Cross-Platform Language Design

Programming languages are increasingly compiled to multiple runtimes, each featuring their own rich structures such as their object model. Furthermore, they need to interact with other languages targeting said runtimes. A language targeting only one runtime can be designed to tailor its semantics to those of that runtime, for easy interoperability with other languages. However, in a language targeting multiple runtimes with differing semantics, it is difficult to cater to each of them while retaining a common behavior across runtimes. We call \emph{cross-platform language} a language that aims at being both \emph{portable} across platforms and \emph{interoperable} with each target platform. Portability is the ability for a program or a library to cross-compile for multiple platforms, and behave the same way on all of them. Interoperability is the ability to communicate with other languages on the same platform. While many cross-compiling languages focus on one of these two properties---only adding support for the other one as an afterthought---, languages that are designed from the ground up to support both are rare.

In this thesis, we present the design of Scala.js, the dialect of Scala targeting the JavaScript platform, which turned Scala into a cross-platform language. On the one hand, Scala programs can be cross-compiled for the JVM and JavaScript with portable semantics. On the other hand, whereas Scala/JVM interoperates with Java, Scala.js interoperates with JavaScript, allowing to use any JavaScript library. Along the dissertation, we give insights that can be transferred to the design of other cross-platform languages, although with a bias towards those targeting the JVM and JavaScript.

The first and most obvious challenge is to reconcile the static nature of Scala's object model with JavaScript's dynamic one. Besides the ability to mutate a class hierarchy at run-time in JavaScript, there are fundamental differences between the two models, in particular the difference between compile-time overloading and run-time overloading. We discuss how such semantic mismatches can live in harmony within the language.

The second challenge is to obtain good performance from a language where interoperability with a dynamic and unknown part of the program is pervasive. To that end, we design and specify an intermediate representation (IR) with first-class support for dynamically typed interoperability features in addition to statically typed JVM-style operations. Despite its tight integration with the open world of JavaScript, most of the IR can be considered as a closed world where advanced whole-program optimizations can be performed. The performance of the overall system is evaluated and shown to be competitive with hand-written JavaScript, and even with Scala/JVM in some cases.

Cross-Platform Language Design in Scala.js (Keynote)

Sébastien Jean R Doeraene

Have you ever wondered what is the secret sauce of Scala.js? What defines Scala.js, above all else, is the overarching will to make it cross-platform. A cross-platform language is both portable-most source code cross-compiles and behaves the same way on multiple platforms-and interoperable-code written in that language can interoperate with other languages on the target platforms. Most multi-platform languages are designed with one of those two properties in mind, and only address the other one as an afterthought. This results in poor integration, difficulty to write portable code, or even the impossibility to use some platform-specific libraries. This talk shows how Scala.js was designed as a cross-platform language. We show how portability and interoperability drive every design decision, from which language feature to include to performance considerations. We state a criteria of completeness for interoperability, and show how Scala.js satisfies it (with the exception of one JavaScript language feature, still work-in-progress). We give insights about what features are critical for portability, and which ones can be relaxed in the name of performance. In particular, we stress the importance of the ecosystem of libraries: existing assumptions in the ecosystem can force the language designer into reproducing behaviors of Scala/JVM, even when they are accidental. As a practical exercise, we study the main language changes that made it into Scala.js 1.x wrt. 0.6.x-as well as possible future developments-and how they fit into the cross-platform language design methodology.

ASSOC COMPUTING MACHINERY2018

ScalaDyno: Making Name Resolution and Type Checking Fault-tolerant

Cédric Bastin, Martin Odersky, Vlad Ureche

The ScalaDyno compiler plugin allows fast prototyping with the Scala programming language, in a way that combines the benefits of both statically and dynamically typed languages. Static name resolution and type checking prevent partially-correct code from being compiled and executed. Yet, allowing programmers to test critical paths in a program without worrying about the consistency of the entire code base is crucial to fast prototyping and agile development. This is where ScalaDyno comes in: it allows partially-correct programs to be compiled and executed, while shifting compile-time errors to program runtime. The key insight in ScalaDyno is that name and type errors affect limited areas of the code, which can be replaced by instructions to output the respective errors at runtime. This allows byte code generation and execution for partially correct programs, thus allowing Python or JavaScript-like fast prototyping in Scala. This is all done without sacrificing name resolution, full type checking and optimizations for the correct parts of the code -- they are still performed, but without getting in the way of agile development. Finally, for release code or sensitive refactoring, runtime errors can be disabled, thus allowing full static name resolution and type checking typical of the Scala compiler.

ACM2014

Semantics-Driven Interoperability between Scala.js and JavaScript

Sébastien Jean R Doeraene, Tobias Schlatter, Nicolas Alexander Stucki

Hundreds of programming languages compile to JavaScript. Yet, most of them fail, at one level or another, to provide satisfactory interoperability with JavaScript APIs. This is limiting, as interoperability is at least required to manipulate web pages through the DOM API, but also to use the ecosystem of existing JavaScript libraries. This paper presents the interoperability features of Scala.js, which solves the shortcomings of previous approaches. Scala. js offers a separate hierarchy of JavaScript types, whose operations have semantics borrowed from ECMAScript 2015. The interoperability features are complete with respect to ECMAScript 2015, save for two exceptions which are still being worked on. This allows Scala. js programs to perform any operation that an ECMAScript program could do, thereby guaranteeing that they can talk to any JavaScript library.

Association for Computing Machinery2016