Unary constraints for efficient context-free parsing

  • Authors:
  • Nathan Bodenstab;Kristy Hollingshead;Brian Roark

  • Affiliations:
  • Oregon Health & Science University, Portland, OR;University of Maryland Institute for Advanced Computer Studies, College Park, MD;Oregon Health & Science University, Portland, OR

  • Venue:
  • HLT '11 Proceedings of the 49th Annual Meeting of the Association for Computational Linguistics: Human Language Technologies: short papers - Volume 2
  • Year:
  • 2011

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Abstract

We present a novel pruning method for context-free parsing that increases efficiency by disallowing phrase-level unary productions in CKY chart cells spanning a single word. Our work is orthogonal to recent work on "closing" chart cells, which has focused on multi-word constituents, leaving span-1 chart cells unpruned. We show that a simple discriminative classifier can learn with high accuracy which span-1 chart cells to close to phrase-level unary productions. Eliminating these unary productions from the search can have a large impact on downstream processing, depending on implementation details of the search. We apply our method to four parsing architectures and demonstrate how it is complementary to the cell-closing paradigm, as well as other pruning methods such as coarse-to-fine, agenda, and beam-search pruning.