A Transfer Model Using a Typed Feature Structure Rewriting System With Inheritance

A TRANSFER MODEL USING A TYPED FEATURE STRUCTURE REWRITING SYSTEM WITH INHERITANCE R6mi Zajac ATR Interpreting Telephony Research Laboratories Sanpeidani lnuidani, Seika-cho~ Soraku-gun, Kyoto 619-02, Japan [zajac%atr-ln.atr.junet@uunet.un.net] ABSTRACT We propose a model for transfer in machine translation which uses a rewriting system for typed feature structures. The grammar definitions describe transfer relations which are applied on the input structure (a typed feaane structure) by the interpreter to produce all possible transfer pairs. The formalism is based on the semantics of typed feature structures as described in [AR-Kaci 84]. INTRODUCTION We propose a new model for transfer in machine translation of dialogues. The goal is twofold: to develop a linguistically-based theory for transfer, and to develop a computer formalism with which we can implement such a theory, and which can be integrated with a unification-based parser. The desired properties of the grammar are (1) to accept as input a feature structure, (2) to produce as output a feature structure, (3) to be reversible, (4) to be as close as possible to current theories and formalisms used for linguistic description. From (1) and (2), we need a rewriting formalism where a rule takes a feature structure as input and gives a feature structure as output. From O), this formalism should be in the class of unification- based formalisms such as PROLOG, and there should be no distinction between input and output. From (4), as the theoretical basis of grammar development in ATR is HPSG [Pollard and Sag 1987], we want the formalism to be as close as possible to HPSG. To meet these requirements, a rewriting system for typed feature structures, based on the semantics of typed feature structures described in [AR-Kaci 84], has been implemented at ATR by Martin Emele and the author [Emele and Zajac 89]. The type system has a lattice structure, and inheritance is achieved through the rewriting mechanism. Type definitions are applied by the interpreter on the input structure (a typed feature structure) using typed unification in a non-deterministic and monotonic way, until no constraint can be applied. Thus, the result is a set of all possible transfer pairs. compatible with the input and with the constraints expressed by the grammar. Thanks to the properties of the rewriting formalism, the transfer grammar is reversible, and can even generate all possible pairs for the grammar, given only the start symbol TRANSLATE. We give an outline of the model on a very simple example. The type inheritance mechanism is mainly used to classify common properties of the bilingual lexicon (sect. 1), and rewriting is fully exploited to describe the relation between a surface structure produced by a unification-based parser and the abstract structme used for transfer (sect. 2), and to describe the relation between Japanese and English structures (sect. 3). An example is detailed in sect. 4. 1. LEXICAL TRANSFER AS A HIERARCHY OF BILINGUAL LEXICAL DEFINITIONS The type system is used to describe a hierarchy of concepts, where a sub-concept inherits all of the properties of its super-concepts. …

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