Rapid Development of Morphological Descriptions for Full Language Processing Systems

Rapid Development of Morphological Descriptions for Full Language Processing Systems David Carter SRI International Cambridge Computer Science Research Centre 23 Millers Yard, Mill Lane Cambridge CB2 1RQ, U:K. dmc~cam, sri. com Abstract I describe a compiler and development environment for feature-augmented two- level morphology rules integrated into a full NLP system. The compiler is optimized for a class of languages in- cluding many or most European ones, and for rapid development and debug- ging of descriptions of new languages. The key design decision is to compose morphophonological and morphosyntac- tic information, but not the lexicon, when compiling the description. This results in typical compilation times of about a minute, and has allowed a rea- sonably full, feature-based description of French inflectional morphology to be de- veloped in about a month by a linguist new to the system. 1 Introduction The paradigm of two-level morphology (Kosken- niemi, 1983) has become popular for handling word formation phenomena in a variety of lan- guages. The original formulation has been ex- tended to allow morphotactic constraints to be ex- pressed by feature specification (Trost, 1990; A1- shawi et al, 1991) rather than Koskenniemi's less perspicuous device of continuation classes. Meth- ods for the automatic compilation of rules from a notation convenient for the rule-writer into finite- state automata have also been developed, allowing the efficient analysis and synthesis of word forms. The automata may be derived from the rules alone (Trost, 1990), or involve composition with the lex- icon (Karttunen, Kaplan and Zaenen, 1992). However, there is often a trade-off between run- time efficiency and factors important for rapid and accurate system development, such as perspicuity of notation, ease of debugging, speed of compi- lation and the size of its output, and the inde- pendence of the morphological and lexical compo- nents. In compilation, one may compose any or all of (a) the two-level rule set, (b) the set of affixes and their allowed combina- tions, and (c) the lexicon; see Kaplan and Kay (1994 / for an exposition of the mathematical basis. The type of compilation appropriate for rapid development and acceptable run-time performance depends on, at least, the nature of the language being described and the number of base forms in the lexicon; that is, on the position in the three-dimensional space defined by (a), (b) and (c). For example, English inflectional morphology is relatively simple; dimensions (a) and (b) are fairly small, so if (c), the lexicon, is known in advance and is of manageable size, then the entire task of morphological anMysis can be carried out at com- pile time, producing a list of analysed word forms which need only be looked up at run time, or a network which can be traversed very simply. Al- ternatively, there may be no need to provide as powerful a mechanism as two-level morphology at all; a simpler device such as affix stripping (A1- shawi, 1992, pll9ff) or merely listing all inflected forms explicitly may be preferable. For agglutinative languages such as Korean, …

Content truncated to 500 words