Parsing Turkish Using the Lexical Functional Grammar Formalism

PARSING TURKISH USING THE LEXICAL FUNCTIONAL GRAMMAR FORMALISM 1 Zelal Giing6rdii Kemal Oflazer Centre for Cognitive Science University of Edinburgh Edinburgh, Scotland, U.K. gungordu @cogsci.ed.ac.uk Department of Computer Engineering Bilkent University Bilkent, Ankara, Turkey ko @cs.bilkent.edu.tr Abstract This paper describes our work on parsing Turk- ish using the lexical-functional grammar formalism. This work represents the first effort for parsing Turkish. Our implementation is based on Tomita's parser developed at Carnegie-Mellon University Center for Machine Transla- tion. The grammar covers a substantial subset of Turkish including simple and complex sentences, and deals with a reasonable amount of word order freeness. The complex agglutinative morphology of Turkish lexical structures is handled using a separate two-level morphological analyzer. After a discussion of key relevant issues regarding Turkish grammar, we discuss aspects of our system and present re- sults fi'om our implementatiou. Our initial results suggest that our system can parse about 82% of the sentences directly and almost all the remaining with very minor pre-editing. 1 INTRODUCTION As part of our ongoing work on the development of compu- tational resources for natural language processing in Turk- ish we have undertakeu the development of a parser for Turkish using the lexical-functional grammar formalism, for use in a mtmber of applications. This work represents the first approach to the computational analysis of Turk- ish, though there have been a number of studies of Turkish syntax fi'om a linguistic perspective (e.g., [Meskill 1970]). Our implementation is based on Tomita's parser developed at Carnegie-Mellon University Center for Machine Transla- tion [Musha et.al. 1988, Tomita 1987]. Our grammar cov- ers a substantial subset of Turkish including simple and complex sentences, and deals with a reasonable amount of word order freeness. Turkish has two characteristics that have to be taken into account: agglutinative morphology, and rather ti'ee word orde r with explicit case marking. We handle the rather com- plex agglutinative morphology of the Turkish lexical struc- tures using a separate morphological processor based on the two-level paradigm [Evans 1990, Otlazer 1993] that we have integrated with the lexical-functional grammar parser. Word order freeness is dealt with by relaxing the order of phrases in the phrase structure parts of lexical-functional gramm.ai" rule by means of generalized phrases. IThis work was done as a part of the first author's M.Sc. degree work at the Department of Computer Engineering of Iqilkent University, Ankara, 06533 Turkey. 2 LEXICAL-FUNCTIONAL GRAMMAR Lexical-functional gramnmr (LFG) is a linguistic theory which fits nicely into computational approaches that use unification IShieber 1986]. A lexical-functional grammar assigns two levels of syntactic description to every sen- tence of a language: a constituent structure and a functional structure. Constituent structures (c-structures) characterize the phrase structure configurations as a conventional phrase structure tree, while surface grammatical functions such as subject, object, and adjuncts are represented in functional structure (f-structure), Because of space limitations we will not go into the details of the theory. One can refer to Kaplan and Bresnan [Kaplan and Bresnan 1982] for a thorough dis- cussion of the …

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