Toccata

Antescofo is recently developed software for musical score following and mixed music: it automatically, and in real-time, synchronizes electronic instruments with a musician playing on a classical instrument. Therefore, it faces some of the same major challenges as embedded systems.

The system provides a programming language used by composers to specify musical pieces that mix interacting electronic and classical instruments. This language is developed with and for musicians and it continues to evolve according to their needs. Yet its semantics has only recently been formally defined. This paper presents a synchronous semantics for the core language of Antescofo and an alternative implementation based on an embedding inside an existing synchronous language, namely ReactiveML. The semantics reduces to a few rules, is mathematically precise and leads to an interpretor of only a few hundred lines. The efficiency of this interpretor compares well with that of the actual implementation: on all musical pieces we have tested, response times have been less than the reaction time of the human ear. Moreover, this embedding permitted the prototyping of several new programming constructs, some of which are described in this paper.

Mixed music is about live musicians interacting with electronic parts which are controlled by a computer during the performance. It allows composers to use and combine traditional instruments with complex synthesized sounds and other electronic devices. There are several languages dedicated to the writing of mixed music scores. Among them, the Antescofo language coupled with an advanced score follower allows a composer to manage the reactive aspects of musical performances: how electronic parts interact with a musician. However these domain specific languages do not offer the expressiveness of functional programming.

We embed the Antescofo language in a reactive functional programming language, ReactiveML. This approach offers to the composer recursion, higher order, inductive types, as well as a simple way to program complex reactive behaviors thanks to the synchronous model of concurrency on which ReactiveML is built. This article presents how to program mixed music in ReactiveML through several examples.

Concurrent and reactive systems often exhibit multiple time scales. For instance, in a discrete simulation, the scale at which agents communicate might be very different from the scale used to model the internals of each agent.

We propose an extension of the synchronous model of concurrency, called reactive domains, to simplify the programming of such systems. Reactive domains allow the creation of local time scales and enable refinement, that is, the replacement of an approximation of a system with a more detailed version without changing its behavior as observed by the rest of the program.

Our work is applied to the ReactiveML language, which extends ML with synchronous language constructs. We present an operational semantics for the extended language and a type system that ensures the soundness of programs.

La concurrence coopérative est un modèle de programmation très répandu. On peut par exemple l'utiliser en OCaml à travers des bibliothèques comme Lwt, Async ou Equeue. Il a de nombreux avantages tels que l'absence de courses critiques et des implantations légères et efficaces. Néanmoins, un des inconvénients majeurs de ce modèle est qu'il dépend de la discipline du programmeur pour garantir que le système est réactif : un processus peut empêcher les autres de s'exécuter.

ReactiveML est un langage qui étend OCaml avec des constructions de concurrence coopérative. Il propose une analyse statique, l'analyse de réactivité, qui permet de détecter les expressions qui risquent de produire des comportements non coopératifs. Dans cet article, nous présentons cette analyse statique qui se définit à l'aide d'un système de types et effets. Ainsi, comme le typage de données aide les programmeurs à détecter des erreurs d'exécution au plus tôt, l'analyse de réactivité aide à détecter des erreurs de concurrence.

The use of constraints in types is quite natural. Yet, integrating constraint based types into the heart of a modern, statically typed, object-oriented programming language is quite tricky. Over the last five years we have designed and implemented the constrained types framework in the programming language X10. In this paper we review the conceptual design, the practical implementation issues, and the many new questions that are raised. We expect the pursuit of these questions to be a profitable area of future work.

Lucy-n is a language for programming networks of processes communicating through bounded buffers. A dedicated type system, termed a clock calculus, automatically computes static schedules of the processes and the sizes of the buffers between them.

In this article, we present a new algorithm which solves the subtyping constraints generated by the clock calculus. The advantage of this algorithm is that it finds schedules for tightly coupled systems. Moreover, it does not overestimate the buffer sizes needed and it provides a way to favor either system throughput or buffer size minimization.

Lucy-n is a dataflow programming language similar to Lustre extended with a buffer operator. This language is based on the n-synchronous model which was initially introduced for programming multimedia streaming applications. In this article, we show that Lucy-n is also applicable to model Latency Insensitive Designs (LID). In order to model relay stations, we have to introduce a delay operator. Thanks to this new operator, a LID can be described by a Lucy-n program. Then, the Lucy-n compiler automatically provides static schedules for computation nodes and buffer sizes needed in the shell wrappers.

Lucy-n est un langage permettant de programmer des réseaux de processus communiquant à travers des buffers de taille bornée. La taille des buffers et les rythmes d'exécution relatifs des processus sont calculés par une phase de typage appelée calcul d'horloge. Ce typage nécessite la résolution d'un ensemble de contraintes de sous-typage. L'an dernier, nous avons proposé un algorithme de résolution de ces contraintes utilisant des méthodes issues de l'interprétation abstraite. Cette année nous présentons un algorithme tirant profit de toute l'information contenue dans les types.

