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‎docs/blog/_posts/2019-06-11-16th-dotty-milestone-release.md

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---
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layout: blog-page
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title: Announcing Dotty 0.16.0-RC1 – the Scala Days 2019 Release
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author: Aggelos Biboudis
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authorImg: /images/aggelos.png
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date: 2019-06-11
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---
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Hello again! Today, we are excited to announce the 16th release of Dotty. The
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development of Dotty continues according to our schedule but today, Tuesday June
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the 11th, we are electrified as it is the first day of [Scala Days 2019](https://scaladays.org/)
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which marks the *10th* anniversary of Scala Days.
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With this release we are getting closer to the _envelop_ of the new features
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that Dotty plans to offer.
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![]({{ site.baseurl }}/images/others/scala-days-logo.png "Scala Days 2019")
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This release serves as a technology preview that demonstrates new
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language features and the compiler supporting them.
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Dotty is the project name for technologies that are being considered for
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inclusion in Scala 3. Scala has pioneered the fusion of object-oriented and
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functional programming in a typed setting. Scala 3 will be a big step towards
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realising the full potential of these ideas. Its main objectives are to
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- become more opinionated by promoting programming idioms we found to work well,
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- simplify where possible,
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- eliminate inconsistencies and surprising behaviours,
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- build on strong foundations to ensure the design hangs together well,
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- consolidate language constructs to improve the language’s consistency, safety, ergonomics, and
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performance.
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You can learn more about Dotty on our [website](https://dotty.epfl.ch).
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<!--more-->
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This is our 16th scheduled release according to our
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[6-week release schedule](https://dotty.epfl.ch/docs/contributing/release.html).
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# What’s new in the 0.16.0-RC1 technology preview?
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<!-- https://github.com/lampepfl/dotty/pulls?q=is%3Apr+closed%3A%3E2019-05-23+is%3Aclosed+sort%3Acomments-desc -->
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## Syntax Change: Type Lambdas
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We reconsider the syntax of type lambdas in an effort to provide an improved
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visual cue for two categories of types: types that relate to normal function
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types and types that operate on a higher level. The _fat_ arrow `=>` definitely
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relates to the first, while we reserve now `->` to mean _pure function_ in the
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future. As a result, we disengage `=>` from type lambdas, which are now
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represented by `=>>`. As a result a function from types to types is written as
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`[X] =>> F[X]`.
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For those who are interested in the discussions,
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[#6558](https://github.com/lampepfl/dotty/pull/6558) introduced the new syntax.
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## Syntax Change: Wildcard Arguments in Types
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The syntax of wildcard arguments in types has changed from `_` to `?`. Example:
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```scala
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List[?]
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Map[? <: AnyRef, ? >: Null]
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```
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Again, in an effort to fine-tune our syntax we put two features, from the world
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of terms and types, side-by-side and drew parallels at the syntactic level.
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Consequently, as `f(_)` is a shorthand for the lambda `x => f(x)` and as we plan
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ahead for making `C[_]` to be a shorthand for the type lambda `[X] =>> C[X]` in
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the future we pick `?` as a replacement syntax for wildcard types, since it
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aligns with Java's syntax.
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For more information please read our documentation on
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[Wildcards](https://dotty.epfl.ch/docs/reference/changed-features/wildcards.html).
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## Polymorphic function types
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We add preliminary support for _polymorphic function types_. Nowadays, when we
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want to write a universally quantified function over elements of lists of type
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`T` we write e.g., `List[T] => List[(T, T)]` where `T` is bound at an enclosing
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definition. With polymorphic function types (PFT hereafter) we can quantify the
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parametric type locally. For example:
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```scala
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[T <: AnyVal] => List[T] => List[(T, T)]
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```
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As you notice, this gives us the ability to impose restrictions on the type
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variable `T` locally. Assume, you have an identity function with `type id = T => T`.
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By writing it as `type id = [T] => T => T` we abstract further the concept
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of a _polymorphic function_ and make it a *true* _family of functions_.
