Question: Does modeling the dependency structure directly via tree encoders outperform a sequential BiLSTM architecture?
Evidence: Semantic parsing is a task of transducing natural language to meaning representations, which in turn can be expressed through many different semantic formalisms including lambda calculus BIBREF1, DCS BIBREF2, Discourse Representation Theory (DRT) BIBREF3, AMR BIBREF4 and so on. This availability of annotated data in English has translated into the development of a plethora of models, including encoder-decoders BIBREF5, BIBREF6 as well as tree or graph-structured decoders BIBREF5, BIBREF7, BIBREF0, BIBREF8.
Whereas the majority of semantic banks focus on English, recent effort has focussed on building multilingual representations, e.g. PMB BIBREF9, MRS BIBREF10 and FrameNetBIBREF11. However, manually annotating meaning representations in a new language is a painstaking process which explains why there are only a few datasets available for different formalisms in languages other than English. As a consequence, whereas the field has made great advances for English, little work has been done in other languages.
We ask: can we learn a semantic parser for English and test it where in another where annotations are not available? What would that require?
To answer this question, previous work have leveraged machine translation techniques to map the semantics from a language to another BIBREF12. However, these methods require parallel corpora to extract automatic alignments which are often noisy or not available at all.
In this paper we explore parameter-shared models instead, where a model is trained on English using language independent features and tested in a target language.
To show how this approach performs, we focus on the Parallel Meaning Bank BIBREF13 – a multilingual semantic bank, where sentences in English, German, Italian and Dutch have been annotated with their meaning representations. The annotations in the PMB are based on Discourse Representation Theory BIBREF3, a popular theory of meaning representation designed to account for intra and inter-sentential phenomena, like temporal expressions and anaphora. Figure 1 shows an example DRT for the sentence `I sat down and opened my laptop' in its canonical `box' representation. A DRS is a nested structure with the top part containing the discourse references and the bottom with unary and binary predicates, as well as semantic constants (e.g. `speaker'). DRS can be linked to each other via logic operator (e.g. $\lnot $, $\rightarrow $, $\diamond $) or, as in this case, discourse relations (e.g. CONTINUATION, RESULT, ELABORATION, etc.).
To test our approach we leverage the DRT parser of liu2018discourse, an encoder-decoder architecture where the meaning representation is reconstructed in three stages, coarse-to-fine, by first building the DRS skeleton (i.e. the `box' structures) and then fill each DRS with predicates and variables. Whereas the original parser utilizes a sequential Bi-LSTM encoder with monolingual lexical features, we experiment with language-independent features in the form of cross-lingual word-embeddings, universal PoS tags and universal dependencies. In particular, we also make use of tree encoders to assess whether modelling syntax can be beneficial in cross-lingual settings, as shown for other semantic tasks (e.g. negation scope detection BIBREF14).
Results show that language-independent features are a valid alternative to projection methods for cross-lingual semantic parsing. We show that adding dependency relation as features is beneficial, even when they are the only feature used during encoding. However, we also show that modeling the dependency structure directly via tree encoders does not outperform a sequential BiLSTM architecture for the three languages we have experimented with.
However, we also show that modeling the dependency structure directly via tree encoders does not outperform a sequential BiLSTM architecture for the three languages we have experimented with.
Answer: No, it doesn't.