Knowledge Engineering Approach to Morphological Analysis

KNOWLEDGE ENGINEERING APPROACH TO MORPHOLOGICAL ANALYSIS* Harri J~ppinen I , Aarno Lehtola, EsaNelimarkka 2, and Matti Ylilammi Helsinki University of Technology Helsinki, Finland ABSTRACT Finnish is a highly inflectional language. A verb can have over ten thousand different surface forms - nominals slightly fewer. Consequently, a morphological analyzer is an important component of a system aiming at "understanding" Finnish. This paper briefly describes our rule-based heu- ristic analyzer for Finnish nominal and verb forms. Our tests have shown it to be quite efficient: the analysis of a Finnish word in a running text takes an average of 15 ms of DEC 20 CPU-time. I INTRODUCTION This paper briefly discusses the application of rule-based systems to the morphological analy- sis of Finnish word forms. Production systems seem to us a convenient way to express the strongly context-sensltive segmentation of Finnish word forms. This work demonstrates that they can be implemented to efficiently perform segmentations and uncover their interpretations. For any computational system aiming at interpreting a highly inflectional language, such as Finnish, the morphological analysis of word forms is an important component. Inflectional suf- fixes carry syntactic and semantic information which is necessary for a syntactic and logical analysis of a sentence. In contrast to major Indo-European languages, such as English, where morphological analysis is often so simple that reports of systems processing these languages usually omit morphological discussion, the analysis of Finnish word forms is a hard problem. A few algorithmic approaches, i.e. methods using precise and fully-informed decisions, to a morphological analysis of Finnish have been reported. Brodda and Karlsson (1981) attempted to find the most probable morphological segmentation for an arbitrary Finnish surface-word form without a reference to a lexicon. They report surprisingly high success, close to 90 %. However, their system neither transforms stems into a basic form, nor finds morphotactic interpretations. Karttunen et *This research is being supported by SITRA (Finnish National Fund for Research and Development) P.O. Box 329, 00121Helsinki 12, Finland IDigitalSystems Laboratory 21nstitu~e of Mathematics al. (1981) report a LISP-program which searches in a root lexicon and in four segment tables for adjacent parts, which generate a given surface- word form. Koskenniami (1983) describes a rela- tional, symmetric model for analysis, as well as for production of Finnish word forms. He, too, uses a word-root lexicon and suffix lexicons to support comparisons between surface and lexical levels. Our morphological analyzer MORFIN was planned to constitute the first component in our forth- coming Finnish natural-language database query system. We therefore rate highly a computationally efficient method which supports an open lexicon. Lexical entries should carry the minimum of morphological information to allow a casual user to add new entries. We relaxed the requirement of fully informed decisions in favor of progressively generated and tested plausible heuristic hypotheses, dressed in production rules. The analysis of a word in our model represents a multi-level heuristic search. The basic control strategy of MORFIN resembles the one more extensively exploited in the Hearsay-II system (Erman et al.,1980). II …

Content truncated to 500 words