Formal Definition Of Semantics Research Articles

AbU: A calculus for distributed event-driven programming with attribute-based interaction

In recent years, event-driven programming languages, in particular those based on Event Condition Action (ECA) rules, have emerged as a promising paradigm for implementing ubiquitous and pervasive systems. These implementations are mostly centralized, where a single server (often in the cloud) collects and processes all the inputs from the environment. In fact, placing the computation on the nodes interacting with the environment requires suitable abstractions for effective communication and coordination of (possibly large) ensembles of these distributed components — abstractions that current ECA languages are still missing.To this end, in this paper we present AbU, a calculus for modeling and reasoning about ECA-based systems with attribute-based communication. The latter is an interaction model recently introduced for the coordination of (possibly large) families of nodes: communication is similar to broadcast but the actual receivers are selected on the spot, by means of predicates over nodes properties. Thus, the programmer can specify interactions between nodes in a declarative way, abstracting from details such as nodes identity, number, or even their existence, without the need for a central server: the computation is moved on the “edge”, thus improving reliability, scalability, privacy and security.After having defined syntax and formal semantics of AbU, we showcase its expressiveness by providing some example applications and the encoding of AbC, the archetypal calculus with attribute-based communication. Then, we focus on two key properties of reactive systems: stabilization (i.e., termination of internal steps) and confluence. For both these properties we provide formal semantic definition, sufficient syntactic conditions on AbU systems, and algorithms to statically check such conditions. Hence, AbU is both a basis for the formal analysis of event-driven architectures with attributed-based interaction, and a reference model for a full-fledged language for IoT and edge computing.

Visualization Tool for Structural Operational Semantics of Simple Imperative Language

We teach the subject of the semantics of programming languages at the graduate level, where we present different approaches to the definition of formal semantics for imperative languages. In this paper, we present a new teaching tool for visualization of semantics computation of a toy imperative language by the small-step semantic method. We have developed a responsive web application that simulates a computation of a program’s semantics as is done by hand on a blackboard.

Semantics-driven extraction of timed automata from Java programs

The automatic verification of time properties of models extracted from programs is challenging, mainly because modern programming languages, such as Java, represent time without a proper semantics. Current approaches to extract time models from source code either represent time only as a tree-like sequence of events or require developers to manually provide a formal model of the time behavior. This makes it difficult for software developers to verify various aspects of their systems, such as timeouts, delays and periodicity of the execution. In this paper, we introduce a formal definition of the time semantics for the Java programming language. Based on the semantics, we present an approach to automatically extract timed automata and their time constraints from Java programs at method level. First, our approach detects the Java statements that involve time, from which it then extracts the timed automata. Our extracted automata are directly amenable to the verification of time properties of the corresponding Java methods. We evaluated the accuracy of our approach on twenty open source Java projects that implement time behavior in their source code. The results show that our approach achieves 100% precision and recall in identifying time related information. They also show that 95% of the timed automata extracted from source code correctly model the time behavior of the method. Finally, we show the applicability of our timed automata to identify eight real errors in four open source Apache systems.

Executable component-based semantics

The potential benefits of formal semantics are well known. However, a substantial amount of work is required to produce a complete and accurate formal semantics for a major language; and when the language evolves, large-scale revision of the semantics may be needed to reflect the changes. The investment of effort needed to produce an initial definition, and subsequently to revise it, has discouraged language developers from using formal semantics. Consequently, many major programming languages (and most domain-specific languages) do not yet have formal semantic definitions.To improve the practicality of formal semantic definitions, the PLanCompS project has developed a component-based approach. In this approach, the semantics of a language is defined by translating its constructs (compositionally) to combinations of so-called fundamental constructs, or ‘funcons’. Each funcon is defined using a modular variant of Structural Operational Semantics, and forms a language-independent component that can be reused in definitions of different languages. A substantial library of funcons has been developed and tested in several case studies. Crucially, the definition of each funcon is fixed, and does not need changing when new funcons are added to the library.For specifying component-based semantics, we have designed and implemented a meta-language called CBS. It includes specification of abstract syntax, of its translation to funcons, and of the funcons themselves. Development of CBS specifications is supported by an integrated development environment. The accuracy of a language definition can be tested by executing the specified translation on programs written in the defined language, and then executing the resulting funcon terms using an interpreter generated from the CBS definitions of the funcons. This paper gives an introduction to CBS, illustrates its use, and presents the various tools involved in our implementation of CBS.

