Lexical Generalization in CCG Grammar Induction for Semantic Parsing
Summary
CCG-based semantic parsers learn lexicons that map words to syntactic categories paired with lambda-calculus meanings. These lexicons are often redundant: words of the same class need several near-identical entries for different syntactic contexts. Such entries must each be learned separately from sparse data. The authors propose a factored lexicon that splits each entry into a lexeme (a word or phrase plus its logical constants) and a lexical template (a shared pattern that turns any lexeme into a full CCG item).
Their system, FUBL, extends the higher-order-unification induction of Kwiatkowski et al. (2010), the UBL approach. It proposes lexemes and templates rather than complete entries. Lexical induction is combined with online estimation of a log-linear parse-selection model, and derivations are treated as hidden variables.
Evaluated on Atis and GeoQuery (including Geo250 in several languages), FUBL reaches at or near state-of-the-art recall without English- or domain-specific rules. It clearly outperforms UBL on the unedited Atis sentences. This shows that factoring lexical knowledge improves generalization in language-independent grammar induction.
Key points
- The factored lexicon (L, T) has two parts:
- lexemes such as (Boston, [bos]);
- templates such as one that yields NP : v₁ from a lexeme.
- On Atis, FUBL's exact-match recall exceeds that of UBL by margins of 16 and 11 points across the two reported evaluations.
- Comparison with ZC07 on Atis is mixed:
- FUBL is 7.5% higher in recall on the development set;
- it is about 2% lower on the test set.
- On Geo250, FUBL gets the highest recall across all languages. However, its precision is notably lower than that of ZC07 and λ-WASP.
- Novel lexical items (non-NP lexeme–template combinations unseen in training) appear in 6% of Atis development parses. Templates that add semantic content contribute only 1.2% of recall there.
Interesting ideas
- I1. Separate content from usage pattern. Store what a word means (its constants) apart from how it combines syntactically (templates). Rare words can then borrow constructions learned from frequent words of the same type, which could help any sparse lexicalized model, such as low-resource Ukrainian grammars.
- I2. Generate unseen entries at test time. Recombining known lexemes with known templates lets the parser handle a word in a construction it never appeared in during training. For example, "cheapest" in mid-sentence position was handled this way. This is a cheap form of compositional generalization.
- I3. Model ellipsis through the lexicon, not the grammar. Lexemes carrying several constants, such as (to, [from, to]), let the parser cope with sloppy, unedited input without relaxing the combinators or writing language-specific rules.
- I4. Expect a recall/precision trade-off. A very general lexicon parses more sentences but makes more wrong guesses. Reporting both metrics, rather than recall alone, exposes this cost of over-generalization.
- I5. Transfer templates across domains. The authors suggest that templates could be reused in new domains, with only the domain-specific lexemes relearned. This is a plausible route to adapting a parser to new applications with little data.
Cited by
- Weakly Supervised Learning of Semantic Parsers for Mapping Instructions to Actions
- Enhancing PTB Universal Dependencies for Grammar-Based Surface Realization
- Evaluating statistical language models as pragmatic reasoners
Cite
@inproceedings{kwiatkowski2011lexical,
title = {{Lexical Generalization in CCG Grammar Induction for Semantic Parsing}},
author = {Kwiatkowski, Tom and Zettlemoyer, Luke and Goldwater, Sharon and Steedman, Mark},
booktitle = {Proceedings of the 2011 Conference on Empirical Methods in Natural Language Processing},
year = {2011},
url = {https://aclanthology.org/D11-1140/}
}References
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