Synchronous functional languages such as Lustre or Lucid Synchrone define a restricted class of Kahn Process Networks which can be executed with no buffer. Every expression is associated to a clock indicating the instants when a value is present. A dedicated type system, the clock calculus, checks that the actual clock of a stream equals its expected clock and thus does not need to be buffered. The n-synchrony relaxes synchrony by allowing the communication through bounded buffers whose size is computed at compile-time. It is obtained by extending the clock calculus with a subtyping rule which defines buffering points.

This paper presents the first implementation of the n-synchronous model inside a Lustre-like language called Lucy-n. The language extends Lustre with an explicit buffer construct whose size is automatically computed during the clock calculus. This clock calculus is defined as an inference type system and is parametrized by the clock language and the algorithm used to solve subtyping constraints. We detail here one algorithm based on the abstraction of clocks, an idea originally introduced in [17]. The paper presents a simpler, yet more precise, clock abstraction for which the main algebraic properties have been proved in Coq. Finally, we illustrate the language on various examples including a video application.

Les langages synchrones flot de données permettent de programmer des réseaux de processus communicant sans buffers. Pour cela, chaque flot est associé à un type d'horloges, qui indique les instants de présence de valeurs sur le flot. La communication entre deux processus f et g peut être faite sans buffer si le type du flot de sortie de f est égal au type du flot d'entrée de g. Un système de type, le calcul d'horloge, infère des types tels que cette condition est vérifiée. Le modèle n-synchrone a pour but de relâcher ce modèle de programmation en autorisant les communications à travers des buffers de taille bornée. En pratique, cela consiste à introduire une règle de sous-typage dans le calcul d'horloge.

Nous avons présenté l'année dernière un article décrivant comment abstraire des horloges pour vérifier la relation de sous-typage. Cette année, nous présentons un langage de programmation n-synchrone : Lucy-n. Dans ce langage, l'inférence des types d'horloges est paramétrable par l'algorithme de résolution des contraintes de sous-typage. Nous montrons ici un algorithme basé sur les travaux de l'an dernier et comment programmer en Lucy-n à travers l'exemple d'une application de traitement multimédia.

Les langages synchrones flot de données tels que Lustre manipulent des séquences infinies de données comme valeurs de base. Chaque flot est associé à une horloge qui définit les instants où sa valeur est présente. Cette horloge est une information de type et un système de types dédié, le calcul d'horloges, rejette statiquement les programmes qui ne peuvent pas être exécutés de manière synchrone. Dans les langages synchrones existants, cela revient à se demander si deux flots ont la même horloge et repose donc uniquement sur l'égalité d'horloges. Des travaux récents ont montré l'intérêt d'introduire une notion relâchée du synchronisme, où deux flots peuvent être composés dès qu'ils peuvent être synchronisés par l'introduction d'un buffer de taille bornée (comme c'est fait dans le modèle SDF d'Edward Lee). Techniquement, cela consiste à remplacer le typage par du sous-typage. Ce papier est une traduction et amélioration technique de [17] qui présente un moyen simple de mettre en oeuvre ce modèle relâché par l'utilisation d'horloges abstraites. Les valeurs abstraites représentent des ensembles d'horloges concrètes qui ne sont pas nécessairement périodiques. Cela permet de modéliser divers aspects des logiciels temps-réel embarqués, tels que la gigue bornée présente dans les systèmes vidéo, le temps d'exécution des processus temps réel et, plus généralement, la communication à travers des buffers de taille bornée. Nous présentons ici l'algèbre des horloges abstraites et leurs principales propriétés théoriques.

Synchronous data-flow languages such as Lustre manage infinite sequences or streams as basic values. Each stream is associated to a clock which defines the instants where the current value of the stream is present. This clock is a type information and a dedicated type system --- the so-called clock-calculus --- statically rejects programs which cannot be executed synchronously. In existing synchronous languages, it amounts at asking whether two streams have the same clocks and thus relies on clock equality only. Recent works have shown the interest of introducing some relaxed notion of synchrony, where two streams can be composed as soon as they can be synchronized through the introduction of a finite buffer (as done in the SDF model of Edward Lee). This technically consists in replacing typing by subtyping. The present paper introduces a simple way to achieve this relaxed model through the use of clock envelopes. These clock envelopes are sets of concrete clocks which are not necessarily periodic. This allows to model various features in real-time embedded software such as bounded jitter as found in video-systems, execution time of real-time processes and scheduling resources or the communication through buffers. We present the algebra of clock envelopes and its main theoretical properties.