lucas
     1  Lucas Jenß
     1  Martijn Hoekstra
     1  bishabosha
     1  brunnerant

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.