Parsing Head-Driven Phrase Structure Grammar

The Head-driven Phrase Structure Grammar project (HPSG) is an English language database query system under development at Hewlett-Packard Laboratories. Unlike other product-oriented efforts in the natural language understanding field, the HPSG system was designed and implemented by linguists on the basis of recent theoretical developments. But, unlike other implementations of linguistic theories, this system is not a toy, as it deals with a variety of practical problems not covered in the theoretical literature. We believe that this makes the HPSG system ,nique in its combination of linguistic theory and practical application. The HPSG system differs from its predecessor GPSG, reported on at the 1982 ACL meeting (Gawron et al. 119821), in four significant respects: syntax, lexical representation, parsing, and semantics. The paper focuses on parsing issues, but also gives a synopsis of the underlying syntactic formalism. 1 Syntax HPSG is a lexically based theory of phrase structure , so called because of the central role played by grammlttical heads and their associated complements.' Roughly speaking, heads are linguistic forms (words and phrases) tl, at exert syntactic and semantic restrictions on the phrases, called complements, that characteristically combine with them to form larger phrases. Verbs are the heads of verb phrm~es (apd sentences), nouns are the heads of noun phra~es, and so forth. As in most current syntactic theories, categories are represented as complexes of feature specifications. But the [IPSG treatment of lcxical subcategorization obviates the need in the theory of categories for the notion of bar-level (in the sense of X-bar theory, prevalent in much current linguistic research}. [n addition, the augmentation of the system of categories with stack-valued features-features whose values ~re sequences of categories-unilies the theory of lexical subcatego-riz~tion with the theory of bi,~ding phenomena. By binding pimnomena we meaa essentially noJL-clause-bounded delmndencies, ,'such a.~ th~rse involving dislocated constituents, relative ~Lnd interrogative pronouns, and reflexive and reciprocal pronouns [12 I. Structulm Gran|n|ar lTI. The detaaJs uf lily tllt~J/-y of HPSG ar~ Nt forth in Ii|[. More precisely, the subcategorization of a head is encoded as the value of a stack-valued feature called ~SUBCAT". For example, the SUBCAT value of the verb persuade is the sequence of three categories IVP, NP, NP I, corresponding to the grammatical relations (GR's): controlled complement, direct object, and subject respectively. We are adopting a modified version of Dowty's [19821 terminology for GR's, where subject "LS last, direct object second-to-last, etc. For semantic reasons …