Ambiguity resolution in a reductionistic parser

Ambiguity resolution in a reductionistic parser * Atro Voutilainen & Pasi Tapanainen Research Unit for Computational Linguistics P.O. Box 4 (Keskuskatu 8) FIN-00014 University of Helsinki Finland Abstract We are concerned with dependency- oriented morphosyntactic parsing of run- ning text. While a parsing grammar should avoid introducing structurally unresolvable distinctions in order to optimise on the ac- curacy of the parser, it also is beneficial for the grammarian to have as expressive a structural representation available as possi- ble. In a reductionistic parsing system this policy may result in considerable ambigu- ity in the input; however, even massive am- biguity can be tackled efficiently with an accurate parsing description and effective parsing technology. 1 Introduction In this paper we are concerned with grammar-based surface-syntactic analysis of running text. Morpho- logical and syntactic analysis is here based on the use of tags that express surface-syntactic relations between functional categories such as Subject, Mod- ifier, Main verb etc.; consider the following simple example: I PRON ~SUBJECT see V PRES @MAINVERB a ART QDET>N bird N ~OBJECT FULLSTOP *The development of ENGCG was supported by TEKES, the Finnish Technological Development Center, and a part of the work on Finite-state syntax has been supported by the Academy of Finland. In this type of analysis, each word gets a mor- phosyntactic analysis I. The present work is closely connected with two parsing formalisms, Constraint Grammar [Karls- son, 1990; Karlsson et aI., 1991; Voutilainen et aI., 1992; Karlsson et aI., 1993] and Finlte-state syn- tax as advocated by [Koskenniemi, 1990; Tapanai- nen, 1991; Koskenniemi et al., 1992]. The Con- straint Grammar parser of English is a sequential modular system that assigns a shallow surface-true dependency-oriented functional analysis on running text, annotating each word with morphological and syntactic tags. The finite-state parser assigns a sim- ilar type of analysis, but it operates on all levels of ambiguity 2 in parallel rather than sequentially, en- abling the grammarian to refer to all levels of struc- tural description in a single uniform rule component. ENGCG, a wide-coverage English Constraint Grammar and lexicon, was written 1989-1992, and the system is currently available 3. The Constraint Grammar framework was proposed by Fred Karls- son, and the English Constraint Grammar was de- veloped by Afro Voutilainen (lexicon, morphological disambiguation), Juha Heikkil~i (lexicon) and Arto Anttila (syntax). There are a few implementations lit consists of a base form, a morphological reading - part-of-speech, inflectional and other morphosyntactic features - and a syntactic-functional tag, flanked by '@'. ~Morphological, clause boundary, and syntactic ambiguities 3The ENGCG parser can currently be tested automatically via E-mail by sending texts of up to 300 words to engcg@ling.Helsinki.FI. The re- ply will contain the analysis as well as informa- tion on usage and availability. Questions can also be directly sent to avoutila@ling.Helsinki.FI or to pt apanai@ling.Helsinki.FI. 394 of the parser, and the latest, written in C by Pasi Tapanainen, analyses more than 1000 words per sec- ond on a Sun SparcStationl0, using a disambiguation grammar of some 1300 constraints. …

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