Processing English with a Generalized Phrase Structure Grammar
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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 can be achieved without detailed syntactic analysis. There is, of course, a massive This paper describes a natural language pragmatic component to human linguistic processing system implemented at Hewlett-Packard's interaction. But we hold that pragmatic inference Computer Research Center. The system's main makes use of a logically prior grammatical and components are: a Generalized Phrase Structure semantic analysis. This can be fruitfully modeled Grammar (GPSG); a top-down parser; a logic and exploited even in the complete absence of any transducer that outputs a first-order logical modeling of pragmatic inferencing capability. representation; and a "disambiguator" that uses However, this does not entail an incompatibility sortal information to convert "normal-form" between our work and research on modeling first-order logical expressions into the query discourse organization and conversational language for HIRE, a relational database hosted in interaction directly= Ultimately, a successful the SPHERE system. We argue that theoretical language understanding system wilt require both developments in GPSG syntax and in Montague kinds of research, combining the advantages of semantics have specific advantages to bring to this precise, grammar-driven analysis of utterance domain of computational linguistics. The syntax structure and pragmatic inferencing based on and semantics of the system are totally discourse structures and knowledge of the world. domain-independent, and thus, in principle, We stress, however, that our concerns at this highly portable. We discuss the prospects for stage do not extend beyond the specification of a extending domain-independence to the lexical system that can efficiently extract literal meaning semantics as well, and thus to the logical semantic from isolated sentences of arbitrarily complex representations. grammatical structure. Future systems will exploit the literal meaning thus extracted in more ambitious applications that involve pragmatic I. INTRODUCTION reasoning and discourse manipulation. This paper is an interim progress report on The system embodies two features that linguistic research carried out at Hewlett-Packard simultaneously promote extensibility, facilitate Laboratories since the summer of 1981. The modification, and increase efficiency. The first is research had three goals: (1) demonstrating the that its grammar is context-free in the informal computational tractability of Generalized Phrase sense sometimes (rather misleadingly) used in Structure Grammar (GPSG), (2) implementing a discussions of the autonomy of grammar and GPSG system covering a large fragment of English, pragmatics: the syntactic rules and the semantic and (3) establishing the feasibility of using GPSG translation rules are independent of the specific for interactions with an inferencing knowledge application domain. Our rules are not devised ad base. hoc with a particular application or type of interaction in mind. Instead, they are motivated Section 2 describes the general architecture by recent theoretical developments in natural of the system. Section 3 discusses the grammar language syntax, and evaluated by the usual and the lexicon. A brief dicussion of the parsing linguistic canons of simplicity and generality. No technique used in found in Section 4. Section 5 changes in the knowledge base or other exigencies discusses the semantics of the system, and Section deriving from a particular context of application 6 presents ~ detailed example of a parse-tree can introduce a problem for the grammar (as complete with semantics. Some typical examples distinct, of course, from the lexicon). that the system can handle are given in the Appendix. The second relevant feature is that the grammar ir the- system is context-free in the sense The system is based on recent developments of formal language theory. This makes the in syntax and semantics, reflecting a modular view extensive mathematical literature on context-free in which grammatical structure an~ abstract logical phrase structure grammars (CF-PSG's) directly structure have independent status. The relevant to the enterprise, and permits utilization understanding of a sentence occurs in a number of of all the well-known techniques for the stages, distinct from each other and governed by computational implementation of context-free different principles of organization. We are grammars. It might seem anachronistic to base a opposed to the idea that language understanding language understanding system on context-free 74 parsing. As Pratt (1975, 423) observes: "It is translation rule. The syntax-semantics match is fashionable these days to want to avoid all realized as follows: each rule is a triple consisting reference to context-free grammars beyond warning of a rule name, a syntactic statement (~ormally a students that they are unfit for computer local condition on node admissibility), and a consumption as far as computational linguistics is semantic translation, specifying how the concerned." Moreover, widely accepted arguments higher-order logic representations of the daughter have been given in the linguistics literature to the nodes combine to yield the correct translation for effect that some human languages are not even the mother. = weakly context-free and thus cannot possibly be described by a CF-PSG. However, Gazdar and The present GPSG system has five Pullum (1982) answer all of these arguments, components : showing that they are either formally invalid or empirically unsupported or both. It seems 1. Grammar appropriate, therefore, to take a renewed interest a. Lexicon in the possibility of CF-PSG description of human b. Rules and Metarules languages, both in computational linguistics and in 2. Parser and Grammar Compiler linguistic research generally. 3. Semantics Handler 4. Disambiguator 5. HIRE database 2. COMPONENTSOF THE SYSTEM 3. GRAMMARAND LEXICON The linguistic basis of the GPSG linguistic system resides in the work reported in Gazdar The grammar that has been implemented thus (1981, 1982) and Gazdar, Pullum, and Sag (1981). 1 far is only a subset of a much larger GPSG These papers argue on empirical and theoretical grammar that we have defined on paper. It grounds that context-freeness is a desirable nevertheless describes a broad sampling of the constraint on grammars. It clearly would not be basic constructions of English, including a variety so desirable, however, if (1) it led to lost of prepositional phrase constructions, noun-noun generalizations or (2) it resulted in an compounds, the auxiliary system, genitives, unmanageable number of rules in the grammar. questions and relative clauses, passives, and Gazdar (1982) proposes a way of simultaneously existential sentences. avoiding these two problems. Linguistic generalizations can be captured in a context-free Each entry in the lexicon contains two kinds grammar with a metagrammor, i.e. a higher-level of information about a lexical item, syntactic and grammar that generates the actual grammar as its semantic. The syntactic part of an entry consists language. The metagrammar has two kinds of of a syntactic feature specification; this includes, statements: inter alia, information about any irregular morphology the item may have, and what is known (1) Rule schemata. These are in the linguistic literature as strict basically like ordinary rules, except that subcategorization information. In our terms the they contain variables ranging over latter is information linking lexical items of a categories and features. particular category to specific environments in which that category is introduced by phrase (2) Metarules. These are implicational structure rules. Presence in the lexical entry for statements, written in the form ===>B, an item I of the feature R (where R is the name which capture relations between rules. A of a rule) indicates that / may appear in metarule ===>t~ is interpreted as saying, structures admitted by R, and absence indicates "for every rule that is an instantiation of that it may not. the schema =, there is a corresponding rule of form [5." Here 13 will be @(~), where 8 The semantic information in a lexical entry is issome mapping specified partly by the sometimes simple, directly linking a lexical item general theory of grammar and partly in with some HIRE predicate or relation. With verbs the metarule formulation. For instance, or prepositions, there is also a specification of it is taken to be part of the theory of what case roles to associate with particular grammar that @ preserves unchanged the arguments (cf. below for discussion of case roles). subcategorization (rule name) features of Expressions that make a complex logical rules (cf. below). contribution to the sentence in which they appear witl in general have complicated translations. Thus every has the translation- The GPSG system also assumes the Rule-to-Rule Hypothesis, first advanced by Richard Montague, which requires that each syntactic rule be associated with a single semantic 2. There is a theoretical issue here about whether semantic translation rules need to be stipulated for each syntactic rule or whether there is a general way of predicting their form. See I. See also Gazdar, Pullum, Sag, and Wasow Klein