Scala Center Community Input

This document outlines some of the ENSIME maintainers’ thoughts for the consideration of the Scala Center, impacting the entire Scala Community.

Dotty and IDEs

It is concerning that Dotty does not yet have a presentation compiler. Without a presentation compiler, it could mean the end of the road for both ENSIME and Scala IDE.

ENSIME and Scala IDE share several common backends including the PresentationCompiler and scala-refactoring. We committed to expand the collaboration through the recently awarded Google Summer of Code project on Source Code Preserving ASTs.

In contrast, IntelliJ IDEA provides Scala editing support via an independently developed type-aware parser. Although the GSoC could allow for collaboration, it is highly likely that IDEA will continue autonomously. IDEA’s commercial backing allows for development on features that could see it become the only editor for Dotty.

We hope that Dotty will shortly provide a replacement for the interactive PresentationCompiler (and something like scalap for TASTY), addressing the shortcomings of the current presentation compiler. We are aware that the amount of effort required to do this is significant. Indeed, it is too much for the volunteers at ENSIME to take on. This initiative requires an experienced full time developer in EPFL / Scala Center, with close access to the Dotty team, in order to complete.


The Scala Contributor License Agreement (the “Scala CLA”) is similar to the Apache CLA: effectively an exclusive Apache 2.0 License Agreement from contributor to EPFL.

The Apache 2.0 is a permissive Free / Libre Software license, recommended by the Free Software Foundation and held in high esteem by Corporates and small companies all over the globe. Importantly, the Apache 2.0 includes a “Grant of Patent License” and a patent retaliation clause, which nullifies any legal wranglings over patents between contributors and users of the project.

However, the Scala distribution is made available under the Scala License, a 3-Clause Simplified BSD License. Although a permissive Free / Libre software license, the Scala License does not provide any grant of patent. This is concerning because there is no guarantee to recipients of the scala compiler and standard libraries that they have the right to use the software without being liable to pay a patent royalty.

It would be consistent, and fair to contributors, for the Scala distribution to be made available under the terms of the Apache 2.0 License. We trust that the Apache 2.0 is a better reflection of EPFL’s approach so far, so this is simply a formal commitment to continue doing the same thing!

Investment in PresentationCompiler

The current PresentationCompiler is sadly lacking in the area of macro support and is unable to keep up with the many boilerplate-removal macro libraries that are becoming more common. As examples, the presentation compiler cannot cope with the macros used in the sbt API (introduced to reduce repetition), or the Typelevel macro projects that reduce boilerplate when using the typeclass / functional style.

The ENSIME authors have set up an exploratory project, with the goal of raising awareness of this limitation to macro authors: A possible short term workaround may be that some macro authors provide a “presentation compiler plugin” that acts only in the earlier phases of the compile.

It would be beneficial to all ENSIME and Scala IDE users if the Scala Center were to improve the support for macros inside the presentation compiler. The first step could be to create a lightweight testing library that macro authors use to test the behaviour of their macros in the presentation compiler. Once tests are easy to write, a standalone regression suite could be produced, allowing for improvements in the presentation compiler itself.

Investment in sbt

It is no secret that the Scala compiler is slow. An efficient incremental compiler is the low hanging fruit that could speed up the workflow for developers.

sbt is a critical part of most build pipelines and it includes an incremental compiler, Zinc. However, independent work on incremental compilation is being undertaken inside IntelliJ and behind closed doors in proprietary and lesser known build tools.

If all hands were improving a single code base, we could avoid duplication of efforts, focus on the bottlenecks, and bring down everybody’s build times. Key to this is the concept of sbt as a server, which is the enabling technology for IntelliJ, Scala IDE and ENSIME.

It would be beneficial to all Scala developers if the Scala Center could provide the resource to help sbt and IDEs integrate with each other via sbt-server, and for the incremental compiler itself to receive further performance improvements. There are many avenues to explore, from improved I/O and caching to better management and persistence of the inter-file dependency structure (perhaps sharing information between developers and a continuous integration server).