Parsing to Noncrossing Dependency Graphs

Marco Kuhlmann,Peter Jonsson

doi:10.1162/tacl_a_00158

Abstract

We study the generalization of maximum spanning tree dependency parsing to maximum acyclic subgraphs. Because the underlying optimization problem is intractable even under an arc-factored model, we consider the restriction to noncrossing dependency graphs. Our main contribution is a cubic-time exact inference algorithm for this class. We extend this algorithm into a practical parser and evaluate its performance on four linguistic data sets used in semantic dependency parsing. We also explore a generalization of our parsing framework to dependency graphs with pagenumber at most k and show that the resulting optimization problem is NP-hard for k ≥ 2.

Highlights

Dependency parsers provide lightweight representations for the syntactic and the semantic structure of natural language
In this paper we address maximum acyclic subgraph parsing under the restriction that the subgraph should be noncrossing, which informally means that its arcs can be drawn on the half-plane above the sentence in such a way that no two arcs cross
We evaluate the performance of our parser on four linguistic data sets: those used in the recent SemEval task on semantic dependency parsing (Oepen et al, 2015), and the dependency graphs extracted from CCGbank (Hockenmaier and Steedman, 2007)

Summary

Introduction

Dependency parsers provide lightweight representations for the syntactic and the semantic structure of natural language. Syntactic dependency parsing has been formalized as the search for maximum spanning trees in weighted digraphs (McDonald et al, 2005b). For semantic dependency parsing, where target representations are not necessarily tree-shaped, it is natural to generalize this view to maximum acyclic subgraphs, with or without the additional requirement of weak connectivity (Schluter, 2014). The main contribution of this paper is an algorithm that finds a maximum noncrossing acyclic subgraph of a (vertex-ordered) weighted digraph on n vertices in time O.n3/. Dependency parsing is the task of mapping a natural language sentence into a formal representation of its syntactic or semantic structure in the form of a dependency graph.

Objectives

Results

Conclusion