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The code below (correctly) fails to type check because `T` needs to be bounded
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in the enclosing class:
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```scala
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val id: T => T = t => t
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println(s"${id(1)} , ${id(7.0d)}")
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```
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With PFTs we can now achieve what we want:
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```scala
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val id = [T] => (t: T) => t
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println(s"${id(1)} , ${id(7.0d)}")
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```
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For those who are interested in the discussions and more test cases,
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[#4672](https://github.com/lampepfl/dotty/pull/4672/) introduced PFTs.
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## lazy vals are now thread-safe by default
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Previously thread-safety was required using `@volatile` but that would not be
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consistent with Scala 2. The old behavior of non-volatile lazy vals can be
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recovered by using the newly-introduced `@threadUnsafe`.
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For more information please read our documentation on the
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[threadUnsafe annotation](https://dotty.epfl.ch/docs/reference/other-new-features/threadUnsafe-annotation.html).
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## Introducing `for` clauses for importing implied imports by type
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Since implied instances can be anonymous it is not always practical to import
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them by their name, and wildcard imports are typically used instead. By-type
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imports provide a more specific alternative to wildcard imports, which makes it
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clearer what is imported. Example:
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```scala
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import implied A.{for TC}
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```
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This imports any implied instance in `A` that has a type which conforms tp `TC`.
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There can be several bounding types following a `for` and bounding types can
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contain wildcards.
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For instance, assuming the object
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```scala
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object Instances {
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implied intOrd for Ordering[Int]
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implied [T: Ordering] listOrd for Ordering[List[T]]
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implied ec for ExecutionContext = ...
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implied im for Monoid[Int]
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}
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```
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the import
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```
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import implied Instances.{for Ordering[_], ExecutionContext}
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```
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would import the `intOrd`, `listOrd`, and `ec` instances but leave out the `im`
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instance, since it fits none of the specified bounds.
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# Let us know what you think!
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If you have questions or any sort of feedback, feel free to send us a message on our
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[Gitter channel](https://gitter.im/lampepfl/dotty). If you encounter a bug, please
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[open an issue on GitHub](https://github.com/lampepfl/dotty/issues/new).
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## Contributing
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Thank you to all the contributors who made this release possible!
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According to `git shortlog -sn --no-merges 0.15.0-RC1..0.16.0-RC1` these are:
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```
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```
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If you want to get your hands dirty and contribute to Dotty, now is a good time to get involved!
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Head to our [Getting Started page for new contributors](https://dotty.epfl.ch/docs/contributing/getting-started.html),
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and have a look at some of the [good first issues](https://github.com/lampepfl/dotty/issues?q=is%3Aissue+is%3Aopen+label%3Aexp%3Anovice).
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They make perfect entry points into hacking on the compiler.
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We are looking forward to having you join the team of contributors.
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## Library authors: Join our community build
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Dotty now has a set of widely-used community libraries that are built against every nightly Dotty
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snapshot. Currently this includes ScalaPB, algebra, scalatest, scopt and squants.
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Join our [community build](https://github.com/lampepfl/dotty-community-build)
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to make sure that our regression suite includes your library.
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[Scastie]: https://scastie.scala-lang.org/?target=dotty
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[@odersky]: https://github.com/odersky
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[@DarkDimius]: https://github.com/DarkDimius
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[@smarter]: https://github.com/smarter
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[@felixmulder]: https://github.com/felixmulder
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[@nicolasstucki]: https://github.com/nicolasstucki
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[@liufengyun]: https://github.com/liufengyun
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[@OlivierBlanvillain]: https://github.com/OlivierBlanvillain
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[@biboudis]: https://github.com/biboudis
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[@allanrenucci]: https://github.com/allanrenucci
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[@Blaisorblade]: https://github.com/Blaisorblade
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[@Duhemm]: https://github.com/Duhemm
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[@AleksanderBG]: https://github.com/AleksanderBG
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[@milessabin]: https://github.com/milessabin
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[@anatoliykmetyuk]: https://github.com/anatoliykmetyuk

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