A reduction semantics for direct-style asynchronous observables

Abstract

Asynchronous programming has gained in importance, not only due to hardware developments like multi-core processors, but also due to pervasive asynchronicity in client-side Web programming and large-scale Web applications. However, asynchronous programming is challenging. For example, control-flow management and error handling are much more complex in an asynchronous than a synchronous context. Programming with asynchronous event streams is especially difficult: expressing asynchronous stream producers and consumers requires explicit state machines in continuation-passing style when using widely-used languages like Java.In order to address this challenge, recent language designs like Google's Dart introduce asynchronous generators which allow expressing complex asynchronous programs in a familiar blocking style while using efficient non-blocking concurrency control under the hood. However, several issues remain unresolved, including the integration of analogous constructs into statically-typed languages, and the formalization and proof of important correctness properties.This paper presents a design for asynchronous stream generators for Scala, thereby extending previous facilities for asynchronous programming in Scala from tasks/futures to asynchronous streams. We present a complete formalization of the programming model based on a reduction semantics and a static type system. Building on the formal model, we contribute a complete type soundness proof, as well as the proof of a subject reduction theorem which establishes that the programming model enforces an important state transition protocol for asynchronous streams.