In-Order Transition-based Constituent Parsing
Summary
Transition-based constituent parsers usually build trees in one of two orders. Bottom-up parsers (post-order traversal) can use features from subtrees they have already built, but they have no lookahead about structure still to come. Top-down parsers (pre-order traversal) give global guidance, but they must commit to a phrase label before seeing its contents, so they depend heavily on the input encoder.
Liu and Zhang propose a third option: a transition system based on in-order traversal. The parser first shifts a phrase's leftmost child, then projects the parent label, then attaches the remaining children before closing the constituent. This mirrors how human readers anticipate upcoming phrases, and it avoids the language-specific binarization rules that bottom-up parsing requires. The authors compare all three strategies inside one stack-LSTM framework that follows Dyer et al. (2016).
On the English WSJ benchmark, the in-order parser outperforms both baselines and sets state-of-the-art results when combined with reranking. The work matters as the first controlled neural comparison of traversal orders, and it offers a simple, portable way to combine bottom-up and top-down evidence.
Key points
- The system uses four actions: SHIFT, PJ-X (project nonterminal X), REDUCE and FINISH. It generalizes to "k-in-order" variants, where top-down corresponds to k=0 and bottom-up to k=∞; only k=1 is studied.
- Greedy fully-supervised parsing scores 91.8 F1 on WSJ (English) and 86.1 F1 on Chinese. Reranking raises the English score to 93.6 F1 (supervised, Choe and Charniak 2016) and 94.2 F1 (semi-supervised).
- After conversion to Stanford dependencies, the output achieves 96.2% UAS and 95.2% LAS, state-of-the-art at the time.
- Per-label analysis shows the in-order parser is best on every constituent type. Among the baselines, top-down does better on WHNP because these phrases begin with a predictable question word.
- Code is released at github.com/LeonCrashCode/InOrderParser.
Topics Syntax & grammar formalisms
Institutions
Cite
Cite as Jiangming Liu and Yue Zhang. 2017. In-Order Transition-based Constituent Parsing. Transactions of the Association for Computational Linguistics.
@article{liu2017order,
title = {{In-Order Transition-based Constituent Parsing}},
author = {Jiangming Liu and Yue Zhang},
journal = {Transactions of the Association for Computational Linguistics},
year = {2017},
doi = {10.1162/tacl\_a\_00070},
url = {https://doi.org/10.1162/tacl_a_00070}
}References
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