Machine Translation Using Isomorphic UCGs

Machine Translation Using Isomorphic UCGs John L. BEAVEN* Pete WHITELOCK Department of Artificial Intelligence University of Edinburgh 80 South Bridge Edinburgh EH1 1HN Scotland Abstract This paper discusses the application of Unification Categorial Grammar (UCG) to the framework of Isomorphic Grammars for Machine Translation pioneered by Landsbergen. The Isomorphic Grammars approach to MT involves developing the grammars of the Source and Target languages in parallel, in order to ensure that SL and TL expressions which stand in the translation rela- tion have isomorphic derivations. The principle advantage of this approach is that knowledge concerning translation equivalence of expressions may be directly exploited, obviating the need for an- swers to semantic questions that we do not yet have. Semantic and other information may still be incorporated, but as constraints on the translation relation, not as levels of textual representation. After introducing this approach to MT system design, and the basics of monolingual UCG, we will show how the two can be integrated, and present an example from an implemented bi- directional Engllsh-Spanish fragment. Finally we will present some outstanding problems with the approach. 1 Background and Introduction The aim of this paper is to explore how the linguistic theory known as Unification Categorial Grammar can be adapted to the general methodology of Machine Translation using Isomorphic Grammars, as pioneered by Landsbergen and others in the ROSETTA team [Landsbergen 87a, b]. UCG is one of several recent grammar formalisms [Calder et al. 86, Karttunen 86, Pollard 85] which are highly lexicalist, i.e. rules of syntactic combination are not a language-specific component of the grammar, but are very general in character, and combinatory information is primarily associated with lexical items. Lexical items are represented by sets of feature-value pairs (where the values may be themselves sets of such pairs}, and are combined by unification into objects of the same type. The lan- guage defined is thus the closure of the lexicon under the combi- natory rules. Landsbergen's work on Isomorphic Grammars follows Monta- gue's approach of having a one-to-one correspondence between syntactic and semantic rules. A syntactic rule Rs/, in the Source Language corresponds to a syntactic rule RTL in the Target Lan- guage if and only if they are both associated with the same seman- tic operation Rsem. The translation relation is then defined in a precise manner and it can be guaranteed that well-formed expres- sions in the Source Language are translatable, as there will be an expression in the Target Language that is derived in a correspond- ing way, and can therefore be considered as a possible translation of it. *Supported by a studentship from the Science and Engineering Research Council. According to Landsbergen, writing isomorphic grammars is a way of being explicit about the "tuning" of SL and TL grarmnars that is essential for reliable MT. The present paper is an attempt to adapt this approach to a type-driven mapping between syntax and semantics. 2 Isomorphic Grammars We can recognise two basic relations of relevance in translation. namely, "possible …

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