An efficient implementation of the head-corner parser

van Noord, G., Sep-1997, In : Computational Linguistics. 23, 3, p. 425-456 32 p.

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This paper describes an efficient and robust implementation of a bidirectional, head-driven parser for constraint-based grammars. This parser is developed for the OVIS system: a Dutch spoken dialogue system in which information about public transport can be obtained by telephone. After a review of the motivation for head-driven parsing strategies, and head-corner parsing in particular, a nondeterministic version of the head-corner parser is presented. A memorization technique is applied to obtain a fast parser. A goal-weakening technique is introduced, which greatly improves average case efficiency, both in terms of speed and space requirements. I argue in favor of such a memorization strategy with goal-weakening in comparison with ordinary chart parsers because such a strategy can be applied selectively and therefore enormously reduces the space requir;ements of the parser, while no practical loss in time-efficiency is observed. On the contrary, experiments are described in which head-corner and left-corner parsers implemented with selective memorization and goal weakening outperform ''standard'' chart parsers. The experiments include the grammar of the OVIS system and the Alvey NL Tools grammar. Head-corner pausing is a mix of bottom-up and top-down processing. Certain approaches to robust parsing require purely bottom-up processing. Therefore, it seems that head-corner parsing is unsuitable for such robust parsing techniques. However, it is shown how underspecification (which arises very naturally in a logic programming environment) can be used in the head-corner parser to allow such robust parsing techniques. A particular robust parsing model, implemented in OVIS, is described.

Original languageEnglish
Pages (from-to)425-456
Number of pages32
JournalComputational Linguistics
Issue number3
Publication statusPublished - Sep-1997

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