A Bidirectional Model for Natural Language Processing

A LANGUAGE i BIDIRECTIONAL MODEL FOR NATURAL PROCESSING Gtinter Neumann Lehrstuhl fur Computerlinguistik Universit~it des Saarlandes Im Stadtwald 15, Bau 17.2 6600 Saarbriicken 11, FRG neumann@coli.uni-sb.de ABSTRACT In this paper* I will argue for a model of grammatical processing that is based on uniform processing and knowledge sources. The main feature of this model is to view parsing and generation as two strongly interleaved tasks performed by a single parametrized deduction process. It will be shown that this view supports flexible and efficient natural language processing. 1 INTRODUCTION The aspect of bidirectionality has been gaining importance since the growing rate of research on natural language generation over the last years offers us deeper insights into this cognitive ability of humans. There are theoretical as well as practical reasons for adopting bidirectionality. Theoretically, the assumption of common knowledge sources for both generation and analysis is essential for the view of language as "an interpersonal medium and an interface to thought" (McDonald 1987). From a psychological point of view, there is a certain amount of empirical evidence for shared processors or facilities (Jackendoff 1987): From a system engineering view, a bidirectional system produces utterances only from that subset of language that it is capable to understand. Therefore, inconsistencies of the language behaviour of the system can be avoided (Jacobs 1988). A fundamental requirement of a bidirectional knowledge base is that it be represented declaratively (Appelt 1987). From this viewpoint one can distinguish two different types of bidirectional natural hmguage systems: ° systems that use uniform knowledge sources, but different processes • systems that use uniform knowledge sources as well as uniform processes 1 Up to now, systems that are capable of analysing and producing language fall into the first class, i.e. they use different operations for both directions (cf. Hoeppner et al. 1983; Busemann and Hauenschild 1988; Allgayer et al. 1989) Currently, it is an open question what degree of bidirectionality should or could be desired (cf. Appelt 1987; Mann 1987; McDonald 1987; Shieber 1988; Jacobs 1988). One of the reasons could be that the formal specification of some tasks (e.g., the determination of content in generation) is currently not well understood in order to decide whether they could be bidirectional in principle. But in some research areas uniform processing models have been developed that are based on formalisms which are well suited for uniform irepresentation and processing, e.g., Koskenniemi's (1984) two-level model of morphology. Recently, there are first approaches to uniform architectures for grammaticallprocessing (e.g., Shieber 1988; Dymetman and Isabelle 1988; Dymetman et al. 1990). These architectures are based on Pereira and lWarren's (1983) paradigm of parsing as deduction. In principle, parsing and generation are viewed as a single parametrizeddeduction process. PROBLEMS OF BIDIRECTIONAL GRAMMATICAL PROCESSING * Thanks to Klaus Netter, Karel Oliva, Norbert Reithinger, Harald Trost and Hans Uszkoreit for fruitful discussions about the aspects of the paper's contents. 1Besides these two classes there are also systems that use different knowledge sources that are compiled from the same source (e.g., …

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