arXiv subjects

Germán Vidal

Publications and source records attributed to Germán Vidal.

18 recordsLinked to original sources

arXiv2026

A Reversible Semantics for Janus

Janus is a paradigmatic example of a reversible programming language. Indeed, Janus programs can be executed backwards as well as forwards. However, its current small-step semantics (useful, e.g., for debugging or as a basis for extensions with concurrency primitives) is not reversible, since it loses information while computing forwards. E.g., it does not satisfy the Loop Lemma, stating that any reduction has an inverse, a main property of reversibility in process calculi, where a small-step semantics is commonly used. We present here a novel small-step semantics which is actually reversible, while remaining equivalent to the previous one. It involves the non-trivial challenge of defining a semantics based on a "program counter" for a high-level programming language.

cs.PL↗

arXiv2025

A Distribution Semantics for Probabilistic Term Rewriting

Probabilistic programming is becoming increasingly popular thanks to its ability to specify problems with a certain degree of uncertainty. In this work, we focus on term rewriting, a well-known computational formalism. In particular, we consider systems that combine traditional rewriting rules with probabilities. Then, we define a novel "distribution semantics" for such systems that can be used to model the probability of reducing a term to some value. We also show how to compute a set of "explanations" for a given reduction, which can be used to compute its probability in a more efficient way. Finally, we illustrate our approach with several examples and outline a couple of extensions that may prove useful to improve the expressive power of probabilistic rewrite systems.

cs.PL↗

arXiv2024

Explaining Explanations in Probabilistic Logic Programming

The emergence of tools based on artificial intelligence has also led to the need of producing explanations which are understandable by a human being. In most approaches, the system is considered a black box, making it difficult to generate appropriate explanations. In this work, though, we consider a setting where models are transparent: probabilistic logic programming (PLP), a paradigm that combines logic programming for knowledge representation and probability to model uncertainty. However, given a query, the usual notion of explanation is associated with a set of choices, one for each random variable of the model. Unfortunately, such a set does not explain why the query is true and, in fact, it may contain choices that are actually irrelevant for the considered query. To improve this situation, we present in this paper an approach to explaining explanations which is based on defining a new query-driven inference mechanism for PLP where proofs are labeled with "choice expressions", a compact and easy to manipulate representation for sets of choices. The combination of proof trees and choice expressions allows us to produce comprehensible query justifications with a causal structure.

cs.AI↗

arXiv2024

Causal-Consistent Reversible Debugging: Improving CauDEr

Causal-consistent reversible debugging allows one to explore concurrent computations back and forth in order to locate the source of an error. In this setting, backward steps can be chosen freely as long as they are "causal consistent", i.e., as long as all the actions that depend on the action we want to undo have been already undone. Here, we consider a framework for causal-consistent reversible debugging in the functional and concurrent language Erlang. This framework considered programs translated to an intermediate representation, called Core Erlang. Although using such an intermediate representation simplified both the formal definitions and their implementation in a debugging tool, the choice of Core Erlang also complicated the use of the debugger. In this paper, we extend the framework in order to deal with source Erlang programs, also including some features that were not considered before. Moreover, we integrate the two existing approaches (user-driven debugging and replay debugging) into a single, more general framework, and develop a new version of the debugging tool CauDEr including all the mentioned extensions as well as a renovated user interface.

cs.PL↗

arXiv2024

An Asynchronous Scheme for Rollback Recovery in Message-Passing Concurrent Programming Languages

Rollback recovery strategies are well-known in concurrent and distributed systems. In this context, recovering from unexpected failures is even more relevant given the non-deterministic nature of execution, which means that it is practically impossible to foresee all possible process interactions. In this work, we consider a message-passing concurrent programming language where processes interact through message sending and receiving, but shared memory is not allowed. In this context, we design a checkpoint-based rollback recovery strategy that does not need a central coordination. For this purpose, we extend the language with three new operators: check, commit, and rollback. Furthermore, our approach is purely asynchronous, which is an essential ingredient to developing a source-to-source program instrumentation implementing a rollback recovery strategy.

cs.PL↗

arXiv2023

From Reversible Computation to Checkpoint-Based Rollback Recovery for Message-Passing Concurrent Programs

