tailieunhanh - Báo cáo khoa học: "Accurate Context-Free Parsing with Combinatory Categorial Grammar"

The definition of combinatory categorial grammar (CCG) in the literature varies quite a bit from author to author. However, the differences between the definitions are important in terms of the language classes of each CCG. We prove that a wide range of CCGs are strongly context-free, including the CCG of CCGbank and of the parser of Clark and Curran (2007). In light of these new results, we train the PCFG parser of Petrov and Klein (2007) on CCGbank and achieve state of the art results in supertagging accuracy, PARSEVAL measures and dependency accuracy. . | Accurate Context-Free Parsing with Combinatory Categorial Grammar Timothy A. D. Fowler and Gerald Penn Department of Computer Science University of Toronto Toronto ON M5S 3G4 Canada tfowler gpenn @ Abstract The definition of combinatory categorial grammar CCG in the literature varies quite a bit from author to author. However the differences between the definitions are important in terms of the language classes of each CCG. We prove that a wide range of CCGs are strongly context-free including the CCG of CCG-bank and of the parser of Clark and Curran 2007 . In light of these new results we train the PCFG parser of Petrov and Klein 2007 on CCGbank and achieve state of the art results in supertagging accuracy PARSEVAL measures and dependency accuracy. 1 Introduction Combinatory categorial grammar CCG is a variant of categorial grammar which has attracted interest for both theoretical and practical reasons. On the theoretical side we know that it is mildly context-sensitive Vijay-Shanker and Weir 1994 and that it can elegantly analyze a wide range of linguistic phenomena Steedman 2000 . On the practical side we have corpora with CCG derivations for each sentence Hockenmaier and Steedman 2007 a wide-coverage parser trained on that corpus Clark and Curran 2007 and a system for converting CCG derivations into semantic representations Bos et al. 2004 . However despite being treated as a single unified grammar formalism each of these authors use variations of CCG which differ primarily on which combinators are included in the grammar and the restrictions that are put on them. These differences are important because they affect whether the mild context-sensitivity proof of Vijay-Shanker and Weir 1994 applies. We will provide a generalized framework for CCG within which the full variation of CCG seen in the literature can be defined. Then we prove that for a wide range of CCGs there is a context-free grammar CFG that has exactly the same derivations. Included .