Categorial Unification Grammars

Categorial Unification Grammars* Hans Uszkoreit Artificial Intelligence Center SRI International and Center for the Study of Language and Information at Stanford University Abstract Categorial unification grammars (CUGs) embody the essential properties of both unification and categorial grammar formalisms. Their efficient and uniform way of encoding linguistic knowledge in well-understood and widely used representations makes them attractive for computational applications and for linguistic research. In this paper, the basic concepts of CUGs and simple examples of their application will be presented. It will be argued that the strategies and potentials of CUGs justify their further exploration in the wider context of research on unification grammars. Approaches to selected linguistic phenomena such as long-distance dependencies, adjuncts, word order, and extraposition are discussed. 0. Introduction The work on merging strategies from unification grammars and categorial grammars has its origins in several research efforst that have been pursued in parallel. One of them is the grammar development on the PATR system (Shieber et al., 1983; Shieber, 1984) at SRI. For quite a while now I have been using the excellent facilities of PATR for the design and testing of experimental[ CUGs. Such grammars currently run on two PATR implementations: Stuart Shieber's Zetalisp version on the Symbolics 3600 and Lauri Karttunen's Interlisp-D w:rsion on the XEROX 1109. The work on CUGs has influenced our efforts to develop a larger PATR grammar, and will do so even more in the future. On the theoretical side, this work is part of ongoing research on such topics as word order variation, modification, and German syntax within projects at SRI and CSLI (Stanford University). The structure of the paper reflects the diverse nature of the enterprise. In the first section, I will introduce the basic notions of CUGs and demonstrate them through examples in PATR notation. The second section discusses the motivation for this work and some of its theoretical implications. The third section sketches a linguistically motivated CUG framework with a strong lexical syntax that accomodates word order variation. The paper concludes with a brief discussion of possible CUG approaches to long-distance dependencies. 1. Basic Notions of Categorial Unification Grammars 1.2. Unification Grammars and Categorial Grammars Both terms, unification grammar (UG) and categorial grammar (CG), stand for whole families of related grammar formalisms whose basic notions are widely known.l Yet, for the characterization of the class of formalisms I want to discuss, it will be useful to review the most central concepts of both UG and CG. Unification grammar formalisms employ complex feature structures as their syntactic representations. These structures encode partial information about constituents. Either term or graph unification is utilized as the main operation for checking, propagating, and merging of the information in these complex representations. Most unification grammars also use the complex feature structures for the linking of syntactic and semantic information. In traditional categorial grammars, all information about possible syntactic combinations of constituents is encoded in their categories. Those grammars allow only binary combinations. One of the two combined constituents, the functor, encodes the …

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