Reversibility and Modularity in Natural Language Generation

REVERSIBILITY AND MODULARITY IN NATURAL LANGUAGE GENERATION Gfinter Neumann Lehrstuhl ffir Computerlinguistik SFB 314, Projekt N3 BiLD Universit£t des Saarlandes Im Stadtwald 15 D-6600 Saarbrficken 11, FRG neumann@coli.uni-sb.de ABSTRACT A consequent use of reversible grammars within natural language generation systems has strong implications for the separation into strategic and tactical components. A central goal of this paper is to make plausible that a uniform architecture for grammatical Processing will serve as a basis to achieve more flexible and efficient generation systems. 1 Introduction In general, the goal of parsing is the derivation of all possible grammatical structures defined by a grammar of a given string a (i.e. especially the determination of all possible logical forms of o~) and the goal of the corresponding generation task is the computation, of all possible strings defined by a grammar of a! given logical form & that are logically equivalent to ~ (see also (Shieber, 1988), (Calder et al., 1989)). Recently, there is a strong tendency to use the same grammar for perform- ing both tasks. Besides more practically moti- vated reasons - obtaining more compact systems or avoidance of inconsistencies between the input and output of a system - there are also theoreti- cal (a single mode! of language behaviour) and psychological evidences (empirical evidence for shared processors or facilities, cf. (Garrett, 1982), (Frazier, 1982), (Ja'ckendoff, 1987)) to adopt this view. From a formal point of view the main interest in obtaining non-directional grammars is the spec- ification of the relationship between strings and logical forms. 1 According to van Noord (1990), a grammar is reversible if the parsing and gen- eration problem is computable and the relation between strings and logical forms is symmetric. In this case parsing and generation are viewed as mutually inverse processes. Furthermore there are also approaches that as- sume that it is possible to use the same algo- rithm for processing the grammar in both direc- tions (e.g. (Hasida and Isizaki, 1987), (Shieber, 1988), (Dymetman et aL, 1990), (Emele and Za- jac, 1990)). A great advantage of a uniform pro- cess is that a discourse and task independent module for grammatical processing is available. This means that during performing both tasks the same grammatical power is potentially dis- posable (regardless of the actual language use). Nevertheless, in most of the 'real' generation systems where all aspects of the generation pro- cess of natural language utterances are consid- ered, grammars are used that are especially de- signed for generation purposes (cf. (Hovy, 1987), (Dale, 1990), (Horacek, 1990), (McKeown el al., 1990), (Reithinger, 1991)). ~ The purpose of this paper is to show that the use of a uniform architecture for grammatical pro- cessing has important influences for the whole generation task. A consequent use of a uniform process within a natural language generation sys- tem affects the separation into strategic and tacti- 11 assume a notion of grammars that integrate phono- logical, syntactical and semantical levels of description, e.g., (Pollard and Sag, 1987). 2But it is important to …

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