Concurrent Lexicalized Dependency Parsing: The ParseTalk Model

CONCURRENT IJ~XICAIJZEI) I)EI)ENI)ENCY PARSING: THE ParseTalk MODEL Norbert Br6keh Udo Hahn & Susanne Schacht Abstract. A grammar ,nodel for concurrent, object-ori- ented natural language l)arsing is introduced. Complete lexical distribution of grmnmatical knowledge is achieved huilding upon the head-oriented notions of valency and dependency, while inheritance mechanisms tire uscd to capture lexic;tl generalizations. The underlying concurrent computation m(xlel relies upon the actor l)aradigm. We consider message passing protocols for establishing de- l)cndency relations and amhiguity handling. 1 INTRODUCTION In this pal)er, we propose a grammar model that combines lexical organization of grammatical knowledge with lexi- calized conlrol of the corresponding parser in an object- oriented specification framework. Recent developments in the lield of linguistic grammar theory have already yielded a rigid lexical modularization. This fine-grained decomlx)- sit|on of linguistic knowledge can he taken as a starting Ix)int for lexic,'tlized control. Current lexicalized grammars (for instance, IlPSG: Pollard & Sag, 1987; CG: t lcpple, 1992; Lexicalized TAG: Sehabes, Abe|lie & Josh|, 1988), however, still consider lexical items as passive data con- taincrs whosc content is uniformly inteq)reled by global control mechanisms (e.g., unification, functional composi- tion, tree adjunction). Divmging front these l)remises, we assign full procedural autonomy to lexical units and treat then~ as active lexical processes communicating with each other by message passing. Thus, they dynamically estal)- lish heterogcncous communication lines in order to deter- mine each lexical item's funclional role. While the issue of lcxicalized control has early I)ecn investigated in the paua- diem of conceptual parsing (Ricsbeck & Schank, 1978), and word expert parsing in particular (Sinall & Ricger, 1982), these prol)osals am limited in several ways. First, they do not provide any general mechanism f()r the sys- tematic incorporation of grammatical knowledge. Second, they do not supply any organizing facility to R)rmuh|te generalizations over sets of lexical items. Third, lexical communication is based on an entirely inform:l[ l)rotocol that lacks any grounding in principles of distributed com- puting. We intend to remedy these metho(Iological shortconl- ings by designing a radically texicalized grammar on lhe basis of valency and dependency (these head-oriented notions aheady tigure in different shal)eS in many modem linguistic theories, e.g., kS suhc.'ltegorizations, case frames, thela roles), by introducing inheritance kS a major organi- zational mechanism (for a survey of at)plying inheritance ('LI'J-- Comtmtational Linguistics Research Group Vreiburg University D-79085 Frciburg, Germany email: {nobi, hahn, sue}@coling.uni-frciburg.dc in in(xtem grammar theory, of. Daelemans, De Smedt & Gazdar, 1992), and by specifying a message passing proto- col that is grounded in the actor computation model (Agha & I lewilt, 1987). As this protocol allows for asynchronous message passing, concurrency enters as a theoretical notion :it the level of grammar specification, not only as an implementatiolml feature. The ParseThlk m(×lel outlined in this paper can tllerefore be considered as an attempt to replace the static, global-control paradigm of natural lan- guage processing hy a dynamic, local-control mc×lel. The design of such a grammar and its associated parser responds to the demands of complex language perfor- mance problems. By …

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