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Towards an Implelnentable Dependency Grammar

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Towards an Implelnentable Dependency Grammar Towards an implementable dependency grammar Timo JKrvinen and Pasi Tapanainen Research Unit for Multilingual Language Technology P.O. Box 4, FIN-00014 University of Helsinki, Finland Abstract such as a context-free phrase structure gram- mar. This leads us to a situation where the Syntactic models should be descriptively ade- parsing problem is extremely hard in the gen- I quate and parsable. A syntactic description is eral, theoretical case, but fortunately parsable autonomous in the sense that it has certain ex- in practise. Our result shows that, while in gen- plicit formal properties. Such a description re- eral we have an NP-hard parsing problem, there i lates to the semantic interpretation of the sen- is a specific solution for the given grammar that tences, and to the surface text. As the formal- can be run quickly. Currently, the speed of the ism is implemented in a broad-coverage syntac- parsing system I is several hundred words per tic parser, we concentrate on issues that must I second. be resolved by any practical system that uses In short, the grammar should be empirically such models. The correspondence between the motivated. We have all the reason to believe structure and linear order is discussed. ! that if a linguistic analysis rests on a solid de- scriptive basis, analysis tools based on the the- 1 Introduction ory would be more useful for practical purposes. i The aim of this paper is to define a dependency We are studying the possibilities of using com- grammar framework which is both linguistically putational implementation as a developing and motivated and computationally parsable. testing environment for a grammatical formal- A linguistically adequate grammar is the pri- ism. We refer to the computational implemen- I mary target because if we fail to define a de- tation of a grammar as a parsing grammar. scriptive grammar, its application is less use- 1.1 Adequacy ful for any linguistically motivated purposes. In I fact, our understanding of the potential bene- A primary requirement for a parsing grammar is fits of the linguistic means can increase only if that it is descriptively adequate. Extreme dis- our practical solutions stand on an adequate de- tortion results if the mathematical properties I scriptive basis. of the chosen model restrict the data. How- Traditionally, grammatical models have been ever, this concern is not often voiced in the dis- constructed by linguists without any consider- cussion. For example, McCawley (1982, p. 92) ation for computational application, and later, notes that such a basic assumption concerning linguistic structures that "strings are more basic I by computationally oriented scientists who have than trees and that trees are available only as a first taken a parsable mathematical model and side product of derivations that operate in terms then forced the linguistic description into the of strings" was attributable to the historical ac- I model which has usually been too weak to de- cident that early transformational grammarians scribe what a linguist would desire. knew some automata theory but no graph the- Our approach is somewhere between these Ory." I two extremes. While we define the grammar One reason for computationally oriented syn- strictly in linguistic terms, we simultaneously tacticians to favour restricted formalisms is that test it in the parsing framework. What is excep- they are easier to implement. Those who began i tional here is that the parsing framework is not restricted by an arbitrary mathematical model l Demo: http://www.conezor.fi/analysers.html | ! i' to use dependency models in the 1960's largely natural language. His main work, (1959) ad- ignored descriptive adequacy in order to develop dresses a large amount of material from typo- models which were mathematically simple and, logically different languages. It is indicative of as a consequence, for which effective parsing al- Tesni~re's empirical orientation that there are gorithms could be presented. These inadequa- examples from some 60 languages, though his cies had to be remedied from the beginning, method was not empirical in the sense that he which resulted in ad hoc theories or engineering would have used external data inductively. As I solutions 2 without any motivation in the theory. Heringer (1996) points out, Tesni~re used data There have been some serious efforts to re- merely as an expository device. However, in solve these problems. Hudson (1989), for exam- order to achieve formal rigour he developed a i ple, has attempted to construct a parser that model of syntactic description, which obviously would reflecs the claims of the theory (Word stems from the non-formal tradition developed Grammar) as closely as possible. However, it since antiquity but without compromising the seems that even linguistically ambitious depen- descriptive needs. We give a brief historical