Scalawags #38

thescalawags's podcast

Scalawags #38: The Sound of Dotty

YouTube link: https://www.youtube.com/watch?v=e687bFp2FT4

Our guest is Martin Odersky, the creator of Scala. We invited him on specifically to discuss Dotty, a new compiler for Scala.

Your hosts this episode: Josh Suereth, Dick Wall, Heather Miller, Seth Tisue

Join us during and between episodes for web-based Scalawags chat on Gitter.

Intro (0:00)

• Martin's been blogging like a maniac, as he promised
• Dick thinks Martin looks like the Milk Tray Man today, in his turtleneck

Where Dotty stands (1:50)

• Dotty is an "advanced research experimental platform for future Scala"
• in the works for about 3 years
• coming soon, an initial alpha release for brave coders who like bugs and mystical error messages
• Dotty is an open-source, volunteer project
  • open source contributors can talk to each other and the core team on Gitter
• how's the SBT support? it works already, but whole-program only, no incremental compilation yet
• Dmitry Petrashko is building an aggressive whole-program optimizer that uses TASTy
• what's TASTy? typed abstract syntax trees, a high-level intermediate format, the serialized output of the type checker, before code generation
• what programs has Dotty been tested on? "a handful". "most" of the Scala standard library, "all" of the collections, "a few" third-party projects
• is there a REPL? yes, doti, but brand new and minimal

Community reaction to Dotty (10:00)

• Martin's working on a new compiler instead of improving the old one, should we worry?
  • "You can't make everyone happy."
  • "Scala is due to have a big jump ahead."
• the experimental-compiler approach has been working well for C#, with their experimental Roslyn compiler: more features, more excitement. "We can move so much faster"
• Mads Torgersen: "We're stealing a lot of things from Scala, have you noticed?"
• change in scalac "has slowed considerably", which is "just right for a lot of people" since "change is the opposite of stability"
• perhaps Dotty will replace Scalac, or perhaps things will be backported to the old compiler

Type inference (12:40)

• Josh: will we get fully spec'ed type inference in Dotty?
• do other languages have this? Standard ML "might be the only one", "maybe OCaml"?
• it's a worthy goal, but there's a danger of fossilization, too

Dotty, the blog posts (15:15)

• the blog posts so far:
  • http://www.scala-lang.org/blog/2016/01/02/new-year-resolutions.html
  • http://www.scala-lang.org/blog/2016/02/03/essence-of-scala.html
  • http://www.scala-lang.org/blog/2016/02/17/scaling-dot-soundness.html
• the blog posts are the essence, what's the essence of the essence?
• the DOT calculus is a mini-language, like the lambda calculus, System J, et al
• proving soundness for DOT was very hard, but now it's done
  • and the proof is verified with Coq
• type projections were trouble area. that feature is "on its way out", to maintain soundness. instead, use implicits.
• can Shapeless still be made to work? it will be need to be adapted, but yes, Miles thinks using implicits instead will work
• Dick: how can Scala programmers prepare their code for Dotty?
  • number one is stay away from clever use of projections. many ordinary uses (e.g. to represent Java inner classes) will still be OK
  • all macros will need to be written, but the new version will be much simpler.
• Josh: what about macros for serializing case classes?
  • Martin: we'll support scrap-your-boilerplate out of the box (but we don't yet), no macros needed

Scala goes multiplatform (27:05)

• transition from JVM/.NET era design to JVM/JavaScript/LLVM era design, is this affecting Dotty decisions?
• aside from macros avoiding reflection (already covered), just "tiny and reasonable" tweaks, e.g.:
  • synchronized will be moved off AnyRef and into a Monitor trait
  • small numerics changes (isInstanceOf/asInstanceOf) motivated by Scala.js
• turning asInstanceOf checking off in Scala.js can give 30% or more speed improvement

Will Dotty replace scalac? (31:00)

• (audience Q from Rajesh Pitty)
• there are "no plans for that yet". if it happens it will be a major version bump, perhaps Scala 3
• other languages are doing this, e.g. Swift 3 ("radically different from Swift 2"), Python 3. we'll have to see how fast the community will be willing to follow
• how compatible is Dotty with Scala 2?
  • there is a switch to enable Scala 2 compatibility, and the hope is to help users with an automatic code rewriting tool (which doesn't exist yet, hey community want to take it on?)
  • if one manual intervention per thousand lines ends up being required, that seems reasonable
• Dick describes his experience moving to Python 3
  • 1 line in 100 needing manual updating wasn't horrible

Will there be a Scala VM? (36:20)

• (another audience Q from Rajesh Pitty)
• "definitely not", it's "too big a project", but TASTy has some of the same advantages by providing a back-end-agnostic platform
• Dick's dog Dottie corrects Martin's spelling

How can we help? (38:15)

• Heather: how can people help with Dotty? is there low-hanging fruit?
• Martin:
  • JLine integration in the REPL
  • the Dotty issue tracker actually has a low-hanging fruit tag
  • other REPL stuff in general
  • adding more sanity checks to the compiler
• user question from Pedro Furlanetto: where to start with reading about Dotty?
  • Martin: there's no language spec yet, it's in our heads, but look at closed issues in the Dotty issue tracker
• current main focus is compatibility with current Scala, so many planned new features aren't implemented yet
• Seth suggests: start trying to get cornerstone third-party projects like ScalaCheck, ScalaTest, and Specs2 to build, with an eye towards eventually getting a minimal community build up

Google Summer of Code (43:25)

• Scala was accepted for GSoC 2016
  • causing Heather to do a seated dance of joy in her chair
• contribute and get paid!
  • https://summerofcode.withgoogle.com/organizations/4954864794730496/
  • http://scala-lang.org/gsoc/2016.html
• suggested projects include two Dotty projects (Dotty documentation, Dotty REPL), and it's not too late for Martin to add more, and not too late for applicants to suggest new things
• Josh reminds us about streamerz, it's got everything you need for a dismally unsatisfying REPL experience
  • https://github.com/jsuereth/streamerz

Conclusion (45:40)

• so long!