Tracking CSP computations

Tracing is one of the most important techniques for program understanding and debugging. A trace gives the user access to otherwise hidden information about a computation. In the context of concurrent languages, computations are particularly complex due to the non-deterministic execution order of processes and to the restrictions imposed on this order by synchronizations; hence, a tracer is a powerful tool to explore, understand and debug concurrent computations. In CSP, traces are sequences of events that define a particular execution. This notion of trace is completely different to the one used in other paradigms where traces are formed by those source code expressions evaluated during a particular execution. We refer to this second notion of traces as tracks. In this work, we introduce the theoretical basis for tracking concurrent and explicitly synchronized computations in process algebras such as CSP. Tracking computations in this kind of systems is a difficult task due to the subtleties of the underlying operational semantics which combines concurrency, non-determinism and non-termination. We define an instrumented operational semantics that generates as a side-effect an appropriate data structure (a track) which can be used to track computations. The formal definition of a tracking semantics improves the understanding of the tracking process, but also, it allows us to formally prove the correctness of the computed tracks.

Compositional execution semantics for business process verification

Service compositions are programmed as executable business processes in languages like WS-BPEL (or BPEL in short). In such programs, activities are nested within concurrency, isolation, compensation and event handling constructs that cause an overwhelming number of execution paths. Program correctness has to be verified based on a formal definition of the language semantics. For BPEL , previous works have proposed execution semantics in formal languages amenable to model checking. Most of the times the service composition structure is not preserved in the formal model, which impedes tracing the verification findings in the original program. Here, we propose a compositional semantics and a structure-preserving translator of BPEL programs onto the BIP component framework. In addition, we verify essential correctness properties that affect process responsiveness, and the compliance with partner services. The scalability of the proposed translation and analysis is demonstrated on BPEL programs of various sizes. Our compositional translation approach can be also applied to other executable languages with nesting syntax.

Categorical model of structural operational semantics for imperative language

Definition of programming languages consists of the formal definition of syntax and semantics. One of the most popular semantic methods used in various stages of software engineering is structural operational semantics. It describes program behavior in the form of state changes after execution of elementary steps of program. This feature makes structural operational semantics useful for implementation of programming languages and also for verification purposes. In our paper we present a new approach to structural operational semantics. We model behavior of programs in category of states, where objects are states, an abstraction of computer memory and morphisms model state changes, execution of a program in elementary steps. The advantage of using categorical model is its exact mathematical structure with many useful proved properties and its graphical illustration of program behavior as a path, i.e. a composition of morphisms. Our approach is able to accentuate dynamics of structural operational semantics. For simplicity, we assume that data are intuitively typed. Visualization and facility of our model is not only a new model of structural operational semantics of imperative programming languages but it can also serve for education purposes.

Semantics-based generation of verification conditions via program specialization

We present a method for automatically generating verification conditions for a class of imperative programs and safety properties. Our method is parametric with respect to the semantics of the imperative programming language, as it generates the verification conditions by specializing, using unfold/fold transformation rules, a Horn clause interpreter that encodes that semantics. We define a multi-step operational semantics for a fragment of the C language and compare the verification conditions obtained by using this semantics with those obtained by using a more traditional small-step semantics. The flexibility of the approach is further demonstrated by showing that it is possible to easily take into account alternative operational semantics definitions for modeling additional language features. We have proved that the verification condition generation takes a number of transformation steps that is linear with respect to the size of the imperative program to be verified. Also the size of the verification conditions is linear with respect to the size of the imperative program. Besides the theoretical computational complexity analysis, we also provide an experimental evaluation of the method by generating verification conditions using the multi-step and the small-step semantics for a few hundreds of programs taken from various publicly available benchmarks, and by checking the satisfiability of these verification conditions by using state-of-the-art Horn clause solvers. These experiments show that automated verification of programs from a formal definition of the operational semantics is indeed feasible in practice.

The Formal Semantics of a Domain-Specific Modeling Language for Semantic Web Enabled Multi-Agent Systems

Development of agent systems is without question a complex task when autonomous, reactive and proactive characteristics of agents are considered. Furthermore, internal agent behavior model and interaction within the agent organizations become even more complex and hard to implement when new requirements and interactions for new agent environments such as the Semantic Web are taken into account. We believe that the use of both domain specific modeling and a Domain-specific Modeling Language (DSML) may provide the required abstraction and support a more fruitful methodology for the development of Multi-agent Systems (MASs) especially when they are working on the Semantic Web environment. Although syntax definition based on a metamodel is an essential part of a modeling language, an additional and required part would be the determination and implementation of DSML constraints that constitute the (formal) semantics which cannot be defined solely with a metamodel. Hence, in this paper, formal semantics of a MAS DSML called Semantic Web enabled Multi-agent Systems (SEA_ML) is introduced. SEA_ML is a modeling language for agent systems that specifically takes into account the interactions of semantic web agents with semantic web services. What is more, SEA_ML also supports the modeling of semantic agents from their internals to MAS perspective. Based on the defined abstract and concrete syntax definitions, we first give the formal representation of SEA_ML's semantics and then discuss its use on MAS validation. In order to define and implement semantics of SEA_ML, we employ Alloy language which is declarative and has a strong description capability originating from both relational and first-order logic in order to easily define complex structures and behaviors of these systems. Differentiating from similar contributions of other researchers on formal semantics definition for MAS development languages, SEA_ML's semantics, presented in this paper, defines both static and dynamic aspects of the interaction between software agents and semantic web services, in addition to the definition of the semantics already required for agent internals and MAS communication. Implementation with Alloy makes definition of SEA_ML's semantics to include relations and sets with a simple notation for MAS model definitions. We discuss how the automatic analysis and hence checking of SEA_ML models can be realized with the defined semantics. Design of an agent-based electronic barter system is exemplified in order to give some flavor of the use of SEA_ML's formal semantics. Lessons learned during the development of such a MAS DSML semantics are also reported in this paper.

