Dotty 0.2.0-RC1 release announcement draft.

docs/blog/_posts/2017-07-12-second-dotty-milestone-release.md

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+---
+layout: blog-page
+title: Announcing Dotty 0.2.0-RC1, with new optimizations, improved stability and IDE support
+author: Dmytro Petrashko
+authorImg: /images/petrashko.jpg
+date: 2017-07-12
+---
+
+Today, we are excited to release Dotty version 0.2.0-RC1. This release
+serves as a technology preview that demonstrates new language features
+and the compiler supporting them.
+
+This release is based on the [previous milestone](/blog/2017/05/31/first-dotty-milestone-release.html).
+The highlights of this release are:
+ - substantial improvement of quality of generated code for pattern matching
+ - improvements in VS Code IDE stability
+ - support Windows in VS Code IDE
+ - improved compatibility with scalac
+ - initial support for reproducible builds
+
+
+<!--more-->
+
+This is our second scheduled release according to our [6-week release schedule](http://dotty.epfl.ch/docs/usage/version-numbers.html).
+
+## What’s in the 0.2.0-RC1 technology preview?
+The [previous technology preview](/blog/2017/05/31/first-dotty-milestone-release.html) has shipped new language features planned for Scala 3:
+[Intersection Types](http://dotty.epfl.ch/docs/reference/intersection-types.html),
+[Union Types](http://dotty.epfl.ch/docs/reference/union-types.html),
+[Trait Parameters](http://dotty.epfl.ch/docs/reference/trait-parameters.html),
+[Enumerations](http://dotty.epfl.ch/docs/reference/enums/enums.html),
+[Algebraic Data Types](http://dotty.epfl.ch/docs/reference/enums/adts.html),
+[By-Name Implicits](http://dotty.epfl.ch/docs/reference/implicit-by-name-parameters.html).
+
+This technology preview is geared towards improving stability and reliability. It includes:
+
+  - [Local optimizations upstreamed from the Dotty Linker](https://github.com/lampepfl/dotty/pull/2513), [2647](https://github.com/lampepfl/dotty/pull/2647) by ([@OlivierBlanvillain](https://github.com/OlivierBlanvillain)). See more details below.
+  - [Optimizing Pattern Matcher](https://github.com/lampepfl/dotty/pull/2829) by ([@odersky](https://github.com/odersky))
+  - [Idempotency checks](https://github.com/lampepfl/dotty/pull/2756) are the first step to reproducible builds
+  - [Faster Base class sets](https://github.com/lampepfl/dotty/pull/2676) by ([@odersky](https://github.com/odersky)) and ([@darkdimius](https://twitter.com/darkdimius))
+  - Numerous fixes to IDE and Dotty Language Server covering:
+
+     - [Windows support for VS Code plugin](https://github.com/lampepfl/dotty/pull/2776)
+     - [Fix hover-on-type for implicitly converted expressions](https://github.com/lampepfl/dotty/pull/2836)
+     - [Fixes to find all references in external projects](https://github.com/lampepfl/dotty/pull/2810), [2773](https://github.com/lampepfl/dotty/pull/2773/files)
+     - [Fix conflict with  dragos-vscode-scala](https://github.com/lampepfl/dotty/pull/2777)
+     - [Fix ide crash on non-parsable file](https://github.com/lampepfl/dotty/pull/2752)
+     - [Fix hover functionality for enum classes](https://github.com/lampepfl/dotty/pull/2722)
+     - [Report errors on Dotty Language Server initialization](https://github.com/lampepfl/dotty/pull/2708)
+     - [Fixes to sbt setting up Dotty IDE](https://github.com/lampepfl/dotty/pull/2690)
+     - General stability improvements [2838](https://github.com/lampepfl/dotty/pull/2838), [2787](https://github.com/lampepfl/dotty/pull/2787), [2692](https://github.com/lampepfl/dotty/pull/2692)
+
+  - Scalac compatibility improvements:
+
