A Finite State Approach to German Verb Morphology

A Finite State Approach to German Verb Morphology Giinther GORZ IBM -- WT LILOG Schlot3str. 70, D-7000 Stuttgart 1, W. Germany (on leave from Univ. of Erlangen-Niirnberg) Goerz@SUMEX-AIM.STANFORD.EDU, GOERZ@DSOLILOG.BITNET Dietrich PAULUS Univ. of Erlangen-Nilrnberg, IMMD V Martensstr. 3, D-8520 Erlangen, W. Germany Paulus@fani53.uucp Abstract This paper presents a new, language independent model for analysis and generation of word forms based on Finite State Transducers (FSTs). It has been completely implemented on a PC and successfully tested with lexicons and rules cover- ing all of German verb morphology and the most interesting subsets of French and Spanish verbs as well. The linguistic databases consist of a'letter-tree structured lexicon with annc~ tated feature lists and a FST which is constructed from a set of morphophonological rules. These rewriting rules operate on complete words unlike other FST-based systems. 1 Introduction Until the beginning of this decade, morphological parsers usually were restricted to on e particular language; in fact we do not know of any one which was language independent or even applicable to a wide class of non-trivial inflected languages. In the meantime, the situa- tion has changed a lot through the usage of Finite State Transducers (FSTs). Although the formalism of generative phonology seems to be powerful enough to cover almost any language, it is very diffi- cult to implement it computationally. Recent approaches to compile the rules of generative phonology into finite automata offer solutions to both problems. In the following, we report on a successful and complete application of this technique to the morphology of German verbs. To demonstrate its generality, it has also been applied to a large subset of French -- in fact the most interesting cases -- and some Spanish verbs. 2 The Finite State Approach As Gazdar/1985/ [1] observed, only very few papers on the math- ematical foundations of modern phonology exist. He quotes from Johnson's/1970/PhD thesis [2], the earliest study of this kind, who states that "any theory which allows phonological rules to simulate arbitrary rewriting systems is seriously defective, for it asserts next to nothing about the sorts of mappings the rules can perform" (/John- son 1970] [2], p. 42). According to Gazdar /1985/ ([1], p. 2) Johnson "proves that a phonology that permits only simultaneous rule appli- cation, as opposed to iterative derivational application, is equivalent to an FST. And he then argues that most of the phonology current around 1970 could eitlmr be formalized or reanalyzed in terms of simultaneous rule application, and could thus be reduced to FSTs." At the Winter LSA meeting at New York in December 1981, R. Kaplan and M. Kay gave a talk -- a written account does nat exist -- in which they showed "how the iteratively applied rules of standard generative phonology could, individually, be algorithmically compiled into FSTs" (/Gazdar 1985/[1], p. 2) under the constraint that rules may not be reapplied to their own outputs. Such a finite ordered cascade of FSTs can be collapsed into a single FST whose behavior is equivalent to …

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