I dency theories, such as Hudson's Word Gram- overview of the formal properties inherent in mar, contain some assumptions which are at- Tesni~re's theory in Section 5 before we proceed tributable to certain mathematical properties of to the implementational issues in Section 6. I an established formalism rather than imposed 1.3 The surface syntactic approach by the linguistic data 3. These kinds of unwar- ranted assumptions tend to focus the discus- We aim at a theoretical framework where we I sion on phenomena which are rather marginal, have a dependency theory that is both descrip- if a complete description of a language is con- tively adequate and formally explicit. The lat- cerned. No wonder that comprehensive descrip- ter is required by the broad-coverage parsing ! tions, such as Quirk et al. (1985), have usually grammar for English that we have implemented. been non-formal. We maintain the parallelism between the syn- tactic structure and the semantic structure in 1.2 The European structuralist our design of the syntactic description: when a i tradition choice between alternative syntactic construc- We argue for a syntactic description that is tions in a specific context should be made, the based on dependency rather than constituency, semantically motivated alternative is selected 4. I and we fully agree with Haji~ovA (1993, p. 1) Although semantics determines what kind of that "making use of the presystemic insights of structure a certain sentence should have., from classical European linguistics, it is then possi- the practical point, of view, we have a completely I ble that constituents may be dispensed with as different problem: how to resolve the syntactic basic elements of (the characterization of) the structure in a given context. Sometimes, the sentence structure." However, we disagree with latter problem leads us back to redefine the syn- the notion of "presystemic" if it is used to imply tactic structure so that it can be detected in I that earlier work is obsolete. From a descriptive the sentence s . Note, however, that this redef- point of view, it is crucial to look at the data inition is now made on a linguistic basis. In that was covered by earlier non-formal gram- order to achieve parsability, the surface descrip- I marians. • As far as syntactic theory is concerned, there 41n such sentence as "I asked John to go home", the is no need to reinvent the wheel. Our de- noun before the infinitive clause is analysed as the (se- ! scription has its basis in the so-called "classi- mantic) subject of the infinitive rather than as a com- plement of the governing verb. cal model" based on the work of the French SFor instance, detecting the distinct roles of the to- linguist Lucien Tesni~re. His structural model infinitive clause in the functional roles of the purpose should be capable of describing any occurring or reason is usually difficult (e.g. Quirk et al. (1985, i p. 564): "Whg did he do itf; purpose: "To relieve his 2See discussion of an earlier engineering art in apply- anger" and reason: "Because he was angry"). In such ing a dependency grammar in Kettunen (1994). sentence as UA man came to the party to have a good 3For instance, the notion of adjacency was redefined time", the interpretation of the infinitive clause depends I in WG, but was still unsuitable for "free~ word order on the interaction of the contextual and lexical semantics languages. rather than a structural distinction. I I 2 I i tion should not contain elements which can not tively adequate constitutional grammars are not be selected by using contextual information. It known. In the remaining sections, we show that is important that the redefinition should not be a descriptively adequate dependency model can made because an arbitrary mathematical model be constructed so that it is formally explicit and denies e.g. crossing dependencies between the parsable. syntactic elements. 3 Parallelism between the syntactic I 2 Constituency vs. dependency and semantic structures A central idea in American structuralism was Obviously, distributional descriptions that do to develop rigorous mechanical procedures, not contribute to their semantic analysis can i i.e. "discovery procedures", which were assumed be given to linguistic strings. Nevertheless, to decrease the grammarians' own, subjective the minimal descriptive requirement should be assessment in the induction of the grammars. that a syntactic description is compatible with I This practice was culminated in Harris (1960, the semantic structure. The question which p. 5), who claimed that "the main research arises is that if the correspondence between of descriptive linguistics, and the only rela- syntactic and semantic structures exists, why tion which will be accepted as relevant in the should these linguistic levels be separated. For I present survey, is the distribution or arrange- example, Sgall (1992, p. 278) has questioned ment within the flow of speech of some parts or the necessity of the syntactic level altogether.
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