The concept of a linguistic variable and its application to approximate reasoning—II☆

The concept of an agent has become important in both Artificial Intelligence (AI) and mainstream computer science. Our aim in this paper is to point the reader at what we perceive to be the most important theoretical and practical issues associated with the design and construction of intelligent agents. For convenience, we divide these issues into three areas (though as the reader will see, the divisions are at times somewhat arbitrary). Agent theory is concerned with the question of what an agent is, and the use of mathematical formalisms for representing and reasoning about the properties of agents. Agent architectures can be thought of as software engineering models of agents;researchers in this area are primarily concerned with the problem of designing software or hardware systems that will satisfy the properties specified by agent theorists. Finally, agent languages are software systems for programming and experimenting with agents; these languages may embody principles proposed by theorists. The paper is not intended to serve as a tutorial introduction to all the issues mentioned; we hope instead simply to identify the most important issues, and point to work that elaborates on them. The article includes a short review of current and potential applications of agent technology.

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Intelligent Agents: Theory and Practice

物件導向軟體之架構(Object-Oriented Software Construction)探討

The Globus system is intended to achieve a vertically integrated treatment of application, middleware, and net work. A low-level toolkit provides basic mechanisms such as communication, authentication, network information, and data access. These mechanisms are used to con struct various higher level metacomputing services, such as parallel programming tools and schedulers. The long- term goal is to build an adaptive wide area resource environment AWARE, an integrated set of higher level services that enable applications to adapt to heteroge neous and dynamically changing metacomputing environ ments. Preliminary versions of Globus components were deployed successfully as part of the I-WAY networking experiment.

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Globus: a Metacomputing Infrastructure Toolkit

The Generative Lexicon

Our objective is to understand the notion of type in programming languages, present a model of typed, polymorphic programming languages that reflects recent research in type theory, and examine the relevance of recent research to the design of practical programming languages.Object-oriented languages provide both a framework and a motivation for exploring the interaction among the concepts of type, data abstraction, and polymorphism, since they extend the notion of type to data abstraction and since type inheritance is an important form of polymorphism. We develop a l-calculus-based model for type systems that allows us to explore these interactions in a simple setting, unencumbered by complexities of production programming languages.The evolution of languages from untyped universes to monomorphic and then polymorphic type systems is reviewed. Mechanisms for polymorphism such as overloading, coercion, subtyping, and parameterization are examined. A unifying framework for polymorphic type systems is developed in terms of the typed l-calculus augmented to include binding of types by quantification as well as binding of values by abstraction.The typed l-calculus is augmented by universal quantification to model generic functions with type parameters, existential quantification and packaging (information hiding) to model abstract data types, and bounded quantification to model subtypes and type inheritance. In this way we obtain a simple and precise characterization of a powerful type system that includes abstract data types, parametric polymorphism, and multiple inheritance in a single consistent framework. The mechanisms for type checking for the augmented l-calculus are discussed.The augmented typed l-calculus is used as a programming language for a variety of illustrative examples. We christen this language Fun because fun instead of l is the functional abstraction keyword and because it is pleasant to deal with.Fun is mathematically simple and can serve as a basis for the design and implementation of real programming languages with type facilities that are more powerful and expressive than those of existing programming languages. In particular, it provides a basis for the design of strongly typed object-oriented languages.

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On understanding types, data abstraction, and polymorphism

Why interaction is more powerful than algorithms

The design space of object-based languages is characterized in terms of objects, classes, inheritance, data abstraction, strong typing, concurrency, and persistence. Language classes (paradigms) associated with interesting subsets of these features are identified and language design issues for selected paradigms are examined. Orthogonal dimensions that span the object-oriented design space are related to non-orthogonal features of real languages. The self-referential application of object-oriented methodology to the development of object-based language paradigms is demonstrated.Delegation is defined as a generalization of inheritance and design alternatives such as non-strict, multiple, and abstract inheritance are considered. Actors and prototypes are presented as examples of classless (delegation based) languages. Processes are classified by their degree of internal concurrency. The potential inconsistency of object-oriented sharing and distributed autonomy is discussed, suggesting that compromises between sharing and autonomy will be necessary in designing strongly typed object-oriented distributed database languages.

Dimensions of object-based language design

We address the following questions for object-oriented programming:What is it?What are its goals?What are its origins?What are its paradigms?What are its design alternatives?What are its models of concurrency?What are its formal computational models?What comes after object-oriented programming?Starting from software engineering goals, we examine the origins and paradigms of object-oriented programming, explore its language design alternatives, consider its models of concurrency, and review its mathematical models to make them accessible to nonmathematical readers. Finally, we briefly speculate on what may come after object-oriented programming and conclude that it is a robust component-based modeling paradigm that is both effective and fundamental. This paper expands on the OOPSLA 89 keynote talk.

Concepts and paradigms of object-oriented programming

Incremental modification is a fundamental mechanism not only in software systems, but also in physical and mathematical systems. Inheritance owes its importance in large measure to its flexibility as a discrete incremental modification mechanism. Four increasingly permissive properties of incremental modification realizable by inheritance are examined: behavior compatibility, signature compatibility, name compatibility, and cancellation. Inheritance for entities with finite sets of attributes is defined and characterized as incremental modification with deferred binding of self-reference. Types denned as predicates for type checking are contrasted with classes defined as templates for object generation. Mathematical, operational, and conceptual models of inheritance are then examined in detail, leading to a discussion of algebraic models of behavioral compatibility, horizontal and vertical signature modification, algorithmically defined name modification, additive and subtractive exceptions, abstract inheritance networks, and parametric polymorphism. Liketypes are defined as a symmetrical general form of incremental modification that provide a framework for modeling similarity. The combination of safe behaviorally compatible changes and less safe radical incremental changes in a single programming language is considered.

Peter Wegner

Papers

On understanding types, data abstraction, and polymorphism

Why interaction is more powerful than algorithms

Dimensions of object-based language design

Concepts and paradigms of object-oriented programming

Inheritance as an Incremental Modification Mechanism or What Like Is and Isn't Like