Span-Based Constituency Parsing with a Structure-Label System and Provably Optimal Dynamic Oracles
Summary
Cross and Huang address the gap between neural greedy parsers, which lead in dependency parsing, and the best constituency parsers. They propose a shift-reduce variant in which stack elements are plain sentence spans, not partial trees. The parser alternates between structural moves (shift, or a non-directional combine of the top two spans) and label moves (assign a nonterminal or a unary chain to the top span, or skip it). Because of this design, n-ary productions need no binarization, and every sentence takes exactly 4n−2 steps. Each span is encoded as the difference of forward and backward outputs from a two-layer bi-directional LSTM, extending LSTM-Minus. The authors also derive a dynamic oracle that is provably optimal for F1, precision and recall and runs in amortized O(1) time. Training uses this oracle together with exploration. With strictly greedy, linear-time decoding, the parser obtains the best results among single-model, closed-data parsers without reranking on both the Penn Treebank and the French Treebank. The work shows that a simple span-centric design can match far more complex parsers.
Key points
- Very few features are needed: structural decisions rely on 4 span features, while label decisions rely on 3. No pre-trained embeddings are used.
- Results on the Penn Treebank (WSJ section 23):
- The dynamic oracle with exploration reaches F1 91.3, compared with 91.0 for the static oracle.
- POS tags come from the Stanford tagger with 10-way jackknifing.
- Results on the PTB development set:
- The static and dynamic oracles alone both give F1 91.38.
- Adding exploration with α=1.0 raises F1 to 91.64, gaining precision at some cost to recall.
- Results on the French Treebank (SPMRL 2014 data, with predicted morphology and lemmas): F1 83.31 with dynamic training versus 83.18 with static training. This exceeds Björkelund et al. (2014) at 82.53, a system that used reranking and external data.
- Code is available at github.com/jhcross/span-parser. The authors list beam search and dynamic programming as future extensions.
Institutions
Cite
@misc{cross2016span,
title = {{Span-Based Constituency Parsing with a Structure-Label System and Provably Optimal Dynamic Oracles}},
author = {Cross, James and Huang, Liang},
year = {2016},
doi = {10.48550/arxiv.1612.06475},
eprint = {1612.06475},
archivePrefix = {arXiv},
url = {https://doi.org/10.48550/arxiv.1612.06475}
}References
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