Parsing a Flexible Word Order Language

PARSING A FLEXIBLE WORD ORDER LANGUAGE Vladimir Pericliev and Alexander Grigorov Institute of Mathematics with Computing Centre, bi.8, 1113 Sofia, Bulgaria, E-mail: BANMAT@BGEARN.BITNET ABSTRACT A logic formalism is presented which increases the expressive power of the ID/LP format of GPSG by enlarging the inventory of ordering relations and extending the domain of their application to non-siblings. This allows a concise, modular and declarative statement of intricate word order regularities. 1. INTRODUCTION Natural languages exhibit significant word order (WO) variation and intricate ordering rules. Despite the fact that specific languages show less varmtion and complexity in such rules (e.g. those characterized by either fixed, or totall 3, free, WO), the vast majority of world languages lie somewhere in-between these two extremes (e.g. Steele 1981). Importantly, even the proclaimed examples of rigid WO languages (English) exhibit variation, whereas those with proclaimed total scrambling (Warlpiri; cf. llale 198.1) show restrictions (Kashket 1987). Therefore, we need general grammar formalism, capable of processing "flexible" WO (i.e. complex WO regularities, including both extremes). There seem to be a number of requirements that such a formalism should (try to) fulfil (e.g. Pericliev and Grigorov 1992). Among these stand out the formalism's: (i) Expressive power, i.e. capability of (reasonably) handling complex WO phenomena, or "flexible" WO. (ii) Linguistic felicity, i.e. capability of stating concisely and declaratively WO roles in a way maximally approximating linguistic parlance in similar situations. (iii) Modularity, i.e. the separation of constituency rules from the rules pertaining to the linearization of these constituents (for there may be many, and diverse, reasons for wanting linearization (and constituency) rules easily modifiable, incl. the transparency of WO statements, the imprecision of our current knowledge of ordering rules or the wish to tailor a system to a domain with specific WO). (iv) Reversibility, i.e. the ability of a system to be used for both parsing and generation (the reason being that, even if the system is originally intended for a parser, complex WO rules may be conveniently tested in the generation mode; in this sense it is not incidental that e.g. Kay & Kartttmen 1984 have first constructed a generator, and used it as a tool in testing the (WO) rules of their grammar, and only then have converted it into a parser). In this paper, we present a logic-based formalism which attempts to satisfy the above requirements. A review shows that most previous approaches to WO within the logic grammars paradigm (Dahl & Abramson 1990) have not been satisfactory. Definite Clause Grammar, DCG, (Pereira & Warren 1980), with their CF-style rules, are not modular (in the sense above), so will have to specify explicitly each ordering of constituents in a separate rule, which results in an intolerably great number of rules in parsing a free WO language (e.g. for 5 constituents, which may freely permute, the number of rules is 5! = 120). Other approaches center around the notion of a "gap" (or "skip"). In Gapping Grammar (GG), for instance (Dahl & Abramson 1984, esp. Dahl 1984), where …

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