Blog Post: Announcing Dotty 0.6.0 and 0.7.0-RC1

docs/blog/_posts/2018-03-05-seventh-dotty-milestone-release.md

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+---
+layout: blog-page
+title: Announcing Dotty 0.6.0 and 0.7.0-RC1
+author: Allan Renucci
+authorImg: /images/allan.jpg
+date: 2018-03-05
+---
+
+Today, we are excited to release Dotty versions 0.6.0 and 0.7.0-RC1. These releases
+serve as a technology preview that demonstrates new language features and the compiler supporting them.
+
+If you’re not familiar with Dotty, it's a platform to try out new language concepts and compiler
+technologies for Scala. The focus is mainly on simplification. We remove extraneous syntax
+(e.g. no XML literals), and try to boil down Scala’s types into a smaller set of more fundamental
+constructs. The theory behind these constructs is researched in
+[DOT](https://infoscience.epfl.ch/record/215280), a calculus for dependent object types.
+You can learn more about Dotty on our [website](http://dotty.epfl.ch).
+
+<!--more-->
+
+This is our seventh scheduled release according to our [6-week release schedule](http://dotty.epfl.ch/docs/usage/version-numbers.html).
+The [previous technology preview](https://github.com/lampepfl/dotty/releases/tag/0.6.0-RC1) focussed
+on bug fixes and stability work.
+
+## What’s new in the 0.7.0-RC1 technology preview?
+
+### Enum Simplification [#4003](https://github.com/lampepfl/dotty/pull/4003)
+The previously introduced syntax and rules for enum were arguably too complex. We can considerably
+simplify them by taking away one capability: that cases can have bodies which can define members.
+Arguably, if we choose an ADT decomposition of a problem, it's good style to write all methods using
+pattern matching instead of overriding individual cases. So this removes an unnecessary choice.
+We now treat enums unequivocally as classes. They can have methods and other statements just like
+other classes can. Cases in enums are seen as a form of constructors. We do not need a
+distinction between enum class and enum object anymore. Enums can have companion objects just like
+normal classes can, of course.
+
+Let's consider how `Option` can be represented as an enum. Previously using an enum class:
+```scala
+enum class Option[+T] {
+   def isDefined: Boolean
+}
+
+object Option {
+  case Some[+T](x: T) {
+     def isDefined = true
+  }
+  case None {
+     def isDefined = false
+  }
+
+  def apply[T](x: T): Option[T] = if (x == null) None else Some(x)
+}
+```
+
+And now:
+```scala
+enum Option[+T] {
+  case Some(x: T)
+  case None
+
+  def isDefined: Boolean = this match {
+    case None => false
+    case Some(_) => true
+  }
+}
+
+object Option {
+  def apply[T](x: T): Option[T] = if (x == null) None else Some(x)
+}
+```
+
+
+For more information about [Enumerations](http://dotty.epfl.ch/docs/reference/enums/enums.html)
+and how to use them to model [Algebraic Data Types](http://dotty.epfl.ch/docs/reference/enums/adts.html),
+visit the respective sections in our documentation.
+
+
+### Erased terms [#3342](https://github.com/lampepfl/dotty/pull/3342)
+The `erased` modifier can be used on parameters, `val` and `def` to enforce that no reference to
+those terms is ever used. As they are never used, they can safely be removed during compilation.
+
+One particular use case is to add implicit type constraints that are only relevant at compilation
+time. For example, let's consider the following implementation of `flatten`.
+
+```scala
+class List[X] {
+  def flatten[Y](implicit erased ev: X <:< List[Y]): List[Y] = {
+    val buffer = new mutable.ListBuffer[Y]
+    this.foreach(e => buffer ++= e.asInstanceOf[List[Y]])
+    buffer.toList
+  }
+}
+
+List(List(1, 2), List(3)).flatten // List(1, 2, 3)
+List(1, 2, 3).flatten             // error: Cannot prove that Int <:< List[Y]
+```
+
+The implicit evidence `ev` is only used to constrain the type parameter `X` of `List` such that we
+can safely cast from `X` to `List[_]`. The usage of the `erased` modifier ensures that the evidence
+is not used and can be safely removed at compilation time.
+
+For more information, visit the [Erased Terms](http://dotty.epfl.ch/docs/reference/erased-terms.html)
+section of our documentation.
+
+**Note**: Erased terms replace _phantom types_: they have similar semantics, but with the added
+advantage that any type can be an erased parameter. See [#3410](https://github.com/lampepfl/dotty/pull/3410).
+
+
+### Improved IDE support [#3960](https://github.com/lampepfl/dotty/pull/3960)
+The Dotty language server now supports context sensitive IDE completions. Completions now include
+local and imported definitions. Members completions take possible implicit conversions into account.
+
+<!-- A GIF would be nice? -->
+
+We also improved the `find references` functionality. It is more robust and much faster!
+
+Try it out in [Visual Studio Code](http://dotty.epfl.ch/docs/usage/ide-support.html)!
