We present the fl-calculus, a computational calculus for higher-order concurrent programming. The calculus can elegantly express higher-order functions (both eager and lazy) and concurrent objects with encapsulated state and multiple inheritance. The primitives of the fl-calculus are logic variables, names, procedural abstraction, and cells. Cells provide a notion of state that is fully compatible with concurrency and constraints. Although it does not have a dedicated communication primitive, the fl-calculus can elegantly express one-to-many and many-to-one communication. There is an interesting relationship between the fl-calculus and the ß-calculus: The fl-calculus is subsumed by a calculus obtained by extending the asynchronous and polyadic ß-calculus with logic variables. The fl-calculus can be extended with primitives providing for constraint-based problem solving in the style of logic programming. A such extended fl-calculus has the remarkable property that it combines first-or...

In previous work, we developed Transaction Logic (or T R), which deals with state changes in deductive databases. T R provides a logical framework in which elementary database updates and queries can be combined into complex database transactions. T R accounts not only for the updates themselves, but also for important related problems, such as the order of update operations, non-determinism, and transaction failure and rollback. In the present paper, we propose Concurrent Transaction Logic (or CT R), which extends Transaction Logic with connectives for modeling the concurrent execution of complex processes. Concurrent processes in CT R execute in an interleaved fashion and can communicate and synchronize themselves. Like classical logic, CT R has a "Horn" fragment that has both a procedural and a declarative semantics, in which users can program and execute database transactions. CT R is thus a deductive database language that integrates concurrency, communication, and updates. All th...

Object oriented databases provide rich structuring capabilities to organise the objects being relevant for a given application. Due to the possible complexity of object structures, path expressions have become accepted as a concise syntactical means to reference objects. Even though known approaches to path expressions provide quite elegant access to objects, there seems to be still a need for more generality. To this end, the rule-language PathLog is introduced. A first contribution of PathLog is to add a second dimension to path expressions in order to increase conci-seness. In addition, a path expression can also be used to reference virtual objects. Both enhancements give rise to interesting semantic implications.

data types are facilitated in Godel by its type and module systems. Thus, in order to describe the meta-programming facilities of Godel, a brief account of these systems is given. Each constant, function, predicate, and proposition in a Godel program must be specified by a language declaration. The type of a variable is not declared but inferred from its context within a particular program statement. To illustrate the type system, we give the language declarations that would be required for the program in Figure 1. BASE Name. CONSTANT Tom, Jerry : Name. PREDICATE Chase : Name * Name; Cat, Mouse : Name. Note that the declaration beginning BASE indicates that Name is a base type. In the statement Chase(x,y) !- Cat(x) & Mouse(y). the variables x and y are inferred to be of type Name. Polymorphic types can also be defined in Godel. They are constructed from the base types, type variables called parameters, and type constructors. Each constructor has an arity 1 attached to it. As an...

Oz is an experimental higher-order concurrent constraint programming system under development at DFKI. It combines ideas from logic and concurrent programming in a simple yet expressive language. From logic programming Oz inherits logic variables and logic data structures, which provide for a programming style where partial information about the values of variables is imposed concurrently and incrementally. A novel feature of Oz is that it accommodates higher-order programming without sacrificing that denotation and equality of variables are captured by first-order logic. Another new feature of Oz is constraint communication, a new form of asynchronous communication exploiting logic variables. Constraint communication avoids the problems of stream communication, the conventional communication mechanism employed in concurrent logic programming. Constraint communication can be seen as providing a minimal form of state fully compatible with logic data structures. Bas...

Abstract. Flora-2 is a rule-based object-oriented knowledge base system designed for a variety of automated tasks on the Semantic Web, ranging from meta-data management to information integration to intelligent agents. The Flora-2 system integrates F-logic, HiLog, and Transaction Logic into a coherent knowledge representation and inference language. The result is a flexible and natural framework that combines rule-based and object-oriented paradigms. This paper discusses the principles underlying the design of the Flora-2 system and describes its salient features, including meta-programming, reification, logical database updates, encapsulation, and support for dynamic modules. 1

The well-founded model provides a natural and robust semantics for logic programs with negative literals in rule bodies. We implemented the well-founded semantics in the SLG-WAM of XSB. Performance results indicate that the overhead of delay and simplification to Prolog --- or tabled --- evaluations is minimal. To compute the well-founded semantics, the SLG-WAM adds to an efficient tabling engine three operations --- negative loop detection, delay and simplification --- which serve to detect, to break and to resolve the cycles through negation that arise in evaluating normal programs XSB is a full Prolog system that closely approximates the ISO standard; additionally, it supports tight integration of tabled predicates with nontabled predicates. 1 Introduction XSB is a research-oriented Logic Programming system for Unix and Windows/DOS-based systems. In addition to providing all the functionality of Prolog, XSB contains several features not usually found in Logic Programming systems, i...

Abstract. A common practice in conceptual modeling is to separate the intensional from the extensional model. Although very intuitive, this approach is inadequate for many complex domains, where the borderline between the two models is not clear-cut. Therefore, OWL-Full, the most expressive of the Semantic Web ontology languages, allows combining the intensional and the extensional model by a feature we refer to as metamodeling. In this paper, we show that the semantics of metamodeling adopted in OWL-Full leads to undecidability of basic inference problems, due to free mixing of logical and metalogical symbols. Based on this result, we propose two alternative semantics for metamodeling: the contextual and the HiLog semantics. We show that SHOIQ — a description logic underlying OWL-DL — extended with metamodeling under either semantics is decidable. Finally, we show how the latter semantics can be used in practice to axiomatize the logical interaction between concepts and metaconcepts. 1

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Abstract. Current technologies for Web Services are based on syntactical descriptions and, therefore, lend themselves to only limited amount of automation. Research efforts in Semantic Web Services, such as WSMO, try to overcome this major deficiency by providing a complete semantic description for Web Services and their related aspects. In this paper we present a logical framework which exploits such formal descriptions in order to dynamically discover Web Services that match requester goals. We consider two kinds of user goals: discovery and contracting. Based on the WSMO conceptual model, we define proof obligations that formalize the concepts of a match in these two cases. We also describe a concrete realization of this framework in the F-Logic reasoning engine F LORA-2. Such a realization requires an extension of F-Logic in order to support rule reification. With this extension, F-logic becomes a suitable framework for describing and reasoning about Semantic Web Services and their capabilities. 1