Robert van Gent

PolyTOIL: A Type-Safe Polymorphic Object-Oriented Language

PolyTOIL is a new statically-typed polymorphic object-oriented programming language which is provably type-safe. 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 which 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.

Acting in Character

In this context, it is not the personality of the actor that interests us, but the personality of the character he or she portrays. Thus, when we say that an actor is “in character,” we mean that the actor is behaving in accordance with a personality created by an author, shaped by a director, and assumed by an audience, for purposes of a particular performance. A good actor creates and communicates a consistent and compelling personality throughout a performance of a given role and creates different personalities for different roles.

Safe and decidable type checking in an object-oriented language

Over the last several years, much interesting work has been done in modelling object-oriented programming languages in terms of extensions of the bounded second-order lambda calculus, Fr. Unfortunately, it has recently been shown by Pierce ([Pie92]) that type checking F< is undecidable. Moreover, he showed that the undecidability arises in t,he seemingly simpler problem of determining whether one type is a subtype of another. In [Bru93a, Bru93b], the fist author introduced a statically-typed, functional, object-oriented programming language, TOOPL, which supports classes, objects, methods, instance variables, subtypes, and inheritance. The semantics of TOOPL is based on F<, so the question arises whether type checking in this language is decidable. In this paper we show that type checking for TOOPLE, a minor variant of TOOPL (Typed Object-Oriented Programming Language), is decidable. The proof proceeds by showing that subtyping is decidable, that all terms of TOOPLE have minimum types (which are in fact computable), and then using these two results to show that type checking is decidable. Our algorithm fails to be polynomial in the size of the term because the size of its type can be exponential in the size of the term. Nevertheless, it performs well in practice. ‘Bruce, Crabtree, Murtagh, and van Gent were partially supported by NSF grant CCR-9121778. Dimock and MuUer were partially supported by DARPA grant F19628-92-C0113. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission. . .

PolyTOIL: A type-safe polymorphic object-oriented language

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.

Staffing the web with interactive characters

Like their human counterparts in the real world, interactive characters will soon play helpful roles, enabling customers of Web enterprises to do the things they go to Web sites to do. They will make it easy for people to interact with them through natural forms of conversation and gesture. They will add interest and warmth to the online experience through their distinctive personas, projecting images and styles designed to appeal specifically to the customers they serve. They will build relationships with customers based on familiarity, affection, and trust. Interactive characters also will surpass their human counterparts in several important ways. They will be able to exploit the Web’s vast information resources. They will deliver desired behavior consistently 24 hours a day, seven days a week. They will scale flexibly in number as needed to give immediate one-to-one service to each and every customer. They will remember important features of past interactions, making customers feel known and valued. They will set a new standard for customer service. We have built several such interactive characters, including Max, who guides visitors at Extempo’s Web site; Merlin, who guides interactive neighborhood tours at Haight Street on the Web; and Jennifer James, who works as a spokesperson at Samson Motors on Extempo’s Web site. All are built on the component-based architecture in Figure 1. The Extempo Improvisational (Imp) engine runs character mind software, providing basic cognitive functions for multiple characters and character instances running simultaneously. Application programming interfaces (APIs) support integration with various knowledge sources. For example, a Webguide character accesses information from a tour database or from a site’s back-end systems. It constructs a personalization database to record and retrieve information it has learned about customers. And it may use the Internet to retrieve information for its dialogs or to send and retrieve email. The mind of an Imp character consists of three pairs of plug-and-play competence/content components: a unique and persistent persona; one of several alternative roles; and the knowledge needed to perform its role in a specific application. Persona competence and content components involve all facets of a character’s identity: backstory; appearance; manner of moving, gesturing, and speaking; voice; knowledge and opinions; and social and emotional dynamics. A character’s persona is revealed through manner, dialog, and behavior. Role competence and content components are the knowledge and skills the character needs to perform his or her job. For example, a character acting as a Web guide has to greet each customer, determine the details of the right “tour,” act as escort to the Staffing the Web with Interactive Characters