将词法标签信息融入神经非连续成分解析,提升语法分析准确率。
Integrating Supertag Features into Neural Discontinuous Constituent Parsing
- 用词法标签作为额外输入或联合训练,增强神经解析器对非连续结构的捕捉能力
- 在德语NeGra和英语DPTB数据集上,引入词法标签后解析准确率显著提升
- 适合关注句法分析、语言结构建模的研究者与开发者
句法分析是自然语言处理的核心任务,成分结构是常用语法描述方式。传统成分结构要求成分由相邻词语构成,难以处理如德语中常见的长距离依赖问题,因此在NeGra、TIGER(德语)和DPTB(英语)等语料库中以交叉边表示非局部依赖。多种语法形式化框架被用于描述非连续树结构,但往往存在高时间复杂度。基于转移的解析通过避免显式语法,利用大规模标注语料上的监督学习训练神经网络生成句法树,有效降低复杂度。Coavoux和Cohen(2019)提出的无栈转移解析器可在最坏情况下以二次时间推导任意非连续成分树。本文探索将词法标签(supertag)信息引入该框架。在如CCG等词法化语法中,词语被赋予指示其结构角色与周围成分关系的标签。研究通过专用词法标签器作为外部输入(流水线)或与解析联合训练(多任务)两种方式引入标签信息,并比较了CCG、LTAG-spinal、LCFRS等框架以及分块、依存解析等序列标注任务作为辅助任务对解析性能的影响。
原文摘要 · Abstract (English)
Syntactic parsing is essential in natural-language processing, with constituent structure being one widely used description of syntax. Traditional views of constituency demand that constituents consist of adjacent words, but this poses challenges in analysing syntax with non-local dependencies, common in languages like German. Therefore, in a number of treebanks like NeGra and TIGER for German and DPTB for English, long-range dependencies are represented by crossing edges. Various grammar formalisms have been used to describe discontinuous trees - often with high time complexities for parsing. Transition-based parsing aims at reducing this factor by eliminating the need for an explicit grammar. Instead, neural networks are trained to produce trees given raw text input using supervised learning on large annotated corpora. An elegant proposal for a stack-free transition-based parser developed by Coavoux and Cohen (2019) successfully allows for the derivation of any discontinuous constituent tree over a sentence in worst-case quadratic time. The purpose of this work is to explore the introduction of supertag information into transition-based discontinuous constituent parsing. In lexicalised grammar formalisms like CCG (Steedman, 1989) informative categories are assigned to the words in a sentence and act as the building blocks for composing the sentence's syntax. These supertags indicate a word's structural role and syntactic relationship with surrounding items. The study examines incorporating supertag information by using a dedicated supertagger as additional input for a neural parser (pipeline) and by jointly training a neural model for both parsing and supertagging (multi-task). In addition to CCG, several other frameworks (LTAG-spinal, LCFRS) and sequence labelling tasks (chunking, dependency parsing) will be compared in terms of their suitability as auxiliary tasks for parsing.
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