Stochastically timed predicate-based communication primitives for autonomic computing

Predicate-based communication allows components of a system to send messages and requests to ensembles of components that are determined at execution time through the evaluation of a predicate, in a multicast fashion. Predicate-based communication can greatly simplify the programming of autonomous and adaptive systems. We present a stochastically timed extension of the Software Component Ensemble Language (SCEL) that was introduced in previous work. Such an extension raises a number of non-trivial design and formal semantics issues with different options as possible solutions at different levels of abstraction. We discuss four of these options, of which two in more detail. We provide a formal semantics definition and an illustration of the use of the language modeling a bike sharing system, together with some preliminary analysis of the system performance.

An evaluation of real-time requirements for automatic sign language recognition using ANNs and HMMs - The LIBRAS use case

Sign languages are the natural way Deafs use to communicate with other people. They have their own formal semantic definitions and syntactic rules and are composed by a large set of gestures involving hands and head. Automatic recognition of sign languages (ARSL) tries to recognize the signs and translate them into a written language. ARSL is a challenging task as it involves background segmentation, hands and head posture modeling, recognition and tracking, temporal analysis and syntactic and semantic interpretation. Moreover, when real-time requirements are considered, this task becomes even more challenging. In this paper, we present a study of real time requirements of automatic sign language recognition of small sets of static and dynamic gestures of the Brazilian Sign Language (LIBRAS). For the task of static gesture recognition, we implemented a system that is able to work on small sub-sets of the alphabet - like A,E,I,O,U and B,C,F,L,V - reaching very high recognition rates. For the task of dynamic gesture recognition, we tested our system over a small set of LIBRAS words and collected the execution times. The aim was to gather knowledge regarding execution time of all the recognition processes (like segmentation, analysis and recognition itself) to evaluate the feasibility of building a real-time system to recognize small sets of both static and dynamic gestures. Our findings indicate that the bottleneck of our current architecture is the recognition phase.

Noninterference in a predicative polymorphic calculus for access control

Polymorphic programming languages have been adapted for constructing distributed access control systems, where a program represents a proof of eligibility according to a given policy. As a security requirement, it is typically stated that the programs of such languages should satisfy noninterference. However, this property has not been defined and proven semantically. In this paper, we first propose a semantics based on Henkin models for a predicative polymorphic access control language based on lambda-calculus. A formal semantic definition of noninterference is then proposed through logical relations. We prove a type soundness theorem which states that any well-typed program of our language meets the noninterference property defined in this paper. In this way, it is guaranteed that access requests from an entity do not interfere with those from unrelated or more trusted entities.

A semantic-web oriented representation of the clinical element model for secondary use of electronic health records data

The clinical element model (CEM) is an information model designed for representing clinical information in electronic health records (EHR) systems across organizations. The current representation of CEMs does not support formal semantic definitions and therefore it is not possible to perform reasoning and consistency checking on derived models. This paper introduces our efforts to represent the CEM specification using the Web Ontology Language (OWL). The CEM-OWL representation connects the CEM content with the Semantic Web environment, which provides authoring, reasoning, and querying tools. This work may also facilitate the harmonization of the CEMs with domain knowledge represented in terminology models as well as other clinical information models such as the openEHR archetype model. We have created the CEM-OWL meta ontology based on the CEM specification. A convertor has been implemented in Java to automatically translate detailed CEMs from XML to OWL. A panel evaluation has been conducted, and the results show that the OWL modeling can faithfully represent the CEM specification and represent patient data.

Open-Environmental Ontology Modeling

Current open information systems need formal semantic definitions to describe and handle information which is dealt within different interaction environments such as human-to-human, human-to-computer, and computer-to-computer interactions. This paper provides a semantic definition of the knowledge used by environmental management information systems. We describe an ontological model in UML class diagrams and present a formalization of the ontology in first-oder logic and encoded in description logic. A model transformation approach is also used to map the UML class diagram elements into web ontology language. A running example is presented to explain the ontological knowledge modeling of EMIS SOLERES cartography and satellite information.