+     - [Support Scala 2.12 traits](https://github.com/lampepfl/dotty/pull/2685)
+     - [Fixes to handling of Scala 2 classfiles](https://github.com/lampepfl/dotty/pull/2834/files)
+     - [Scalac parser crashes on Dotty.jar](https://github.com/lampepfl/dotty/pull/2719)
+
+  - Java compatibility improvements:
+
+     - [Fixes to handing of Java generic signatures](https://github.com/lampepfl/dotty/pull/2831)
+     - [java.lang.System.out is final but that's a lie](https://github.com/lampepfl/dotty/pull/2781)
+
+  - Improved error messages:
+
+     - [Nicer error message for "implicit function type needs non-empty parameter list"](https://github.com/lampepfl/dotty/pull/2821)
+     - [Nicer error message for nonsensical modifier combination](https://github.com/lampepfl/dotty/pull/2807/files), [2747](https://github.com/lampepfl/dotty/pull/2747)
+     - [Nicer error message for supercall inside @inline method](https://github.com/lampepfl/dotty/pull/2740)
+     - [Check that case classes don't inherit case classes](https://github.com/lampepfl/dotty/pull/2790)
+     - [Check that named parameters don't conflict with positional ones](https://github.com/lampepfl/dotty/pull/2785)
+
+  - Improved command line handling:
+
+     - [Support params in a file like @file.txt](https://github.com/lampepfl/dotty/pull/2765)
+
+  - Type system stability:
+
+     - [Handle wildcard types in unions and intersections](https://github.com/lampepfl/dotty/pull/2742)
+
+  - Fixes to implicit search:
+
+     - [Fix shadowing of higher order implicits](https://github.com/lampepfl/dotty/pull/2739)
+
+
+### Better generated code:
+
+As was [spotted](https://twitter.com/gkossakowski/status/870243464528744449) by [@gkossakowski](https://twitter.com/gkossakowski)
+in the previous release Dotty was on par with Scala 2.11 in speed. But why is that?
+The reason is that Dotty compiled by Dotty had really horrible code generated for pattern matching.
+
+Let's illustrate on a simple example:
+
+```
+case class CC(a: Int, b: Object)
+
+  def foo(x: Any): Int = {
+    val (a, b) = x match {
+      case CC(s @ 1, CC(t, _)) =>
+        (s , 2)
+      case _ => (42, 43)
+    }
+    a + b
+  }
+
+  def booleans(a: Object) = {
+    val (b1, b2) = (a.isInstanceOf[CC], a.isInstanceOf[List[Int]])
+    (b1, b2) match {
+      case (true, true) => true
+      case (false, false) => true
+      case _ => false
+    }
+  }
+```
+
+
+The Dotty that was released in the previous milestone didn't contain any optimizations and generated inefficient code for it.
+The java-with-goto code below is equivalent to what Dotty generated.
+
+```
+// output of dotc 0.1.2-RC1
+    public int foo(Object x) {
+        var3_2 = x;
+        if (!(var3_2 instanceof CC)) ** GOTO lbl-1000
+        var4_3 = (CC)var3_2;
+        if (CC$.MODULE$.unapply((CC)var3_2) == null) ** GOTO lbl-1000
+        var5_4 = CC$.MODULE$.unapply((CC)var3_2);
+        s = var5_4._1();
+        var7_6 = var5_4._2();
+        if (1 != s) ** GOTO lbl-1000
+        var8_7 = s;
+        if (!(var7_6 instanceof CC)) ** GOTO lbl-1000
+        var9_8 = (CC)var7_6;
+        if (CC$.MODULE$.unapply((CC)var7_6) != null) {
+            var10_9 = CC$.MODULE$.unapply((CC)var7_6);
+            var11_10 = var10_9._2();
+            v0 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToInteger((int)1), (Object)BoxesRunTime.boxToInteger((int)2));
+        } else lbl-1000: // 5 sources:
+        {
+            v0 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToInteger((int)42), (Object)BoxesRunTime.boxToInteger((int)43));
+        }
+        var2_11 = v0;
+        a = BoxesRunTime.unboxToInt((Object)var2_11._1());
+        b = BoxesRunTime.unboxToInt((Object)var2_11._2());
