Processing English With a Generalized Phrase Structure Grammar

PROCESSING ENGLISH WITH A GENERALIZED PHRASE STRUCTURE GRAMMAR Jean Mark Gawron, Jonathan King, John Lamping, Egon Loebner, Eo Anne Paulson, Geoffrey K. Pullum, Ivan A. Sag, and Thomas Wasow Computer Research Center Hewlett Packard Company 1501 Page Mill Road Palo Alto, CA 94304 ABSTRACT This paper describes a natural language processing system implemented at Hewlett-Packard's Computer Research Center. The system's main components are: a Generalized Phrase Structure Grammar (GPSG); a top-down parser; a logic transducer that outputs a first-order logical representation; and a "disambiguator" that uses sortal information to convert "normal-form" first-order logical expressions into the query language for HIRE, a relational database hosted in the SPHERE system. We argue that theoretical developments in GPSG syntax and in Montague semantics have specific advantages to bring to this domain of computational linguistics. The syntax and semantics of the system are totally domain-independent, and thus, in principle, highly portable. We discuss the prospects for extending domain-independence to the lexical semantics as well, and thus to the logical semantic representations. I. INTRODUCTION This paper is an interim progress report on linguistic research carried out at Hewlett-Packard Laboratories since the summer of 1981. The research had three goals: (1) demonstrating the computational tractability of Generalized Phrase Structure Grammar (GPSG), (2) implementing a GPSG system covering a large fragment of English, and (3) establishing the feasibility of using GPSG for interactions with an inferencing knowledge base. Section 2 describes the general architecture of the system. Section 3 discusses the grammar and the lexicon. A brief dicussion of the parsing technique used in found in Section 4. Section 5 discusses the semantics of the system, and Section 6 presents ~ detailed example of a parse-tree complete with semantics. Some typical examples that the system can handle are given in the Appendix. The system is based on recent developments in syntax and semantics, reflecting a modular view in which grammatical structure an~ abstract logical structure have independent status. The understanding of a sentence occurs in a number of stages, distinct from each other and governed by different principles of organization. We are opposed to the idea that language understanding can be achieved without detailed syntactic analysis. There is, of course, a massive pragmatic component to human linguistic interaction. But we hold that pragmatic inference makes use of a logically prior grammatical and semantic analysis. This can be fruitfully modeled and exploited even in the complete absence of any modeling of pragmatic inferencing capability. However, this does not entail an incompatibility between our work and research on modeling discourse organization and conversational interaction directly= Ultimately, a successful language understanding system wilt require both kinds of research, combining the advantages of precise, grammar-driven analysis of utterance structure and pragmatic inferencing based on discourse structures and knowledge of the world. We stress, however, that our concerns at this stage do not extend beyond the specification of a system that can efficiently extract literal meaning from isolated sentences of arbitrarily complex grammatical structure. Future systems will exploit the literal meaning thus …

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