Compiling Trace & Unification Grammar for Parsing and Generation

COMPILING TRACE & UNIFICATION GRAMMAR FOR PARSING AND GENERATION Hans Ulrich Block Siemens AG, Corporate Research, ZFE IS INF 23 Otto Hahn-Ring 6 D-8000 Miinchen 83 Germany block@ztivax.uucp ABSTRACT This paper presents Trace & Unification Gram- mar (TUG), a declarative and reversible grammar formalism that brings together Unification Gram- mar (uG) and ideas of Government & Binding Theory (GB) in an undogmatic way. A grammar compiler is presented that transforms a grammar written in the TUG formalism into two different forms, one being useful for parsing, the other be- ing useful for generation. 1 INTRODUCTION During the last years there has been a growing interest in NL systems that can be used for both parsing and generation. The invention of unifica- tion grammar that allows for a declarative descrip- tion of language made it possible to use the same grammar for both tasks. The main goal of a gram- mar then is to describe a relation between nor- malized (semantic) representations and language strings. A grammar that can be used in both di- rections is called "reversible". We can distinguish three levels of reversibility. On the first level, not only the same grammar is used for parsing and generation, but also the in- terpreter for parsing and generation is reversible. This approach is taken in Shieber (1988). Besides elegance the approach has the advantage that the reversibility is guaranteed. Further advantages are mentioned in Neumann (1991). As a disadvantage, it is yet unclear whether and how these systems can be made efficient. On the second level we find systems where the same reversible grammar is processed by two dif- ferent interpreters, one for parsing, one for gener- ation. The advantage of these systems is that the grammar can be changed and tested easily, which helps to shorten the development cycle. The dis- advantage again is that grammar interpreters are usually too slow to be used in realistic systems. On the third level we finally find systems, where the linguistic description is given in a reversible declarative grammar. This grammar is then com- piled into two different forms, one being usefull only for parsing, the other only for generation. Whereas here we have to face the disadvantage that compiling can take some time and therefore prolongs the development cycle, the advantage lies in the efficient processing that can be achieved with compiled grammars. Strzalkowski (1990) and Strzalkowski/Peng (1990) describe a compiler that transforms a grammar originally written for pars- ing into an efficient generator. In the follwing section I will present a system of the third type and show by means of which com- piling methods a grammar written in a perspicious formalism, TRACE AND UNIFICATION GRAMMAR (TUG) can be transformed to fast parsers and gen- erators. The proposed compilers and their mod- ular architecture have the further advantage that most of their parts can be used also for other for- malisms than the one described, e.g. DCGS. The whole system is part of a polyfunctional linguistic processor for German called …

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