+        return a + b;
+    }
+
+    public boolean booleans(Object a) {
+        Tuple2 tuple2 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToBoolean((boolean)(a instanceof CC)), (Object)BoxesRunTime.boxToBoolean((boolean)(a instanceof List)));
+        boolean b1 = BoxesRunTime.unboxToBoolean((Object)tuple2._1());
+        boolean b2 = BoxesRunTime.unboxToBoolean((Object)tuple2._2());
+        Tuple2 tuple22 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToBoolean((boolean)b1), (Object)BoxesRunTime.boxToBoolean((boolean)b2));
+        Option option = Tuple2..MODULE$.unapply(tuple22);
+        if (!option.isEmpty()) {
+            Tuple2 tuple23 = (Tuple2)option.get();
+            boolean bl = BoxesRunTime.unboxToBoolean((Object)tuple23._1());
+            boolean bl2 = BoxesRunTime.unboxToBoolean((Object)tuple23._2());
+            if (bl) {
+                boolean bl3 = bl;
+                if (bl2) {
+                    boolean bl4 = bl2;
+                    return true;
+                }
+            }
+        }
+        Option option2 = Tuple2..MODULE$.unapply(tuple22);
+        if (option2.isEmpty()) return false;
+        Tuple2 tuple24 = (Tuple2)option2.get();
+        boolean bl = BoxesRunTime.unboxToBoolean((Object)tuple24._1());
+        boolean bl5 = BoxesRunTime.unboxToBoolean((Object)tuple24._2());
+        if (bl) return false;
+        boolean bl6 = bl;
+        if (bl5) return false;
+        boolean bl7 = bl5;
+        return true;
+    }
+```
+
+Due to the new optimizing pattern matcher, Dotty now is able to generate the code below without `-optimise`
+
+```
+// output of 0.2.0-RC1 without -optimise
+    public int foo(Object x) {
+        var3_2 = x;
+        if (!(var3_2 instanceof CC)) ** GOTO lbl-1000
+        var4_3 = CC$.MODULE$.unapply((CC)var3_2);
+        s = var5_4 = var4_3._1();
+        if (1 == var5_4 && (var7_6 = var4_3._2()) instanceof CC) {
+            t = CC$.MODULE$.unapply((CC)var7_6)._1();
+            v0 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToInteger((int)1), (Object)BoxesRunTime.boxToInteger((int)2));
+        } else lbl-1000: // 2 sources:
+        {
+            v0 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToInteger((int)42), (Object)BoxesRunTime.boxToInteger((int)43));
+        }
+        var2_8 = v0;
+        a = BoxesRunTime.unboxToInt((Object)var2_8._1());
+        b = BoxesRunTime.unboxToInt((Object)var2_8._2());
+        return a + b;
+    }
+
+    public boolean booleans(Object a) {
+        Tuple2 tuple2 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToBoolean((boolean)(a instanceof CC)), (Object)BoxesRunTime.boxToBoolean((boolean)(a instanceof List)));
+        boolean b1 = BoxesRunTime.unboxToBoolean((Object)tuple2._1());
+        boolean b2 = BoxesRunTime.unboxToBoolean((Object)tuple2._2());
+        Tuple2 tuple22 = Tuple2..MODULE$.apply((Object)BoxesRunTime.boxToBoolean((boolean)b1), (Object)BoxesRunTime.boxToBoolean((boolean)b2));
+        if (tuple22 != null) {
+            boolean bl;
+            boolean bl2 = BoxesRunTime.unboxToBoolean((Object)tuple22._1());
+            if (!bl2) {
+                bl = bl2;
+            } else {
+                if (BoxesRunTime.unboxToBoolean((Object)tuple22._2())) {
+                    return true;
+                }
+                bl = bl2;
+            }
+            if (!bl) {
+                if (false != BoxesRunTime.unboxToBoolean((Object)tuple22._2())) return false;
+                return true;
+            }
+        }
+        return false;
+    }
+```
+
+You can clearly see that it's shorter ;-) and it actually does less work.
