ScalaDyno: Making Name Resolution and Type Checking Fault-tolerant

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.

Cross-Platform Language Design

Sébastien Jean R Doeraene

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.

EPFL2018

Programming language abstractions for mobile code

Stéphane Micheloud

Scala is a general-purpose programming language developed at EPFL. It combines the most important concepts found in object-oriented and functional languages. Scala is a statically typed language; in particular it features an advanced type system and supports local type inference. Furthermore it integrates well with the Java and .net platforms: their libraries are accessible without glue code and the Scala compiler generates code for both execution environments. The Scala programming language has several features that make it desirable as a language for distributed application programming. In particular, it supports first-class functions which are useful in relation with the notions of distributed scope and code mobility. In that context, the missing support for run-time types is one important drawback of the Java run-time environment as a target platform. This thesis focuses on the realisation of a new concept combining essential notions from the functional and distributed programming and implying the extension of the notion of lexical scoping to the distributed context. In short, we claim that the notion of lambda abstraction provides an elegant way for dealing with the dynamic rebinding of local references in a distributed execution environment. The key ideas exposed in this research work have been implemented in our Scala compiler. This helped us to evaluate the used techniques, in particular their impact on the reliability and the performance of distributed programs. So far, most research works related to the present subject have focused on functional programming languages, in particular on the ML language family.

EPFL2009

Compiling Scala for Performance

Iulian Dragos

Scala is a new programming language bringing together object-oriented and functional programming. Its defining features are uniformity and extensibility. Scala offers great flexibility for programmers, allowing them to grow the language through libraries. Oftentimes what seems like a language feature is in fact implemented in a library, effectively giving programmers the power of language designers. The downside of this flexibility is that familiar looking code may hide unexpected performance costs. It is important for Scala compilers to bring down this cost as much as possible. We identify several areas of impact for Scala performance: higher-order functions and closures, and generic containers used with primitive types. We present two complementary approaches for improving performance in these areas: optimizations and specialization. Compiler optimization can bring down the cost through a combination of aggressive inlining of higher-order functions, an extended version of copy-propagation and dead-code elimination. Both anonymous functions and boxing can be eliminated by this approach. We show on a number of benchmarks that these language features can be up to 5 times faster when properly optimized, on current day JVMs. We propose a new approach to compiling parametric polymorphism for performance at primitive types. We mix a homogeneous translation scheme with user-directed specialization for primitive types. Type parameters may be annotated to require specialization of code depending on them. We propose definition-site specialization for primitive types, achieving separate compilation and no boxing when both the definition and call site are specialized. Specialized classes are compatible with unspecialized code, and specialization agnostic code can work with specialized instances, meaning that specialization is opportunistic. We present a formalism of a small subset of Scala with specialization and prove that specialization preserves types. We implemented this translation in the Scala compiler and report on improvements on a set of benchmarks, showing that specialization can make programs more than two times faster.