Concurrent Lexicalized Dependency Parsing: A Behavioral View on ParseTalk Events

CONCURRENT LEXICALIZED DEPENDENCY PARSING: A BEHAVIORAL VIEW ON ParseTalk EVENTS Susanne Schacht, Udo tlahn & Norbert Br6ker CLI]- Computational Linguistics Research Group Freiburg University D-79085 I:reiburg, Germany email: {sue, hahn, nobi }@coling.uni-freiburg.dc Abstract. The behavioral specification of an obj~t-ori- cnted grammar model is considered. The model is based on full lexicalization, hcad-oricntation via valency con- straints and dependency relations, inhcritancc as a means for non-redundant lexicon specification, and conct, rrcncy of computation. The compntation model relies upon the actor l)aradigm, with concurrency entering throt~gh asyn- ~:hronous message passing between actors. In pa,ticular, we here elaborate on principlcs of how the global lyzhavior of a lexically distributexl grammar and its corresponding parser can be specified in terms of evcnt type networks and cvent networks, resp. 1 INTRODUCTION In this paper, we propose a grammar model that combines lexical organization of grammatical knowledge with lexi- calized control of the corresponding parser in a coherent object-oriented specification framework. We build upon recent developments in the lield of linguistic grammar the- ory which have already yiekled a rigid lexical mod,lariza- tion, but extend thcm by assigning full procedural auton- omy to lexical units. In particular, we treat lexic,'ll items as active lexieal processes commtnticating with each other by message passing. Titus, they dynamically establish hetero- geneous communication lines in order to dctcnninc each lexical item's fimctional role. While the issue of lexieali- ze(I control has early becn invcstigatcd in the l)aradigm of conceptual parsers (e.g., Riesbcck & Schank 1978), and word expell parsing in particular (Small & Ricger, 1982), we here elaborate on improving ils lcxical commtmication facilities by formalizing the parscr's message passing pro- tocol according to actor computation principles. As this protocol allows for asynchronous message passing, con- currency enters as a theoretical notion at the level of grant- mar specification, not only as an implcmentational fcatttrc. Correspondingly, we introduce a behavioral description in terms of event type networks which rcpresent grammatical interrelations at the level of actor definitions, while event networks represent the I)arsing process in terms of actual messages exchanged between instantiated actors. The Par- seTalk model outlined in this paper can thereft)re be con- sidcred as an attempt to rcmedy the lack of theoretical integration of parallelism at the level of grammar design. 2 ParseTalk's GRAMMAR MODEL The ParseThlk grammar mcxlel (of. Br6kcr, Hahn & Schacht (1994) for a more comprehensive treatment) con- siders dependency relations between words as the funda- meatal notion of lingnistic analysis. This corresponds to the head-orientation found in most modern grammar theo- ries. Grammatical spccilications are given in the format of valency constraints attached to each lexical unit, on which lhe comptttalion of concrete dependency relations is based. A modilicr is said to depend on its head if the mod- ifier satisfies the constraints placed on it. These constraints incorl×)rate iulormation about the hierarchy of word classes (encapsulating declarative and behavioral proper- ties of lexical items), nmrphosyntax (containing Ihe gram- malical conditions of the combination of lcxical items to l)hrascs as expressed by a …

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