+If you additionally enable local optimizations, you get decent generated code:
+
+```
+// output of 0.2.0-RC1 with -optimise
+
+    public int foo(Object x) {
+        int n;
+        Tuple2 tuple2;
+        CC cC;
+        Object object;
+        if (x instanceof CC && 1 == (n = (cC = (CC)x)._1()) && (object = cC._2()) instanceof CC) {
+            ((CC)object)._1();
+            tuple2 = new Tuple2((Object)BoxesRunTime.boxToInteger((int)1), (Object)BoxesRunTime.boxToInteger((int)2));
+        } else {
+            tuple2 = new Tuple2((Object)BoxesRunTime.boxToInteger((int)42), (Object)BoxesRunTime.boxToInteger((int)43));
+        }
+        Tuple2 tuple22 = tuple2;
+        return BoxesRunTime.unboxToInt((Object)tuple22._1()) + BoxesRunTime.unboxToInt((Object)tuple22._2());
+    }
+
+    public boolean booleans(Object a) {
+        boolean bl = a instanceof CC;
+        boolean bl2 = a instanceof List;
+        new Tuple2((Object)BoxesRunTime.boxToBoolean((boolean)bl), (Object)BoxesRunTime.boxToBoolean((boolean)bl2));
+        new Tuple2((Object)BoxesRunTime.boxToBoolean((boolean)bl), (Object)BoxesRunTime.boxToBoolean((boolean)bl2));
+        if (bl && bl2) {
+            return true;
+        }
+        boolean bl3 = bl;
+        if (bl3) return false;
+        if (bl2) return false;
+        return true;
+    }
+```
+
+This code still has a major inefficiency; it allocates tuples.
+We plan to continue the migration of local optimizations from the Dotty Linker that should allow us to generate code that is as
+good the code generated by the Dotty Linker with global analysis disabled:
+
+```
+  // output of Dotty linker https://github.com/dotty-linker/dotty/tree/opto
+    public int foo(Object x) {
+         CC cC;
+         int n = 0;
+         int n2 = 0;
+         if (x instanceof CC && 1 == (cC = (CC)x)._1() && cC._2() instanceof CC) {
+             n = 1;
+             n2 = 2;
+         } else {
+             n = 42;
+             n2 = 43;
+         }
+         return n + n2;
+     }
+
+     public boolean booleans(Object a) {
+         boolean bl = a instanceof CC;
+         boolean bl2 = a instanceof List;
+         if (bl && bl2 || !bl && !bl2) {
+             return true;
+         }
+         return false;
+ }
+```
+
+## How can you try it out?
+We ship with tools that help you try out the Dotty platform:
+
+  - [IDE features for Visual Studio Code](http://dotty.epfl.ch/docs/usage/ide-support.html)
+  - [sbt support, including retro-compatibility with Scala 2](https://github.com/lampepfl/dotty-example-project)
+
+
+You have several alternatives; use the `sbt-dotty` plugin, get a standalone
+installation, or try it online on [Scastie].
+
+### sbt
+Using sbt 0.13.13 or newer, do:
+
+```
+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).
+
+### 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:
+
+```
+brew install lampepfl/brew/dotty
+```
+
+In case you have already installed Dotty via brew, you should instead update it:
+
+```
+brew upgrade dotty
+```
+
+### Scastie
+
+[Scastie], the online Scala playground,
+supports Dotty.
+You can try it out there without installing anything.
+
+
+## What are the next steps?
+
+Over the coming weeks and months, we plan to work on the following topics:
+
+ - [Add support for using Dotty generated classes with Scala 2.12](https://github.com/lampepfl/dotty/pull/2827)
+ - [Add Language-level support for HMaps and HLists](https://github.com/lampepfl/dotty/pull/2199);
+ - Upstream more optimizations from Dotty Linker
+ - [Add support for existing in the same classpath with Scala 2.12](https://github.com/lampepfl/dotty/pull/2827)
+
+If you want to get your hands dirty with any of this, now is a good
+moment to get involved! Join the team of contributors, including
+Martin Odersky ([@odersky](https://twitter.com/odersky))
+Dmitry Petrashko ([@DarkDimius](https://twitter.com/DarkDimius)),
+Guillaume Martres ([@smarter](https://github.com/smarter)),
+Felix Mulder ([@felixmulder](https://twitter.com/felixmulder)),
+Nicolas Stucki ([@nicolasstucki](https://github.com/nicolasstucki)),
+Liu Fengyun ([@liufengyun](https://github.com/liufengyun)),
+Olivier Blanvillain ([@OlivierBlanvillain](https://github.com/OlivierBlanvillain)),
+and others!
+
+## Library authors: Join our community build
+
+Dotty now has a set of libraries that are built against every nightly snapshot.
+Currently this includes scalatest, squants and algebra.
+Join our [community build](https://github.com/lampepfl/dotty-community-build)
+ to make sure that our regression suite includes your library.
+
+
+To get started, see [https://github.com/lampepfl/dotty](https://github.com/lampepfl/dotty).
+
+
+[Scastie]: https://scastie.scala-lang.org/?target=dotty