Abstract
Abstract syntax is a semantic tree representation that lies between parse trees and logical forms. It abstracts away from word order and lexical items, but contains enough information to generate both surface strings and logical forms. Abstract syntax is commonly used in compilers as an intermediate between source and target languages. Grammatical Framework (GF) is a grammar formalism that generalizes the idea to natural languages, to capture cross-lingual generalizations and perform interlingual translation. As one of the main results, the GF Resource Grammar Library (GF-RGL) has implemented a shared abstract syntax for over 30 languages. Each language has its own set of concrete syntax rules (morphology and syntax), by which it can be generated from the abstract syntax and parsed into it. This paper presents a conversion method from abstract syntax trees to dependency trees. The method is applied for converting GF-RGL trees to Universal Dependencies (UD), which uses a common set of labels for different languages. The correspondence between GF-RGL and UD turns out to be good, and the relatively few discrepancies give rise to interesting questions about universality. The conversion also has potential for practical applications: (1) it makes the GF parser usable as a rule-based dependency parser; (2) it enables bootstrapping UD treebanks from GF treebanks; (3) it defines formal criteria to assess the informal annotation schemes of UD; (4) it gives a method to check the consistency of manually annotated UD trees with respect to the annotation schemes; (5) it makes information from UD treebanks available.
Highlights
Computational syntax can work on different levels of abstraction
We focus on evaluating two things: the mapping proposed in this paper to convert Grammatical Framework (GF) trees into Universal Dependencies (UD) dependency trees and subsequently how much the abstract syntax in GF Resource Grammar Library (GF-Resource Grammar Library (RGL)) and the mapping is useful in bootstrapping treebanks
We investigate the similarities and differences between GF-RGL and Universal Dependencies (UDs)
Summary
Computational syntax can work on different levels of abstraction. The lowest level normally used when processing written text is strings of tokens ("words"). It is often useful to work with more abstract structures: part-of-speech (POS) tagged lemma sequences, phrase structure trees, dependency trees, or some kind of logical forms. Raising the level of abstraction often gives new ways to relate different languages to each other. Tagged lemmas enable the separation of surface strings from word senses, which can be useful, for instance, in factored machine translation. Traditional phrase structure trees preserve surface words and constituent order and are language-dependent. Dependency trees are often a bit more abstract, treating the word order as irrelevant and lemmatizing the words. Sharing the POS tags and dependency labels between languages increases this potential to abstract over languages
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