+
+### Better and safer types in pattern matching (improved GADT support)
+
+Consider the following implementation of an evaluator for a very simple
+language containing only integer literals (`Lit`) and pairs (`Pair`):
+```scala
+sealed trait Exp
+case class Lit(value: Int) extends Exp
+case class Pair(fst: Exp, snd: Exp) extends Exp
+
+object Evaluator {
+  def eval(e: Exp): Any = e match {
+    case Lit(x) =>
+      x
+    case Pair(a, b) =>
+      (eval(a), eval(b))
+  }
+
+  eval(Lit(1))                             // 1: Any
+  eval(Pair(Pair(Lit(1), Lit(2)), Lit(3))) // ((1, 2), 3) : Any
+}
+```
+
+This code is correct but it's not very type-safe since `eval` returns a value
+of type `Any`, we can do better by adding a type parameter to `Exp` that
+represents the result type of evaluating the expression:
+
+```scala
+sealed trait Exp[T]
+case class Lit(value: Int) extends Exp[Int]
+case class Pair[A, B](fst: Exp[A], snd: Exp[B]) extends Exp[(A, B)]
+
+object Evaluator {
+  def eval[T](e: Exp[T]): T = e match {
+    case Lit(x) =>
+      // In this case, T = Int
+      x
+    case Pair(a, b) =>
+      // In this case, T = (A, B) where A is the type of a and B is the type of b
+      (eval(a), eval(b))
+  }
+
+  eval(Lit(1))                             // 1: Int
+  eval(Pair(Pair(Lit(1), Lit(2)), Lit(3))) // ((1, 2), 3) : ((Int, Int), Int)
+}
+```
+
+Now the expression `Pair(Pair(Lit(1), Lit(2)), Lit(3)))` has type `Exp[((Int,
+Int), Int)]` and calling `eval` on it will return a value of type `((Int,
+Int), Int)` instead of `Any`.
+
+Something subtle is going on in the definition of `eval` here: its result type
+is `T` which is a type parameter that could be instantiated to anything, and
+yet in the `Lit` case we are able to return a value of type `Int`, and in the
+`Pair` case a value of a tuple type. In each case the typechecker has been able
+to constrain the type of `T` through unification (e.g. if `e` matches `Lit(x)`
+then `Lit` is a subtype of `Exp[T]`, so `T` must be equal to `Int`). This is
+usually referred to as **GADT support** in Scala since it closely mirrors the
+behavior of [Generalized Algebraic Data
+Types](https://en.wikipedia.org/wiki/Generalized_algebraic_data_type) in
+Haskell and other languages.
+
+GADTs have been a part of Scala for a long time, but in Dotty 0.7.0-RC1 we
+significantly improved their implementation to catch more issues at
+compile-time. For example, writing `(eval(a), eval(a))` instead of `(eval(a),
+eval(b))` in the example above should be an error, but it was not caught by
+Scala 2 or previous versions of Dotty, whereas we now get a type mismatch error
+as expected. More work remains to be done to fix the remaining [GADT-related
+issues](https://github.com/lampepfl/dotty/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+gadt),
+but so far no show-stopper has been found.
+
+## Trying out Dotty
+### Scastie
+[Scastie], the online Scala playground, supports Dotty.
+This is an easy way to try Dotty without installing anything.
+
+### sbt
+Using sbt 0.13.13 or newer, do:
+
+```shell
+sbt new lampepfl/dotty.g8
+```
+
+This will setup a new sbt project with Dotty as compiler. For more details on
+using Dotty with sbt, see the
+[example project](https://github.com/lampepfl/dotty-example-project).
+
+### IDE support
+It is very easy to start using the Dotty IDE in any Dotty project by following
+the [IDE guide](http://dotty.epfl.ch/docs/usage/ide-support.html).
+
+
+### Standalone installation
+Releases are available for download on the _Releases_
+section of the Dotty repository:
+[https://github.com/lampepfl/dotty/releases](https://github.com/lampepfl/dotty/releases)
+
+We also provide a [homebrew](https://brew.sh/) package that can be installed by running:
+
+```shell
+brew install lampepfl/brew/dotty
+```
+
+In case you have already installed Dotty via brew, you should instead update it:
+
+```shell
+brew upgrade dotty
+```
+
+## 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](https://gitter.im/lampepfl/dotty). If you encounter a bug, please
+[open an issue on GitHub](https://github.com/lampepfl/dotty/issues/new).
+
+## Contributing
+Thank you to all the contributors who made this release possible!
+
+According to `git shortlog -sn --no-merges 0.6.0..0.7.0-RC1` these are:
+
+```
+   182  Martin Odersky
+    94  Nicolas Stucki
+    48  Olivier Blanvillain
+    38  liu fengyun
+    16  Allan Renucci
+    15  Guillaume Martres
+    11  Aggelos Biboudis
+     5  Abel Nieto
+     5  Paolo G. Giarrusso
+     4  Fengyun Liu
+     2  Georg Schmid
+     1  Jonathan Skowera
+     1  Fedor Shiriaev
+     1  Alexander Slesarenko
+     1  benkobalog
+     1  Jimin Hsieh
+```
+
+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](http://dotty.epfl.ch/docs/contributing/getting-started.html),
+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).
+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](https://github.com/lampepfl/dotty-community-build)
+to make sure that our regression suite includes your library.
+
+
+[Scastie]: https://scastie.scala-lang.org/?target=dotty
+
+[@odersky]: https://github.com/odersky
+[@DarkDimius]: https://github.com/DarkDimius
+[@smarter]: https://github.com/smarter
+[@felixmulder]: https://github.com/felixmulder
+[@nicolasstucki]: https://github.com/nicolasstucki
+[@liufengyun]: https://github.com/liufengyun
+[@OlivierBlanvillain]: https://github.com/OlivierBlanvillain
+[@biboudis]: https://github.com/biboudis
+[@allanrenucci]: https://github.com/allanrenucci
+[@Blaisorblade]: https://github.com/Blaisorblade
+[@Duhemm]: https://github.com/duhemm
+