AspectJ-TM is a simple and practical aspect-oriented extension to Java-TM. With just a few new constructs, AspectJ provides support for modular implementation of a range of crosscutting concerns. In AspectJ's dynamic join point model, join points are well-defined points in the execution of the program

. We introduce MetaML, a practically-motivated, staticallytyped multi-stage programming language. MetaML is a "real" language. We have built an implementation and used it to solve multi-stage problems. MetaML allows the programmer to construct, combine, and execute code fragments in a type-safe manner. Code fragments can contain free variables, but they obey the static-scoping principle. MetaML performs typechecking for all stages once and for all before the execution of the first stage. Certain anomalies with our first MetaML implementation led us to formalize an illustrative subset of the MetaML implementation. We present both a big-step semantics and type system for this subset, and prove the type system's soundness with respect to a big-step semantics. From a software engineering point of view, this means that generators written in the MetaML subset never generate unsafe programs. A type system and semantics for full MetaML is still ongoing work. We argue that multi-...

This paper presents a generic meta-model of multi-agent systems based on organizational concepts such as groups, roles and structures. This model, called AALAADIN, defines a very simple description of coordination and negotiation schemes through multi-agent systems. Aalaadin is a meta-model of artificial organization by which one can build multi-agent systems with different forms of organizations such as market-like and hierarchical organizations. We show that this meta-model allows for agent heterogeneity in languages, applications and architectures. We also introduce the concept of organizational reflection which uses the same conceptual model to describe systemlevel tasks such as remote communication and migration of agents. Finally, we briefly describe a platform, called MADKIT, based on this model. It relies on a minimal agent kernel with platform-level services implemented as agents, groups and roles. 1 Introduction Whereas organization has been presented as a major issue of m...

We present a language framework for describing dependable systems which emphasizes modularity and composition. Dependability and functionality aspects of an application may be described separately providing a separation of design concerns. Futhermore, the dependability protocols of an application may be constructed bottom-up as simple protocols that are composed into more complex protocols. Composition makes it easier to reason about dependability and supports the construction of general reusable dependability schemes. A significant aspect of our language framework is that dependability protocols may be loaded into a running application and installed dynamically. Dynamic installation makes it possible to impose additional dependability protocols on a server as clients with new dependability demands are integrated into a system. Similarly, if a given dependability protocol is only necessary during some critical phase of execution, it may be installed during that period only. 1 Introduc...

In an important series of papers [8, 9], Brian Smith has discussed the nature of programs that know about their text and the context in which they are executed. He called this kind of knowledge reflection. Smith proposed a programming language, called 3-LISP, which embodied such self-knowledge in the domain of metacircular interpreters. Every 3-LISP program is interpreted by a metacircular interpreter, also written in 3-LISP. This gives rise to a picture of an infinite tower of metacircular interpreters, each being interpreted by the one above it. Such a metaphor poses a serious challenge for conventional modes of understanding of programming languages. In our earlier work on reflection [4], we showed how a useful species of reflection could be modeled without the use of towers. In this paper, we give a semantic account of the reflective tower. This account is self-contained in the sense that it does not employ reflection to explain reflection. 1. Modeling reflection Reflective programming languages were introduced in [8, 9] to study programs that need knowledge of their own behavior. In artificial intelligence, this kind of knowledge is needed, for example, in programs that must explain their behavior to

Recent years have seen a shift in perception of the nature of HCI and interactive systems. As interface work has increasingly become a focus of attention for the social sciences, we have expanded our appreciation of the importance of issues such as work practice, adaptation, and evolution in interactive systems. The reorientation in our view of interactive systems has been accompanied by a call for a new model of design centered around user needs and participation. This article argues that a new process of design is not enough and that the new view necessitates a similar reorientation in the structure of the systems we build. It outlines some requirements for systems that support a deeper conception of interaction and argues that the traditional system design techniques are not suited to creating such systems. Finally, using examples from ongoing work in the design of an open toolkit for collaborative applications, it illustrates how the principles of computational reflection and metaobject protocols can lead us toward a new model based on open abstraction that holds great promise in addressing these issues.

Over the past ten years, the use of sociological methods and sociological reasoning have become more prominent in the analysis and design of interactive systems. For a variety of reasons, one form of sociological enquiry, ethnomethodology, has become something of a favoured approach. Our goal in this paper is to investigate the consequences of approaching system design from the ethnomethodological perspective. In particular, we are concerned with how ethnomethodology can take a foundational place in the very notion of system design, rather than simply being employed as a resource in aspects of the process such as requirements elicitation and specification. We begin by outlining the basic elements of ethnomethodology, and discussing the place that it has come to occupy in CSCW and, increasingly, in HCI. We discuss current approaches to the use of ethnomethodology in systems design, and point to the contrast between the use of ethnomethodology for critique and for design. Currently, und...

A micro-world is described, in which many analogies involving strikingly different concepts and levels of subtlety can be made. The question "What differentiates the good ones from the bad ones?" is discussed, and then the problem of how to implement a computational model of the human ability to come up with such analogies (and to have a sense for their quality) is consicered. A key part of the proposed system, now under development, is its dependence on statistically emergent properties of stochastically interacting "codelets" (small pieces of ready-to-run code created by the system, and selected at random to run with probability proportional to heuristically assigned "urgencies"). Another key element is a network of linked concepts of varying levels of "semanticity", in which activation spreads and indirectly controls the urgencies of new codelets. There is pressure in the syste.rn toward maximizing the degree of "semanticity" or "intensionality" of descriptions of structures, but many such pressures, often conflicting, m.,u$ interact with one another, and compromises must be made. The shifting of (1) percei/v'bd boundaries insi,d,e stru,c, tures, (2) descriptive concepts chosen to apply to structures, and (3)4eatures perceived as salient or not, is called "slippage". What can slip, and how, are emergent consequences of the interaction of (1) the temporary ("cytoplasmic") structures involved in the analogy with (2) the permanent ("Platonic") concepts and links in the conceptual proximity network, or "slippability network". The architecture of this system is postulated as a general architecture suitable for dealing not only with fluid analogies, but also with other types of abstract perception and categorization tasks, such as musical perception, scientific theorizing,...

For intelligent interactive systems to communicate with humans in a natural manner, they must have knowledge about the system users. This paper explores the role of user modeling in such systems. It begins with a characterization of what a user model is and how it can be used. The types of information that a user model may be required to keep about a user are then identified and discussed. User models themselves can vary greatly depending on the requirements of the situation and the implementation, so several dimensions along which they can be classified are presented. Since acquiring the knowledge for a user model is a fundamental problem in user modeling, a section is devoted to this topic. Next, the benefits and costs of implementing a user modeling component for a system are weighed in light of several aspects of the interaction requirements that may be imposed by the system. Finally, the current state of research in user modeling is summarized, and future research topics that must be addressed in order to achieve powerful, general user modeling systems are assessed. 1

computing with the physical world via sensors and actuators, physical computing systems promise to give society an improved living standard, greater security, and unparalleled convenience and efficiency.