arXiv subjects

Luís Cruz-Filipe

Publications and source records attributed to Luís Cruz-Filipe.

At least 19 recordsLinked to original sources

arXiv2026

FÆRDXEL: An Expert System for Danish Traffic Law

We present FÆRDXEL, a tool for symbolic reasoning in the domain of Danish traffic law. FÆRDXEL combines techniques from logic programming with a novel interface that allows users to navigate through its reasoning process, thereby ensuring the system's explainability. Towards the goal of better understanding the value of FÆRDXEL, two evaluations of the system have been performed: (1) An empirical evaluation showing that for a selection of court cases, the conclusions of FÆRDXEL align with those of Danish judges. (2) A qualitative evaluation from legal experts indicating that this work has potential to become a foundation for real-world AI tools supporting professionals in the Danish legal sector.

cs.AI↗

arXiv2024

Hypothetical answers to continuous queries over data streams

Continuous queries over data streams may suffer from blocking operations and/or unbound wait, which may delay answers until some relevant input arrives through the data stream. These delays may turn answers, when they arrive, obsolete to users who sometimes have to make decisions with no help whatsoever. Therefore, it can be useful to provide hypothetical answers - "given the current information, it is possible that X will become true at time t" - instead of no information at all. In this paper we present a semantics for queries and corresponding answers that covers such hypothetical answers, together with an online algorithm for updating the set of facts that are consistent with the currently available information.

cs.PL↗

arXiv2023

Reasoning about Choreographic Programs

Choreographic programming is a paradigm where a concurrent or distributed system is developed in a top-down fashion. Programs, called choreographies, detail the desired interactions between processes, and can be compiled to distributed implementations based on message passing. Choreographic languages usually guarantee deadlock-freedom and provide an operational correspondence between choreographies and their compiled implementations, but until now little work has been done on verifying other properties. This paper presents a Hoare-style logic for reasoning about the behaviour of choreographies, and illustrate its usage in representative examples. We show that this logic is sound and complete, and discuss decidability of its judgements. Using existing results from choreographic programming, we show that any functional correctness property proven for a choreography also holds for its compiled implementation.

cs.PL↗

arXiv2023

Certified Compilation of Choreographies with hacc

Programming communicating processes is challenging, because it requires writing separate programs that perform compatible send and receive actions at the right time during execution. Leaving this task to the programmer can easily lead to bugs. Choreographic programming addresses this challenge by equipping developers with high-level abstractions for codifying the desired communication structures from a global viewpoint. Given a choreography, implementations of the involved processes can be automatically generated by endpoint projection (EPP). While choreographic programming prevents manual mistakes in the implementation of communications, the correctness of a choreographic programming framework crucially hinges on the correctness of its complex compiler, which has motivated formalisation of theories of choreographic programming in theorem provers. In this paper, we build upon one of these formalisations to construct a toolchain that produces executable code from a choreography.

cs.PL↗

arXiv2023

Now It Compiles! Certified Automatic Repair of Uncompilable Protocols

Choreographic programming is a paradigm where developers write the global specification (called choreography) of a communicating system, and then a correct-by-construction distributed implementation is compiled automatically. Unfortunately, it is possible to write choreographies that cannot be compiled, because of issues related to an agreement property known as knowledge of choice. This forces programmers to reason manually about implementation details that may be orthogonal to the protocol that they are writing. Amendment is an automatic procedure for repairing uncompilable choreographies. We present a formalisation of amendment from the literature, built upon an existing formalisation of choreographic programming. However, in the process of formalising the expected properties of this procedure, we discovered a subtle counterexample that invalidates the original published and peer-reviewed pen-and-paper theory. We discuss how using a theorem prover led us to both finding the issue, and stating and proving a correct formulation of the properties of amendment.

cs.LO↗

arXiv2022

A Formal Theory of Choreographic Programming

Choreographic programming is a paradigm for writing coordination plans for distributed systems from a global point of view, from which correct-by-construction decentralised implementations can be generated automatically. Theory of choreographies typically includes a number of complex results that are proved by structural induction. The high number of cases and the subtle details in some of these proofs has led to important errors being found in published works. In this work, we formalise the theory of a choreographic programming language in Coq. Our development includes the basic properties of this language, a proof of its Turing completeness, a compilation procedure to a process language, and an operational characterisation of the correctness of this procedure. Our formalisation experience illustrates the benefits of using a theorem prover: we get both an additional degree of confidence from the mechanised proof, and a significant simplification of the underlying theory. Our results offer a foundation for the future formal development of choreographic languages.

