Philipp Haller

Scala Actors: Unifying thread-based and event-based programming

There is an impedance mismatch between message-passing concurrency and virtual machines, such as the JVM. VMs usually map their threads to heavyweight OS processes. Without a lightweight process abstraction, users are often forced to write parts of concurrent applications in an event-driven style which obscures control flow, and increases the burden on the programmer. In this paper we show how thread-based and event-based programming can be unified under a single actor abstraction. Using advanced abstraction mechanisms of the Scala programming language, we implement our approach on unmodified JVMs. Our programming model integrates well with the threading model of the underlying VM.

Event-Based programming without inversion of control

Concurrent programming is indispensable. On the one hand, distributed and mobile environments naturally involve concurrency. On the other hand, there is a general trend towards multi-core processors that are capable of running multiple threads in parallel.

Actors that unify threads and events

There is an impedance mismatch between message-passing concurrency and virtual machines, such as the JVM. VMs usually map their threads to heavyweight OS processes. Without a lightweight process abstraction, users are often forced to write parts of concurrent applications in an event-driven style which obscures control flow, and increases the burden on the programmer. In this paper we show how thread-based and event-based programming can be unified under a single actor abstraction. Using advanced abstraction mechanisms of the Scala programming language, we implemented our approach on unmodified JVMs. Our programming model integrates well with the threading model of the underlying VM.

Implementing joins using extensible pattern matching

Join patterns are an attractive declarative way to synchronize both threads and asynchronous distributed computations. We explore joins in the context of extensible pattern matching that recently appeared in languages such as F# and Scala. Our implementation supports join patterns with multiple synchronous events, and guards. Furthermore, we integrated joins into an existing actor-based concurrency framework. It enables join patterns to be used in the context of more advanced synchronization modes, such as future-type message sending and token-passing continuations.

Scala-Virtualized: linguistic reuse for deep embeddings

Scala-Virtualized extends the Scala language to better support hosting embedded DSLs. Scala is an expressive language that provides a flexible syntax, type-level computation using implicits, and other features that facilitate the development of embedded DSLs. However, many of these features work well only for shallow embeddings, i.e. DSLs which are implemented as plain libraries. Shallow embeddings automatically profit from features of the host language through linguistic reuse: any DSL expression is just as a regular Scala expression. But in many cases, directly executing DSL programs within the host language is not enough and deep embeddings are needed, which reify DSL programs into a data structure representation that can be analyzed, optimized, or further translated. For deep embeddings, linguistic reuse is no longer automatic.Scala-Virtualized defines many of the language’s built-in constructs as method calls, which enables DSLs to redefine the built-in semantics using familiar language mechanisms like overloading and overriding. This in turn enables an easier progression from shallow to deep embeddings, as core language constructs such as conditionals or pattern matching can be redefined to build a reified representation of the operation itself.While this facility brings shallow, syntactic, reuse to deep embeddings, we also present examples of what we call deep linguistic reuse: combining shallow and deep components in a single DSL in such a way that certain features are fully implemented in the shallow embedding part and do not need to be reified at the deep embedding level.

Lightweight polymorphic effects

Type-and-effect systems are a well-studied approach for reasoning about the computational behavior of programs. Nevertheless, there is only one example of an effect system that has been adopted in a wide-spread industrial language: Javas checked exceptions. We believe that the main obstacle to using effect systems in day-to-day programming is their verbosity, especially when writing functions that are polymorphic in the effect of their argument. To overcome this issue, we propose a new syntactically lightweight technique for writing effect-polymorphic functions. We show its independence from a specific kind of side-effect by embedding it into a generic and extensible framework for checking effects of multiple domains. Finally, we verify the expressiveness and practicality of the system by implementing it for the Scala programming language.

FlowPools: A Lock-Free Deterministic Concurrent Dataflow Abstraction

Implementing correct and deterministic parallel programs is challenging. Even though concurrency constructs exist in popular programming languages to facilitate the task of deterministic parallel programming, they are often too low level, or do not compose well due to underlying blocking mechanisms. In this paper, we present the design and implementation of a fundamental data structure for composable deterministic parallel dataflow computation through the use of functional programming abstractions. Additionally, we provide a correctness proof, showing that the implementation is linearizable, lock-free, and deterministic. Finally, we show experimental results which compare our FlowPool against corresponding operations on other concurrent data structures, and show that in addition to offering new capabilities, FlowPools reduce insertion time by 49 − 54% on a 4-core i7 machine with respect to comparable concurrent queue data structures in the Java standard library.

On the integration of the actor model in mainstream technologies: the scala perspective

Integrating the actor model into mainstream software platforms is challenging because typical runtime environments, such as the Java Virtual Machine, have been designed for very different concurrency models. Moreover, to enable integration with existing infrastructures, execution modes and constructs foreign to the pure actor model have to be supported. This paper provides an overview of past and current efforts to address these challenges in the context of the Scala programming language.

Spores: A Type-Based Foundation for Closures in the Age of Concurrency and Distribution

Functional programming FP is regularly touted as the way forward for bringing parallel, concurrent, and distributed programming to the mainstream. The popularity of the rationale behind this viewpoint has even led to a number of object-oriented OO programming languages outside the Smalltalk tradition adopting functional features such as lambdas and thereby function closures. However, despite this established viewpoint of FP as an enabler, reliably distributing function closures over a network, or using them in concurrent environments nonetheless remains a challenge across FP and OO languages. This paper takes a step towards more principled distributed and concurrent programming by introducing a new closure-like abstraction and type system, called spores, that can guarantee closures to be serializable, thread-safe, or even have custom user-defined properties. Crucially, our system is based on the principle of encoding type information corresponding to captured variables in the type of a spore. We prove our type system sound, implement our approach for Scala, evaluate its practicality through a small empirical study, and show the power of these guarantees through a case analysis of real-world distributed and concurrent frameworks that this safe foundation for closures facilitates.

Functional programming for all! scaling a MOOC for students and professionals alike

Massive open online courses (MOOCs) have launched a scale shift in higher education, with several individual MOOCs now boasting tens or hundreds of thousands of participants worldwide. Our MOOC on the principles of functional programming has more than 100,000 registered students to date, and boasts one of the highest rates of completion (19.2%) for its size. In this paper, we describe our experience organizing this popular MOOC, and demonstrate how providing innovative supporting tools (IDE plugins, testing frameworks, interactive build tools, automated cloud-based graders, style checkers) and considering key human-computer interaction factors potentially contributed to this markedly high completion rate. We collect an unprecedented volume of course statistics and survey results and have made them available, along with scripts for generating interactive web-based visualizations, as an open-source project.