A Psycholinguistically Motivated Parser for CCG

A Psycholinguistically Motivated Parser for CCG Michael Niv* Technion - Israel Institute of Technology Haifa, Israel Internet: niv@linc.cis.upenn.edu Abstract Considering the speed in which humans resolve syn- tactic ambiguity, and the overwhelming evidence that syntactic ambiguity is resolved through selec- tion of the analysis whose interpretation is the most 'sensible', one comes to the conclusion that inter- pretation, hence parsing take place incrementally, just about every word. Considerations of parsimony in the theory of the syntactic processor lead one to explore the simplest of parsers: one which repre- sents only analyses as defined by the grammar and no other information. Toward this aim of a simple, incremental parser I explore the proposal that the competence gram- mar is a Combinatory Categorial Grammar (CCG). I address the problem of the proliferating analyses that stem from CCG's associativity of derivation. My solution involves maintaining only the max- imally incremental analysis and, when necessary, computing the maximally right-branching analysis. I use results from the study of rewrite systems to show that this computation is efficient. 1 Introduction The aim of this paper is to work towards a compu- tational model of how humans syntactically process the language that they hear and read. The endpoint of this enterprise is a precise characterization of the process that humans follow, getting details such as timing and garden pathing exactly right. *The research reported here was conducted as part of my Ph.D. thesis work at the University of Pennsyl- vania and supported by the following grants: DARPA N00014-90-J-1863, ARO DAAL03-89-C-0031, NSF IRI 90-16592, Ben Franklin 91S.3078C-1. Preparation of this paper was supported by a postdoctoral fellowship at the Technion in Israel. I am grateful to Mark Hepple, Mitch Marcus, Mark Steedman, VM Tannen, and Henry Thompson for helpful suggestions, and to Jeff Siskind for help with typesetting CCG derivations. Any errors are my own. 1.1 Ambiguity Resolution Recently, a great deal of evidence has accumu- lated that humans resolve syntactic ambiguity by considering the meaning of the available analyses and selecting the 'best' one. Various criteria for goodness of meaning have been advanced in the psycholinguistic literature: e.g. thematic compat- ibility and lexical selection (Trueswell and Tanen- haus 1994), discourse felicity of definite expressions (Altmann et al. 1994), temporal coherence in dis- course (Trueswell and Tanenhaus 1991), grammati- cal function vis avis given/new status (Niv 1993b), and general world-knowledge (Kawamoto and Far- rar 1993). Many of the works cited above consider the tim- ing of the ambiguity resolution decision. The evi- dence is overwhelming that ambiguity is resolved within a word or two of the arrival of disambiguat- ing information-- that is, when there is a meaning- based criterion which militates toward one or an- other syntactically available analysis, that analysis is selected. Should the other analysis turn out to be the ultimately correct analysis, a garden path will result. Given that the various analyses available are compared on various criteria of sensibleness, it fol- lows that these analyses are constructed and main- tained in parallel until …

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