cs.LO↗

arXiv2022

Functional Choreographic Programming

Choreographic programming is an emerging programming paradigm for concurrent and distributed systems, whereby developers write the communications that should be enacted and then a distributed implementation is automatically obtained by means of a compiler. Theories of choreographic programming typically come with strong theoretical guarantees about the compilation process, most notably: the generated implementations operationally correspond to their source choreographies and are deadlock-free. Currently, the most advanced incarnation of the paradigm is Choral, an object-oriented choreographic programming language that targets Java. Choral deviated significantly from known theories of choreographies, and introduced the possibility of expressing higher-order choreographies (choreographies parameterised over choreographies) that are fully distributed. As a consequence, it is unclear if the usual guarantees of choreographies can still hold in the more general setting of higher-order ones. We introduce Chorλ, the first functional choreographic programming language: it introduces a new formulation of the standard communication primitive found in choreographies as a function, and it is based upon the λ-calculus. Chorλ is the first theory that explains the core ideas of higher-order choreographic programming (as in Choral). Bridging the gap between practice and theory requires developing a new evaluation strategy and typing discipline for λ terms that accounts for the distributed nature of computation in choreographies. We illustrate the expressivity of Chorλ with a series of examples, which include reconstructions of the key examples from the original presentation of Choral. Our theory supports the expected properties of choreographic programming and bridges the gap between the communities of functional and choreographic programming.

cs.PL↗

arXiv2022

From Infinity to Choreographies: Extraction for Unbounded Systems

Choreographies are formal descriptions of distributed systems, which focus on the way in which participants communicate. While they are useful for analysing protocols, in practice systems are written directly by specifying each participant's behaviour. This created the need for choreography extraction: the process of obtaining a choreography that faithfully describes the collective behaviour of all participants in a distributed protocol. Previous works have addressed this problem for systems with a predefined, finite number of participants. In this work, we show how to extract choreographies from system descriptions where the total number of participants is unknown and unbounded, due to the ability of spawning new processes at runtime. This extension is challenging, since previous algorithms relied heavily on the set of possible states of the network during execution being finite.

cs.PL↗

arXiv2021

Certifying Choreography Compilation

Choreographic programming is a paradigm for developing concurrent and distributed systems, where programs are choreographies that define, from a global viewpoint, the computations and interactions that communicating processes should enact. Choreography compilation translates choreographies into the local definitions of process behaviours, given as terms in a process calculus. Proving choreography compilation correct is challenging and error-prone, because it requires relating languages in different paradigms (global interactions vs local actions) and dealing with a combinatorial explosion of proof cases. We present the first certified program for choreography compilation for a nontrivial choreographic language supporting recursion.

cs.PL↗

arXiv2021

Formalising a Turing-Complete Choreographic Language in Coq

Theory of choreographic languages typically includes a number of complex results that are proved by structural induction. The high number of cases and the subtle details in some of them lead to long reviewing processes, and occasionally to errors being found in published proofs. In this work, we take a published proof of Turing completeness of a choreographic language and formalise it in Coq. Our development includes formalising the choreographic language and its basic properties, Kleene's theory of partial recursive functions, the encoding of these functions as choreographies, and proving this encoding correct. With this effort, we show that theorem proving can be a very useful tool in the field of choreographic languages: besides the added degree of confidence that we get from a mechanised proof, the formalisation process led us to a significant simplification of the underlying theory. Our results offer a foundation for the future formal development of choreographic languages.

cs.LO↗

arXiv2019

Implementing choreography extraction

Choreography extraction deals with the generation of a choreography (a global description of communication behaviour) from a set of local process behaviours. In this work, we implement a previously proposed theory for extraction and show that, in spite of its theoretical exponential complexity, it is usable in practice. We discuss the data structures needed for an efficient implementation, introduce some optimizations, and perform a systematic practical evaluation.

cs.PL↗

arXiv2018

A Core Model for Choreographic Programming

Choreographic Programming is a programming paradigm for building concurrent programs that are deadlock-free by construction, as a result of programming communications declaratively and then synthesising process implementations automatically. Despite strong interest on choreographies, a foundational model that explains which computations can be performed with the hallmark constructs of choreographies is still missing. In this work, we introduce Core Choreographies (CC), a model that includes only the core primitives of choreographic programming. Every computable function can be implemented as a choreography in CC, from which we can synthesise a process implementation where independent computations run in parallel. We discuss the design of CC and argue that it constitutes a canonical model for choreographic programming.