Bidirectionalizing graph transformations

Bidirectional transformations provide a novel mechanism for synchronizing and maintaining the consistency of information between input and output. Despite many promising results on bidirectional transformations, these have been limited to the context of relational or XML (tree-like) databases. We challenge the problem of bidirectional transformations within the context of graphs, by proposing a formal definition of a well-behaved bidirectional semantics for UnCAL, i.e., a graph algebra for the known UnQL graph query language. The key to our successful formalization is full utilization of both the recursive and bulk semantics of structural recursion on graphs. We carefully refine the existing forward evaluation of structural recursion so that it can produce sufficient trace information for later backward evaluation. We use the trace information for backward evaluation to reflect in-place updates and deletions on the view to the source, and adopt the universal resolving algorithm for inverse computation and the narrowing technique to tackle the difficult problem with insertion. We prove our bidirectional evaluation is well-behaved. Our current implementation is available online and confirms the usefulness of our approach with nontrivial applications.

Towards the compositional verification of real-time UML designs

Current techniques for the verification of software as e.g. model checking are limited when it comes to the verification of complex distributed embedded real-time systems. Our approach addresses this problem and in particular the state explosion problem for the software controlling mechatronic systems, as we provide a domain specific formal semantic definition for a subset of the UML 2.0 component model and an integrated sequence of design steps. These steps prescribe how to compose complex software systems from domain-specific patterns which model a particular part of the system behavior in a well-defined context. The correctness of these patterns can be verified individually because they have only simple communication behavior and have only a fixed number of participating roles. The composition of these patterns to describe the complete component behavior and the overall system behavior is prescribed by a rigorous syntactic definition which guarantees that the verification of component and system behavior can exploit the results of the verification of individual patterns.

A formal semantic definition of DEVIL

In this paper we present a formal semantic definition of a device interface language called DEVIL (DEVice Interface Language). The semantic model is based on a high-order logic system supported by a mathematical theorem proof development system called PowerEpsilon . This work is intended for fully understanding semantics of DEVIL and will be used as a reference-guide in implementation project for the embedded real-time operating system δ- OS .

PolyTOIL

PolyTOIL is a new statically typed polymorphic object-oriented programming language that is provably typesafe. By separating the definitions of subtyping and inheritance, providing a name for the type of self, and carefully defining the type-checking rules, we have obtained a language that is very expressive while supporting modular type-checking of classes. The matching relation on types, which is related to F-bounded quantification, is used both in stating type-checking rules and expressing the bounds on type parameters for polymorphism. The design of PolyTOIL is based on a careful formal definition of type-checking rules and semantics. A proof of type safety is obtained with the aid of a subject reduction theorem.

A Model-Based Concurrent Specification Language over CML: Semantic Foundations

In this paper we address the problem of specification and design of concurrent systems. More accurately, we present the definition of a new specification language that is formal, wide-spectrum, model-based, concurrent, polymorphic and strongly implicitly typed. The language is built upon a concurrent, funtional and imperative programming language: Concurrent ML. Specification aspects are supported thanks to the addition of some specification constructs and also by allowing axioms to ML structures and signatures. The resulting specification language is thus highly expressive though it embodies a restricted number of concepts. We present here the motivations underlying the definition of such a language as well as the design choices. Furthermore, we introduce the specification and development methology and illustrate it on various examples. We will see that many specification styles are allowed: algebraic, applicative, state-based, concurrent applicative and concurrent imperative. We show that the language rests on secure theoretical foundations exemplified by formal syntactic and semantic definitions. The latter consists in a static semantics together with a dynamic semantics. The static semantics reconstructs not only principal types but also minimal side and communication effects. This is done thanks to an extension of the type and effect discipline. The language is also endowed with a dynamic denotational semantics. The underlying model is based on an extension of the acceptance trees model to handle value-passing, communication, assignment, sequencing, return of results and higher order objects.

Equivalence of formal semantics definition methods

Abstract There are numerous methods of formally defining the semantics of computer languages. Each method has been designed to fulfil a different purpose. For example, some have been designed to make reasoning about languages as easy as possible; others have been designed to be accessible to a large audience and some have been designed to ease implementation of languages. Given two semantics definitions of a language written using two separate semantics definition methods, we must be able to show that the two are in fact equivalent. If we cannot do this then we either have an error in one of the semantics definitions, or more seriously we have a problem with the semantics definition methods themselves. Three methods of defining the semantics of computer languages have been considered, i.e. Denotational Semantics, Structural Operational Semantics and Action Semantics. An equivalence between these three is shown for a specific example language by first defining its semantics using each of the three definition methods. The proof of the equivalence is then constructed by selecting pairs of the semantics definitions and showing that they define the same language.