A Symmetrical Approach to Parsing and Generation

Abstract. Lexicat Grammars are a class of unification grammars which share a fixed rule component, for which there exists a simple left-recursion elimination transformation. The parsing and generation programs ale seen as two dual non-left-recursive versions of the original grammar, and are implemented through a standard top-down Prolog interpreter. Formal criteria for termination are given as conditions on lexical entries: during parsing as well as during generation the processing of a lexical entry constimes some amount of a guide; the guide used for parsing is a list of words remaining to be analyzed, while the guide for generation is a list of the semantics of constituents waiting to be generated. I. In t roduct ion Symmetry between pars ing and generat ion. There is a natural appeal to the attempt to characterize parsing and ge~;era~ion i a symmetrical way. This is because the statement of the problem of reversibil ity is naturally synlmetrical: parsing is concerned with recovering semantic content from phonological content, generation phonological conten