The reliability of concurrent and distributed systems often depends on some well-known techniques for fault tolerance. One such technique is based on checkpointing and rollback recovery. Checkpointing involves processes to take snapshots of their current states regularly, so that a rollback recovery strategy is able to bring the system back to a previous consistent state whenever a failure occurs. In this paper, we consider a message-passing concurrent programming language and propose a novel rollback recovery strategy that is based on some explicit checkpointing operators and the use of a (partially) reversible semantics for rolling back the system.

cs.PL↗

arXiv2023

Explanations as Programs in Probabilistic Logic Programming

The generation of comprehensible explanations is an essential feature of modern artificial intelligence systems. In this work, we consider probabilistic logic programming, an extension of logic programming which can be useful to model domains with relational structure and uncertainty. Essentially, a program specifies a probability distribution over possible worlds (i.e., sets of facts). The notion of explanation is typically associated with that of a world, so that one often looks for the most probable world as well as for the worlds where the query is true. Unfortunately, such explanations exhibit no causal structure. In particular, the chain of inferences required for a specific prediction (represented by a query) is not shown. In this paper, we propose a novel approach where explanations are represented as programs that are generated from a given query by a number of unfolding-like transformations. Here, the chain of inferences that proves a given query is made explicit. Furthermore, the generated explanations are minimal (i.e., contain no irrelevant information) and can be parameterized w.r.t. a specification of visible predicates, so that the user may hide uninteresting details from explanations.

cs.AI↗

arXiv2022

Computing Race Variants in Message-Passing Concurrent Programming with Selective Receives

Message-passing concurrency is a popular computation model that underlies several programming languages like, e.g., Erlang, Akka, and (to some extent) Go and Rust. In particular, we consider a message-passing concurrent language with dynamic process spawning and selective receives, i.e., where messages can only be consumed by the target process when they match a specific constraint (e.g., the case of Erlang). In this work, we introduce a notion of trace that can be seen as an abstraction of a class of causally equivalent executions (i.e., which produce the same outcome). We then show that execution traces can be used to identify message races. We provide constructive definitions to compute message races as well as to produce so-called race variants, which can then be used to drive new executions which are not causally equivalent to the previous ones. This is an essential ingredient of state-space exploration techniques for program verification.

cs.PL↗

arXiv2022

Reversible Debugging in Logic Programming

Reversible debugging is becoming increasingly popular for locating the source of errors. This technique proposes a more natural approach to debugging, where one can explore a computation from the observable misbehaviour backwards to the source of the error. In this work, we propose a reversible debugging scheme for logic programs. For this purpose, we define an appropriate instrumented semantics (a so-called Landauer embedding) that makes SLD resolution reversible. An implementation of a reversible debugger for Prolog, rever, has been developed and is publicly available.

cs.PL↗

arXiv2022

A Lightweight Approach to Computing Message Races with an Application to Causal-Consistent Reversible Debugging

This paper presents a lightweight formalism (a trace) to model message-passing concurrent executions where some common common problems can be identified, like lost or delayed messages, some forms of deadlock, etc. In particular, we consider (potential) message races that can be useful to analyze alternative executions. We consider a particular application for our developments in the context of a causal-consistent reversible debugging framework for Erlang programs

cs.PL↗

arXiv2021

A Program Instrumentation for Prefix-Based Tracing in Message-Passing Concurrency

The execution of concurrent programs generally involves some degree of nondeterminism, mostly due to the relative speeds of the concurrent processes. As a consequence, reproducibility is often challenging. This problem has been traditionally tackled by a combination of tracing and replay. In this paper, we introduce a program instrumentation for "prefix-based tracing" that combines both tracing and replay. In the general case, the program is instrumented with a partial trace, so that the execution first follows the partial trace (replay) and, then, proceeds nondeterministically, eventually producing a trace of the complete execution as a side effect. Observe that traditional tracing and replay are particular cases of our approach when an empty trace is provided (pure tracing) and when a full trace is provided (pure replay), respectively.

