Fast and Accurate Non-Projective Dependency Tree Linearization
What it is about
The paper treats linearization as a travelling-salesman problem. A biaffine model gives the cost of putting one token before another, and the costs are solved per subtree with a greedy TSP solver. The subtree orders are then combined into a projective tree. A transition system, borrowed from non-projective parsing, runs afterwards to restore the crossing edges. The method beats the previous best linearizer and is about ten times faster in training and decoding. The ablations are useful. Lemma and dependency relation carry most of the word-order signal, while UPOS and morphology add little. The greedy solver is almost as good as a more expensive metaheuristic, so speed costs little accuracy.
Ideas
- I1. Ablate the feature set of
en/src/lin.rs. The paper finds that relation and lemma matter and the tags add little. Remove each POS and FEATS table in turn and report word-order accuracy on dev. This tells which tables are worth their size. - I2. Count-based cost table. Set the cost of "a before b" to a smoothed count of that order for the same relation and head tag, drawn from the data. Solve each subtree greedily, then combine. This keeps the structure and drops the network, and it is fully deterministic.
- I3. Non-projective repair as a separate step. The paper's transition pass fixes crossing edges after projective decoding. The translator can use the same step when a meta token's head differs from the target tree (open problem 1), instead of re-attaching the token by hand.
For Mova
The paper bears most on en/src/lin.rs and on the target-side linearization of tools/translate. The biaffine scorer is not for Mova, but the cost-table form and the post-hoc repair are. Measure on dev, run twice for identical output, and keep the test set for the release run.
Cite
@inproceedings{yu2020fast,
title = {{Fast and Accurate Non-Projective Dependency Tree Linearization}},
author = {Yu, Xiang and Tannert, Simon and Vu, Ngoc Thang and Kuhn, Jonas},
booktitle = {Proceedings of the 58th Annual Meeting of the Association for Computational Linguistics},
year = {2020},
doi = {10.18653/v1/2020.acl-main.134},
url = {https://doi.org/10.18653/v1/2020.acl-main.134}
}References
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