Carolyn L. Talcott

A foundation for actor computation

We present an actor language which is an extension of a simple functional language, and provide an operational semantics for this extension. Actor configurations represent open distributed systems, by which we mean that the specification of an actor system explicitly takes into account the interface with external components. We study the composability of such systems. We define and study various notions of testing equivalence on actor expressions and configurations. The model we develop provides fairness. An important result is that the three forms of equivalence, namely, convex, must, and may equivalences, collapse to two in the presence of fairness. We further develop methods for proving laws of equivalence and provide example proofs to illustrate our methodology.

The maude 2.0 system

This paper gives an overviewof the Maude 2.0 system. We emphasize the full generality with which rewriting logic and membership equational logic are supported, operational semantics issues, the new built-in modules, the more general Full Maude module algebra, the new META-LEVEL module, the LTL model checker, and new implementation techniques yielding substantial performance improvements in rewriting modulo. We also comment on Maudes formal tool environment and on applications.

Equivalence in functional languages with effects

Traditionally the view has been that direct expression of control and store mechanisms and clear mathematical semantics are incompatible requirements. This paper shows that adding objects with memory to the call-by-value lambda calculus results in a language with a rich equational theory, satisfying many of the usual laws. Combined with other recent work this provides evidence that expressive, mathematically clean programming languages are indeed possible. 1. Overview Real programs have eeects|creating new structures, examining and modifying existing structures, altering ow of control, etc. Such facilities are important not only for optimization, but also for communication, clarity, and simplicity in programming. Thus it is important to be able to reason both informally and formally about programs with eeects, and not to sweep eeects either to the side or under the store parameter rug. Recent work of Talcott, Mason, Felleisen, and Moggi establishes a mathematical foundation for studying notions of program equivalence for programming languages with function and control abstractions operating on objects with memory. This work extends work of Landin, Reynolds, Morris and Plotkin. Landin 14] and Reynolds 35] describe high-level abstract machines for deening language semantics. Morris 29] deenes an extensional equivalence relation for the classical lambda calculus. Plotkin 33] extends these ideas to the call-by-value lambda calculus and deenes the operational equivalence relation. Operational approximation is the pre-ordering induced by an operational semantics. Operational equivalence is the equivalence naturally associated with this pre-ordering. One expression operationally approximates another if for all closing program contexts either the rst expression is undeened or both expressions are deened and their values are indistinguishable (with respect to some primitive means of testing equality). Operational approximation and equivalence are congruence relations on expressions and hence closed under substitution and abstraction. Mason and Talcott in 39, 17, 18, 40] study operational approximation and equivalence for subsets of a language with function and control abstractions and objects with memory. Felleisen 8] deenes reduction calculi extending the call-by-value lambda calculus to languages with control and assignment abstractions. These calculi are simpliied and extended by Felleisen and Hieb in 9]. Talcott, Mason, and Felleisen all apply their theories to expressing and proving properties of program constructs and of particular programs. Moggi 27, 28] introduces the notion of computational monad as a framework for axiomatizing features of programming languages. Computational monads are categories with certain additional structure that accommodate a wide variety of language features including assignment, exceptions, and …

Pathway Logic: Executable Models of Biological Networks

Abstract In this paper we describe the use of the rewriting logic based Maude tool to model and analyze mammalian signaling pathways. We discuss the representation of the underlying biological concepts and events and describe the use of the new search and model checking capabilities of Maude 2.0 to analyze the modeled network. We also discuss the use of Maudes reflective capability for meta modeling and analyzing the models themselves. The idea of symbolic biological experiments opens up an exciting new world of challenging applications for formal methods in general and for rewriting logic based formalisms in particular.

Towards a Theory of Actor Computation

In this paper we present preliminary results of a rigorous development of the actor model of computation. We present an actor language which is an extension of a simple functional language, and provide a precise operational semantics for this extension. Our actor systems are open distributed systems, meaning we explicitly take into account the interface with external components in the specification of an actor system. We define and study various notions of equivalence on actor expressions and systems. We show that the usual tripartite family of testing equivalence relations collapses to two in the presence of fairness. We define a notion of operational bisimulation as a tool for establishing equivalence under fairness assumptions, and illustrate its use.

Electron spin resonance spectra of the radical anions of pyridine and related nitrogen heterocyclics

The E.S.R. spectra of the radical anions of pyridine, 4-picoline, 3,5-lutidine, 2,6-lutidine, pyrazine, pyrimidine, pyridine N-oxide, 4-picoline N-oxide and 2,6-lutidine N-oxide have been observed in liquid ammonia. The data for pyridine, pyrazine and pyrimidine can be combined to evaluate Q N N = +27·3, Q CN N = -1·7 and Q H = -24·5 gauss independently of molecular orbital spin densities. Good agreement is found for the observed coupling constants for the N-heterocyclics and McLachlans theory with methyl group coupling consistent with both Levys hyperconjugation equations and with Q CH3 eff. = 25·7 gauss. The coupling constants for the N-oxides are in satisfactory agreement with an arbitrary set of molecular orbital parameters.

Specification and Analysis of the AER/NCA Active Network Protocol Suite in Real-Time Maude

This paper describes the application of the Real-Time Maude tool and the Maude formal methodology to the specification and analysis of the AER/NCA suite of active network multicast protocol components. Because of the time-sensitive and resource-sensitive behavior, the presence of probabilistic algorithms, and the composability of its components, AER/NCA poses challenging new problems for its formal specification and analysis. Real-Time Maude is a natural extension of the Maude rewriting logic language and tool for the specification and analysis of real-time object-based distributed systems. It supports a wide spectrum of formal methods, including: executable specification; symbolic simulation; breadth-first search for failures of safety properties in infinite-state systems; and linear temporal logic model checking of time-bounded temporal logic formulas. These methods complement those offered by network simulators on the one hand, and timed-automaton-based tools and general-purpose theorem provers on the other. Our experience shows that Real-Time Maude is well-suited to meet the AER/NCA modeling challenges, and that its methods have proved effective in uncovering subtle and important errors in the informal use case specification.

Cyber-Physical Systems and Events

This paper discusses event-based semantics in the context of the emerging concept of Cyber Physical Systems and describes two related formal models concerning policy-based coordination and Interactive Agents.

Semantic Models for Distributed Object Reflection

A generic formal model of distributed object reflection is proposed, that combines logical reflection with a structuring of distributed objects as nested configurations of metaobject that can control subobjects under them. The model provides mathematical models for a good number of existing models of distributed reflection and of reflective middleware. To illustrate the ideas, we show in some detail how two important models of distributed actor reflection can be naturally obtained as special cases of our generic model, and discuss how several recent models of reflective middleware can be likewise formalized as instances of our model.

Pathway logic modeling of protein functional domains in signal transduction.

Protein functional domains (PFDs) are consensus sequences within signaling molecules that recognize and assemble other signaling components into complexes. Here we describe the application of an approach called Pathway Logic to the symbolic modeling signal transduction networks at the level of PFDs. These models are developed using Maude, a symbolic language founded on rewriting logic. Models can be queried (analyzed) using the execution, search and model-checking tools of Maude. We show how signal transduction processes can be modeled using Maude at very different levels of abstraction involving either an overall state of a protein or its PFDs and their interactions. The key insight for the latter is our algebraic representation of binding interactions as a graph.