cs.PL↗

arXiv2018

A Theory of Reversibility for Erlang

In a reversible language, any forward computation can be undone by a finite sequence of backward steps. Reversible computing has been studied in the context of different programming languages and formalisms, where it has been used for testing and verification, among others. In this paper, we consider a subset of Erlang, a functional and concurrent programming language based on the actor model. We present a formal semantics for reversible computation in this language and prove its main properties, including its causal consistency. We also build on top of it a rollback operator that can be used to undo the actions of a process up to a given checkpoint.

cs.PL↗

arXiv2017

Reversible Computation in Term Rewriting

Essentially, in a reversible programming language, for each forward computation from state $S$ to state $S'$, there exists a constructive method to go backwards from state $S'$ to state $S$. Besides its theoretical interest, reversible computation is a fundamental concept which is relevant in many different areas like cellular automata, bidirectional program transformation, or quantum computing, to name a few. In this work, we focus on term rewriting, a computation model that underlies most rule-based programming languages. In general, term rewriting is not reversible, even for injective functions; namely, given a rewrite step $t_1 \rightarrow t_2$, we do not always have a decidable method to get $t_1$ from $t_2$. Here, we introduce a conservative extension of term rewriting that becomes reversible. Furthermore, we also define two transformations, injectivization and inversion, to make a rewrite system reversible using standard term rewriting. We illustrate the usefulness of our transformations in the context of bidirectional program transformation.

cs.PL↗

arXiv2016

Towards Reversible Computation in Erlang

In a reversible language, any forward computation can be undone by a finite sequence of backward steps. Reversible computing has been studied in the context of different programming languages and formalisms, where it has been used for debugging and for enforcing fault-tolerance, among others. In this paper, we consider a subset of Erlang, a concurrent language based on the actor model. We formally introduce a reversible semantics for this language. To the best of our knowledge, this is the first attempt to define a reversible semantics for Erlang.

cs.PL↗

arXiv2016

Tuning Fuzzy Logic Programs with Symbolic Execution

Fuzzy logic programming is a growing declarative paradigm aiming to integrate fuzzy logic into logic programming. One of the most difficult tasks when specifying a fuzzy logic program is determining the right weights for each rule, as well as the most appropriate fuzzy connectives and operators. In this paper, we introduce a symbolic extension of fuzzy logic programs in which some of these parameters can be left unknown, so that the user can easily see the impact of their possible values. Furthermore, given a number of test cases, the most appropriate values for these parameters can be automatically computed.

cs.PL↗

arXiv2015

Concolic Testing in Logic Programming

Software testing is one of the most popular validation techniques in the software industry. Surprisingly, we can only find a few approaches to testing in the context of logic programming. In this paper, we introduce a systematic approach for dynamic testing that combines both concrete and symbolic execution. Our approach is fully automatic and guarantees full path coverage when it terminates. We prove some basic properties of our technique and illustrate its practical usefulness through a prototype implementation.

cs.PL↗

arXiv2012

Annotation of Logic Programs for Independent AND-Parallelism by Partial Evaluation

Traditional approaches to automatic AND-parallelization of logic programs rely on some static analysis to identify independent goals that can be safely and efficiently run in parallel in any possible execution. In this paper, we present a novel technique for generating annotations for independent AND-parallelism that is based on partial evaluation. Basically, we augment a simple partial evaluation procedure with (run-time) groundness and variable sharing information so that parallel conjunctions are added to the residual clauses when the conditions for independence are met. In contrast to previous approaches, our partial evaluator is able to transform the source program in order to expose more opportunities for parallelism. To the best of our knowledge, we present the first approach to a parallelizing partial evaluator.

cs.PL↗

arXiv2006

Forward slicing of functional logic programs by partial evaluation

Program slicing has been mainly studied in the context of imperative languages, where it has been applied to a wide variety of software engineering tasks, like program understanding, maintenance, debugging, testing, code reuse, etc. This work introduces the first forward slicing technique for declarative multi-paradigm programs which integrate features from functional and logic programming. Basically, given a program and a slicing criterion (a function call in our setting), the computed forward slice contains those parts of the original program which are reachable from the slicing criterion. Our approach to program slicing is based on an extension of (online) partial evaluation. Therefore, it provides a simple way to develop program slicing tools from existing partial evaluators and helps to clarify the relation between both methodologies. A slicing tool for the multi-paradigm language Curry, which demonstrates the usefulness of our approach, has been implemented in Curry itself.

cs.PL↗