A Transition-Based System for Joint Lexical and Syntactic Analysis

Proceedings of the 54th Annual Meeting of the Association for Computational Linguistics, pages 161–171, Berlin, Germany, August 7-12, 2016. c⃝2016 Association for Computational Linguistics A Transition-Based System for Joint Lexical and Syntactic Analysis Matthieu Constant Universit´e Paris-Est, LIGM (UMR 8049) Alpage, INRIA, Universit´e Paris Diderot Paris, France Matthieu.Constant@u-pem.fr Joakim Nivre Uppsala University Dept. of Linguistics and Philology Uppsala, Sweden joakim.nivre@lingfil.uu.se Abstract We present a transition-based system that jointly predicts the syntactic structure and lexical units of a sentence by building two structures over the input words: syntactic dependency tree and a forest ofa lexical units including multiword expres- sions (MWEs). This combined represen- tation allows us to capture both the syn- tactic and semantic structure of MWEs, which in turn enables deeper downstream semantic analysis, especially for semi- compositional MWEs. The proposed sys- tem extends the arc-standard transition system for dependency parsing with tran- sitions for building complex lexical units. Experiments on two different data sets show that the approach significantly im- proves MWE identification accuracy (and sometimes syntactic accuracy) compared to existing joint approaches. 1 Introduction Multiword expressions (MWEs) are sequences of words that form non-compositional semantic units. Their identification is crucial for semantic analysis, which is traditionally based on the prin- ciple of compositionality. For instance, the mean- ing of cut the mustard cannot be compositionally derived from the meaning of its elements and the expression therefore has to be treated as a single unit. Since Sag et al. (2002), MWEs have attracted growing attention in the NLP community. Identifying MWEs in running text is challeng- ing for several reasons (Baldwin and Kim, 2010; Seretan, 2011; Ramisch, 2015). First, MWEs en- compass very diverse linguistic phenomena, such as complex grammatical words (in spite of, be- cause of), nominal compounds (light house), non- canonical prepositional phrases (above board), verbal idiomatic expressions (burn the midnight oil), light verb constructions (have a bath), multi- word names (New York), and so on. They can also be discontiguous in the sense that the sequence can include intervening elements (John pulled Mary’s leg). They may also vary in their morphologi- cal forms (hot dog, hot dogs), in their lexical el- ements (lose one’s mind/head), and in their syn- tactic structure (he took a step, the step he took). The semantic processing of MWEs is further complicated by the fact that there exists a contin- uum between entirely non-compositional expres- sions (piece of cake) and almost free expressions (traffic light). Many MWEs are indeed semi- compositional. For example, the compound white wine denotes a type of wine, but the color of the wine is not white, so the expression is only par- tially transparent. In the light verb construction take a nap, nap keeps its usual meaning but the meaning of the verb take is bleached. In addition, the noun can be compositionally modified as in take a long nap. Such cases show that MWEs may be decomposable and partially analyzable, which implies the need for predicting their internal struc- ture in order to compute …

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