Coping With Derivation in a Morphological Component

Coping With Derivation in a Morphological Component * Harald Trost Austrian Research Institute for Artificial Intelligence Schottengasse 3, A-1010 Wien Austria email: harald@ai.univie.ac.at Abstract In this paper a morphological component with a limited capability to automatically interpret (and generate) derived words is presented. The system combines an ex- tended two-level morphology [Trost, 1991a; Trost, 1991b] with a feature-based word grammar building on a hierarchical lexicon. Polymorphemic stems not explicitly stored in the lexicon are given a compositional in- terpretation. That way the system allows to minimize redundancy in the lexicon be- cause derived words that are transparent need not to be stored explicitly. Also, words formed ad-hoc can be recognized correctly. The system is implemented in CommonLisp and has been tested on examples from Ger- man derivation. 1 Introduction This paper is about words. Since word is a rather fuzzy term we will first try to make clear what word means in the context of this paper. Following [di Sci- ullo and Williams, 1989] we discriminate two senses. One is the morphological word which is built from morphs according to the rules of morphology. The other is the syntactic word which is the atomic entity from which sentences are built according to the rules of syntax. *Work on this project was partially sponsored by the Austrian Federal Ministry for Science and Research and the "Fonds zur FSrderung der wissenschaftlichen Forschung" grant no.P7986-PHY. I would also like to thank John Nerbonne, Klaus Netter and Wolfgang Heinz for comments on earlier versions of this paper. These two views support two different sets of infor- mation which are to be kept separate but which are not disjunctive. The syntactical word carries infor- mation about category, valency and semantics, infor- mation that is important for the interpretation of a word in the context of the sentence. It also carries in- formation like case, number, gender and person. The former information is basically the same for all dif- ferent surface forms of the syntactic word 1 the latter is conveyed by the different surface forms produced by the inflectional paradigm and is therefore shared with the morphological word. Besides this shared information the morphologi- cal word carries information about the inflectional paradigm, the stem, and the way it is internally structured. In our view the lexicon should be a me- diator between these two views of word. Traditionally, the lexicon in natural language pro- cessing (NLP) systems is viewed as a finite collection of syntactic words. Words have stored with them their syntactic and semantic information. In the most simple case the lexicon contains an entry for every different word form. For highly inflecting (or agglutinating) languages this approach is not feasible for realistic vocabulary sizes. Instead, morphological components are used to map between the different surface forms of a word and its canonical form stored in the lexicon. We will call this canonical form and the information associated with it lezeme. There are problems with such a static view of the lexicon. In the …

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