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Recent developments in generation algorithms have enabled work in nnificafion-based computational linguistics to approach more closely the ideal of grammars as declarative statements of linguistic facts, neutral between analysis an0_synthesis, x-"~-oui this perspective, however, the situation is still far from perfect; all known methods of generation impose constraints on the grammars they assume. We briefly consider a number of proposals for generation, outlining their consequences for the form of grammacs, and then report on experience arising from the addition of a generator to an existing unification environment. . | Asymmetry in Parsing and Generating with Unification Grammars Case Studies From ELU Graham Russell Susan Warwick and John Canollt ISSCO 54 rte. des Acacias 1227 Geneva Switzerland njssell@divsun.unige.ch t Cambridge University Computer Laboratory New Museums Site Pembroke Street Cambridge CB23QG Abstract Recent developments in generation algorithms have enabled work in unification-based computational linguistics to approach more closely the ideal of grammars as declarative statements of linguistic facts neutral between analysis and synthesis. From this perspective however the situation is still far from perfect all known methods of generation impose constraints on the grammars they assume. We briefly consider a number of proposals for generation outlining thefr consequences for the form of grammars and then report on experience arising from the addition of a generator to an existing unification environment. The algorithm in question based on that of Shieber et al. 1989 though among the most permissive currently available excludes certain classes of parsable analyses. 1. Introduction Parsing and generation are both concerned with the relation between texts and representations and in so far as a grammar defines this relation without reference to direction it may be regarded as reversible. Yet in practice the program which applies a grammar for the purpose of parsing is quite distinct from the one which performs generation.1 The essential difference between parsing and generating lies in the nature of the input. The text as a string of words traditionally establishes the starting poiffl of parsing whether the processing is top-down or bottom-up the basis for selecting grammar rules is information associated with words m the lexicon. In the case of generation there is in general no guarantee that the constituents of an input representation correspond to words a portion of the input may be related directly to a given word or it may be die result of combining .
