Modularity, Parallelism, and Licensing in a Principle-Based Parser for German

MODULARITY~ PARALLELISM~ AND LICENSING IN A PRINCIPLE'BASED PARSER FOR GERMAN SEBASTIAN M1LLIES Saarbriieken University Dept. of Computational Linguistics D-6600 Saarbriicken Germany e-mail: millies@coli.uni-sb.de Abstract This paper presents a direct implementation of Government-Binding theory in a parser for German, which faithfully models the modular structure of the theory. The modular design yields a flexible environment, in which it is possible to define and test various versions of principles and parameters. The several modules of linguistic theory and the parser proper are interleaved in parallel fashion for early elimination of ungrammatical structures. Efficient processing of global constraints is made possible by the concept of licensing, and the use of tree indexing techniques. 1 Introduction Government-Binding theory 1 (henceforth "GB") seeks to describe human knowledge of language by positing a small number of highly general principles, which interact to produce highly specific effects. Most of these principles are regarded as universal principles. Specific construction types in different human languages result from applying language-particular versions of the universal principles, derived from them by parametrizafion. GB tries to avoid language-particular and construction specific rules. Only recently has the idea of "principle- By that term 1 will mean not only the particular version of the theory set forth in [ChomSl], but rather the entire tamily of theories of the principles- and-parameters type inspired by Chomsky's work. based" parsers, which derive structures by deduction from an explicit representation of the principles, come into the focus of attention. Importantly, however, GB does not specify any particular relation between the principles and a parser which is supposed to use them. As a consequence, extant GB-parsers reflect the internal organization of GB-theory to varying degrees. This paper reports on an implemen~ tation of a GB-parser for German, which faith- fully mirrors the modular structure of (mucb of) GB-theory in the way it represents linguistic knowledge. In discussing the parser, l will presuppose a basic familiarity with GB-theory. 2 According to Mark Johnson (cf. [John88, John89]), the most direct relation between a parser and linguistic theory can be observed in a "parsing-as-deduction" approach. Johnson's project is to forntalize linguistics in some suitable subset of first-order logic, arid use this formalization as inpnt for an antomatic theorem prover, such as Prolog, without any intervening recoding. This proposal, however, suffers from some well-known difficulties, such as undecidability, left-recursion (in Prolog), and a tendency to produce generate-and-test algo- 2 The reader is referred to [Se185] for a short introduction. For a detailed di~ussion, see one of the standard texu% e.g. [LIJ881. AcrEs Dr. COL1NG-92. NANTF~S. 23-28 AOm' 1992 I 6 3 PROC. OF COLING-92. N^~rES, Auu. 23-28, 1992 rithms (with modules such as X'-theory and move-c~ as generators, and other parts of grammar as filters). Furthermore, there is no place in the model for those aspects of language processing which do not have to do with knowledge of grammar, but rather with procedural considerations (resolution of ambiguities in PP-attachment and the like). Johnson proposes to cope with the difficulty about indeterminacy by using the …

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