Generalizing Dimensionality in Combinatory Categorial Grammar

Generalizing Dimensionality in Combinatory Categorial Grammar Geert-Jan M. Kruijff Computational Linguistics Saarland University Saarbr¨ucken, Germany gj@coli.uni-sb.de Jason Baldridge ICCS, Division of Informatics University of Edinburgh Edinburgh, Scotland jbaldrid@inf.ed.ac.uk Abstract We extend Combinatory Categorial Grammar (CCG) with a generalized notion of multi- dimensional sign, inspired by the types of rep- resentations found in constraint-based frame- works like HPSG or LFG. The generalized sign allows multiple levels to share information, but only in a resource-bounded way through a very restricted indexation mechanism. This improves representational perspicuity without increasing parsing complexity, in contrast to full-blown unification used in HPSG and LFG. Well-formedness of a linguistic expressions re- mains entirely determined by the CCG deriva- tion. We show how the multidimensionality and perspicuity of the generalized signs lead to a simplification of previous CCG accounts of how word order and prosody can realize infor- mation structure. 1 Introduction The information conveyed by linguistic utterances is diverse, detailed, and complex. To properly ana- lyze what is communicated by an utterance, this in- formation must be encoded and interpreted at many levels. The literature contains various proposals for dealing with many of these levels in the description of natural language grammar. Since information flows between different levels of analysis, it is common for linguistic formalisms to bundle them together and provide some means for communication between them. Categorial gram- mars, for example, normally employ a Saussurian sign that relates a surface string with its syntactic category and the meaning it expresses. Syntactic analysis is entirely driven by the categories, and when information from other levels is used to affect the derivational possibilities, it is typically loaded as extra information on the categories. Head-driven Phrase Structure Grammar (HPSG) (Pollard and Sag, 1993) and Lexical Functional Grammar (LFG) (Kaplan and Bresnan, 1982) also use complex signs. However, these signs are mono- lithic structures which permit information to be freely shared across all dimensions: any given di- mension can place restrictions on another. For ex- ample, variables resolved during the construction of the logical form can block a syntactic analysis. This provides a clean, unified formal system for dealing with the different levels, but it also can adversely af- fect the complexity of parsing grammars written in these frameworks (Maxwell and Kaplan, 1993). We thus find two competing perspectives on com- munication between levels in a sign. In this paper, we propose a generalization of linguistic signs for Combinatory Categorial Grammar (CCG) (Steed- man, 2000b). This generalization enables different levels of linguistic information to be represented but limits their interaction in a resource-bounded man- ner, following White (2004). This provides a clean separation of the levels and allows them to be de- signed and utilized in a more modular fashion. Most importantly, it allows us to retain the parsing com- plexity of CCG while gaining the representational advantages of the HPSG and LFG paradigms. To illustrate the approach, we use it to model var- ious aspects of the realization of information struc- ture, an inherent aspect of the (linguistic) …

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