A Simple Reconstruction of GPSG

A Simple Reconstruction of GPSG Stuart M. Shieber Artificial Intelligence Center SRI International and Center for the Study of Language and Information Stanford University Abstract Like most linguistic theories, the theory of generalized phrase structure grammar (GPSG) has described language axiomati- cally, that is, as a set of universal and language-specific con- attaints on the well-formedncss of linguistic elements of some sort. The coverage atttl detailed analysis of English grammar in the ambitious recent volume by Gazdar, Klein, Pullum, and Sag enti- tled Generalized Phrase Structure Grammar [2] are impressive, in part because of the complexity of the axiomatic system developed by the authors. In this paper, we examine the possibility that simpler descriptions of the same theory can be achieved through a slightly different, albeit still axiomatic, method. Rather than characterize the well-formed trees directly, we progress in two stages by procedurally characterizing the well-formedness axioms themselves, which in turn characterize the trees. 1 Introduction I Like most llngafistic theories, the theory of generalized phrase structure grammar (GPSG) has described language axiomati- cally, that is, as a set of universal and language-specific con- straints on the we[l-formedncss of linguistic elements of some sort. In the case of GPSG, these elements are trees whose nodes are themselves structured entltics from a domain of categories (a type of feature ~trueture [6]). The proposed axioms have be- come quite complex, culminating in the ambitious recent volume by Gazdar, Klein, Pullum, and Sag entitled Generalized Phrase Structure Grammar [2]. The coverage and detailed analysis of English grammar in this work are impressive, in part because of the complexity of the axiomatic system developed by the author. In this paper, we examine the possibility that simpler descrip- tions of the same theory can be achieved through a slightly dif- ferent, albeit still axiomatic, method. Rather than characterize the well-formed trees driectly, we progress in two stages by pro- cedurally characterizing tim well-formedaess axioms themselves, which in turn charaetei'ize the trees. In particular, we give a pro- cedure which converts GPSG gramma~ into gramma~ written lThls research was m~de possible by a gift. from the System Development Foundation. I am indebted to Lauri K~rttuncn and Ray Perrault for their eomrael~te on earlier drafts, and to Roger Evans, Gerald Gszdsr~ Ivan S~.$t ltenry Thompson, and members of the Foundations of Grammar project at the Center for the Study of Language and Information for their helpful dis- cussions during the development of this work. in a unification-b~qed formalism, the PATR-II formalism devel- oped at SRI International (henceforth PATR) [5], which h~s its own declarative semmltics, and which can therefore be viewed &s an axiomatization of string well-formedness constraints. 2 The characterization of GPSG thus obtained is simpler and better defined than the version described by Gazdar et al. The semantics of the formalism is given directly through the reduction to PATR. Also, the PATR axiomatization has a clear construe- tire interpretation, unlike that used in Gazdar et al., thus mak- ing the system more amenable to computational …

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