Design and Implementation of a Lexical Data Base

DESIGN AND IMPLEMENTATION OF A LLXICAL DATA BASE Eric Wehrli Department of Linguistics U.C.L.A. 405 Hilgard Ave, Los Angeles, CA 90024 ABSTRACT This paper is concerned with the specifications and the implementation of a particular concept of word-based lexicon to be used for large natural language processing systems such as machine translation systems, and compares it with the morpheme-based conception of the lexicon traditionally assumed in computational linguistics. It will be argued that, although less concise, a relational word-based lexicon is superior to a morpheme-based lexicon from a theoretical, computational and also practical viewpoint. INTRODUCTION It has been traditionally assumed by computational linguists and particularly by designers of large natural language processing systems such as machine translation systems that the lexicon should be limited to lexical information that cannot be derived by rules. According to this view, a lexicon consists of a list of basic morphemes along with irregular or unpredictable words. In this paper, I would like to reexamine this traditional view of the lexicon and point out some of the problems it faces which seriously question the general adequacy of this model for natural language processing. As a trade-off between the often conflicting linguistic, computational and also practical considerations, an alternative conception of the lexicon will be discussed, largely based on Jackendoff's (1975) proposal. According to this view, lexical entries are fully-specified but related to one another. First developed for a French parser (cf. Wehrli, 1984), this model has been adopted for an English parser in development, as well as for the prototype of a French-English translation system. This paper is organized as follows: the first section addresses the general issue of what constitutes a lexical entry as well as the question of the relation between lexicon and morphology from the point of view of both theoretical linguistics and computational linguistics. Section 2 discusses the relational word-based model of the lexicon and the role morphology is assigned in this model. Finally, it spells out some of the details of the implementation of this model. OVERVIEW OF THE PROBLSM One of the well-known characteristic features of natural languages is the size and the complexity of their lexicons. This is in sharp constrast with artificial languages, which typically have small lexicons, in most cases made up of simple, unambiguous lexical items. Not only do natural languages have a huge number of lexical elements -- no matter what precise definition of this latter term one chooses -- but these lexical elements can furthermore (i) be ambiguous in several ways (ii) have a non-trivial internal structure, or (iii) be part of compounds or idiomatic expressions, as illustrated in (1)-(A): (I) ambiguous words: can, fly, bank, pen, race, etc. (2) internal structure: use-ful-ness, mis-understand-ing, lake-s, tri-ed (3) compounds: milkman, moonlight, etc. (4) idiomatic expressions: to kick the bucket, by and large, to pull someone's leg, etc. In fact, the notion of word, itself, is not all that clear, as numerous linguists -- theoreticians and/or computational linguists -- have acknowledged. Thus, to take an …

Content truncated to 500 words