cs.PL↗

arXiv2018

Machine-Assisted Proofs (ICM 2018 Panel)

This submission to arXiv is the report of a panel session at the 2018 International Congress of Mathematicians (Rio de Janeiro, August). It is intended that, while v1 is that report, this stays a living document containing the panelists', and others', reflections on the topic.

math.HO↗

arXiv2018

Connectors meet Choreographies

We present Cho-Reo-graphies (CR), a new language model that unites two powerful programming paradigms for concurrent software based on communicating processes: Choreographic Programming and Exogenous Coordination. In CR, programmers specify the desired communications among processes using a choreography, and define how communications should be concretely animated by connectors given as constraint automata (e.g., synchronous barriers and asynchronous multi-casts). CR is the first choreography calculus where different communication semantics (determined by connectors) can be freely mixed; since connectors are user-defined, CR also supports many communication semantics that were previously unavailable for choreographies. We develop a static analysis that guarantees that a choreography in CR and its user-defined connectors are compatible, define a compiler from choreographies to a process calculus based on connectors, and prove that compatibility guarantees deadlock-freedom of the compiled process implementations.

cs.PL↗

arXiv2017

On Asynchrony and Choreographies

Choreographic Programming is a paradigm for the development of concurrent software, where deadlocks are prevented syntactically. However, choreography languages are typically synchronous, whereas many real-world systems have asynchronous communications. Previous attempts at enriching choreographies with asynchrony rely on ad-hoc constructions, whose adequacy is only argued informally. In this work, we formalise the properties that an asynchronous semantics for choreographies should have: messages can be sent without the intended receiver being ready, and all sent messages are eventually received. We explore how out-of-order execution, used in choreographies for modelling concurrency, can be exploited to endow choreographies with an asynchronous semantics. Our approach satisfies the properties we identified. We show how our development yields a pleasant correspondence with FIFO-based asynchronous messaging, modelled in a process calculus, and discuss how it can be adopted in more complex choreography models.

cs.PL↗

arXiv2017

Communications in Choreographies, Revisited

Choreographic Programming is a paradigm for developing correct-by-construction concurrent programs, by writing high-level descriptions of the desired communications and then synthesising process implementations automatically. So far, choreographic programming has been explored in the monadic setting: interaction terms express point-to-point communications of a single value. However, real-world systems often rely on interactions of polyadic nature, where multiple values are communicated among two or more parties, like multicast, scatter-gather, and atomic exchanges. We introduce a new model for choreographic programming equipped with a primitive for grouped interactions that subsumes all the above scenarios. Intuitively, grouped interactions can be thought of as being carried out as one single interaction. In practice, they are implemented by processes that carry them out in a concurrent fashion. After formalising the intuitive semantics of grouped interactions, we prove that choreographic programs and their implementations are correct and deadlock-free by construction.

cs.PL↗

arXiv2017

That's Enough: Asynchrony with Standard Choreography Primitives

Choreographies are widely used for the specification of concurrent and distributed software architectures. Since asynchronous communications are ubiquitous in real-world systems, previous works have proposed different approaches for the formal modelling of asynchrony in choreographies. Such approaches typically rely on ad-hoc syntactic terms or semantics for capturing the concept of messages in transit, yielding different formalisms that have to be studied separately. In this work, we take a different approach, and show that such extensions are not needed to reason about asynchronous communications in choreographies. Rather, we demonstrate how a standard choreography calculus already has all the needed expressive power to encode messages in transit (and thus asynchronous communications) through the primitives of process spawning and name mobility. The practical consequence of our results is that we can reason about real-world systems within a choreography formalism that is simpler than those hitherto proposed.

cs.PL↗

arXiv2017

The Paths to Choreography Extraction

Choreographies are global descriptions of interactions among concurrent components, most notably used in the settings of verification (e.g., Multiparty Session Types) and synthesis of correct-by-construction software (Choreographic Programming). They require a top-down approach: programmers first write choreographies, and then use them to verify or synthesize their programs. However, most existing software does not come with choreographies yet, which prevents their application. To attack this problem, we propose a novel methodology (called choreography extraction) that, given a set of programs or protocol specifications, automatically constructs a choreography that describes their behavior. The key to our extraction is identifying a set of paths in a graph that represents the symbolic execution of the programs of interest. Our method improves on previous work in several directions: we can now deal with programs that are equipped with a state and internal computation capabilities; time complexity is dramatically better; we capture programs that are correct but not necessarily synchronizable, i.e., they work because they exploit asynchronous communication.

cs.PL↗