Lexicase Parsing: A Lexicon-driven Approach to Syntactic Analysis

Lexicase Parsing: A Lexicon-driven App.roach. to Syntactic Analysis Stanley STAROSTA University of lIawaii Social Science Research Institute and Pacific international Center for lIigh Technology Research Hoaohdu, Hawaii 96822, U.S.A. Abstract This paper presents a lexicon-based approach to syntactic analysis, l,exicase, and applies it to a lexicon driven computatlolml parsing system. The basic descriptive mechanism in a l,exicase grammar is lexieal features. The properties of lexieal items are represented by contextual and non-contextual features, and generalizations are expressed as relationships among sets of these features and among sets of lexieal entries. Syntactic tree strue{,uros are representaed as networks of pairwise dependency relationships among the words in a sentence. Possible dependencies are marked as contextual features on individual lexical items, and Lexicase parsing ix a process of picking out words in a string and attaching dependents to them in accordance with their contextual features. Lexiease is an appropriate vehicle for parsing because I,exicase analyses are monostratal, fiat, and relatively non-abstract, and it is well suited to machine translation because grammatical rel)resentations for corresponding smrtences in two languages will Im very similar to each other in structure and inter-constituent relations, and teas far easier to interconvert. 1. Introduction There are a number of current frameworks of syntactic analysis which have been used as the basis for natural language processing. Many suffer from serious metatheoretical or pra(:tieal defects, especially in the areas of power and descriptive adequacy. Several more recent syntactic fl'ameworks, including I,exical-Functional Grammar [1], Generalized Phrase Structure Grammm" [2], and Lexiease [3] have begun to take these problems seriously, and to consider al)plications to natural language processi~g. This paper will be concerned with the application of lexiease grammatical theory to computer parsing of natural language texts. The point of view which we will adopt here is a very simple one: sentences are hierarchically structured strings of words, and grammar is a statement about the internal composition and external distributions of words. Proceeding from this basis, it is possible to construct a fornml and explicit grannnatical fl'amework of limited generative power which is capable of stating language-specific and universal generalizations in a natural way, unhh}dered by pretheoretieal a priori assumptions about VP's, etc. The fl'amework so constructed, lexiease [13], [4], [5], turns out to have a significant 1)otential for application in the processing of natural language [6], The basic descriptive mechanism in a lexiease grammar is lexieal features. The properties of lexieal items are represented by eantextual and non-.contexttlal features, aud generalizations are expressed as relationships among sets of these features. The ways in which words can combine together are strongly restricted by the Sisterhead Constraint [3], which states that a word can contract a grammatleal relationship only with the head of a dependent sister eonstruetlon, and tbe One-bar Constraint lop. ell,l, which requires every construction to have at least one lexical head. The result is syntaetie tree representations which are flatter, since there are no intermediate nodes between lexical entries and their maximal projections, and more universal, since there are only a …

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