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The ubiquity of multicore computers has forced programming language designers to rethink how languages express parallelism and concurrency. This has resulted in new language constructs and new combinations or revisions of existing constructs. In this line, we extended the programming languages Encore (actor-based), and Clojure (functional) with an asynchronous parallel abstraction called ParT, a data structure that can dually be seen as a collection of asynchronous values (integrating with futures) or a handle to a parallel computation, plus a collection of combinators for manipulating the data structure. The combinators can express parallel pipelines and speculative parallelism. This paper presents a typed calculus capturing the essence of ParT, abstracting away from details of the Encore and Clojure programming languages. The calculus includes tasks, futures, and combinators similar to those of Orc but implemented in a non-blocking fashion. Furthermore, the calculus strongly mimics how ParT is implemented, and it can serve as the basis for adaptation of ParT into different languages and for further extensions.

In many actor-based programming models, asynchronous method calls communicate their results using futures, where the fulfilment occurs under-the-hood. Promises play a similar role to futures, except that they must be explicitly created and explicitly fulfilled; this makes promises more flexible than futures, though promises lack fulfilment guarantees: they can be fulfilled once, multiple times or not at all. Unfortunately, futures are too rigid to exploit many available concurrent and parallel patterns. For instance, many computations block on a future to get its result only to return that result immediately (to fulfil their own future). To make futures more flexible, we explore a construct, forward, that delegates the responsibility for fulfilling the current implicit future to another computation. Forward reduces synchronisation and gives futures promise-like capabilities. This paper presents a formalisation of the forward construct, defined in a high-level source language, and a compilation strategy from the high-level language to a low-level, promised-based target language. The translation is shown to preserve semantics. Based on this foundation, we describe the implementation of forward in the parallel, actor-based language Encore, which compiles to C.

Concurrent programs often make use of futures, handles to the results of asynchronous operations. Futures provide means to communicate not yet computed results, and simplify the implementation of operations that synchronise on the result of such asynchronous operations. Futures can be characterised as implicit or explicit, depending on the typing discipline used to type them. Current future implementations suffer from "future proliferation", either at the type-level or at run-time. The former adds future type wrappers, which hinders subtype polymorphism and exposes the client to the internal asynchronous communication architecture. The latter increases latency, by traversing nested future structures at run-time. Many languages suffer both kinds. Previous work offer partial solutions to the future proliferation problems; in this paper we show how these solutions can be integrated in an elegant and coherent way, which is more expressive than either system in isolation. We describe our proposal formally, and state and prove its key properties, in two related calculi, based on the two possible families of future constructs (data-flow futures and control-flow futures). The former relies on static type information to avoid unwanted future creation, and the latter uses an algebraic data type with dynamic checks. We also discuss how to implement our new system efficiently.

Dynamic languages like Erlang, Clojure, JavaScript, and E adopted data-race freedom by design. To enforce data-race freedom, these languages either deep copy objects during actor (thread) communication or proxy back to their owning thread. We present Dala, a simple programming model that ensures data-race freedom while supporting efficient inter-thread communication. Dala is a dynamic, concurrent, capability-based language that relies on three core capa- bilities: immutable values can be shared freely; isolated mutable objects can be transferred between threads but not aliased; local objects can be aliased within their owning thread but not dereferenced by other threads. The addition of a fourth capability, unsafe, allows data races and we show how data-race free programs interact with unsafe programs. We present a formal model of Dala, prove data race-freedom and the dynamic gradual guarantee. These theorems guarantee data race-freedom when using safe capabilities and show that the addition of capabili- ties is semantics preserving modulo permission and cast errors.