A Lazy way to Chart-Parse with Categorial Grammars


There has recendy been a revival of interest in Categorial Grammars (CG) among computational linguists. The various versions noted below which extend pure CG by including operations such as functional composition have been claimed to offer simple and uniform accounts of a wide range of natural language (NL) constructions involving bounded and unbounded "movement" and coordination "reduction" in a number of languages. Such grammars have obvious advantages for computational applications, provided that they can be parsed efficiently. However, many of the proposed extensions engender proliferating semantically equivalent surface syntactic analyses. These "spurious analyses" have been claimed to compromise their efficient parseability. The present paper descn~oes a simple parsing algorithm for our own "combinatory" extension of CG. This algorithm offers a uniform treatment for "spurious" syntactic ambiguities and the "genuine" structural ambiguities which any processor must cope with, by exploiting the assodativRy of functional composition and the procedural neutrality of the combinatory rules of grammar in a bottom-up, left-to-fight parser which delivers all semantically distinct analyses via a novel unification-based extension of chart-parsing. 1. Combinatory Categorial Grammars "Pure" categorial grammar (CG) is a grammatical notation, equivalent in power to context-free grammars, which puts all syntactic information in the lexicon, via the specification of all grammatical entities as either functions or arguments. For example, such a grammar might capture the obvious intuitions concerning constituency in a sentence like John must leave by identifying the VP leave and the NP John as the arguments of the tensed verb must, and the verb itself as a function combining to its right with a VP, to yield a predicate -that is, a leftward-combining function-from-NPs-into-sentences. One common "slash" notation for the types of such functions expresses them as triples of the for~ <result, direction, argu. merit>, where result and argument are themselves syntactic types, and direction is indicated by "/" (for rightwardcombining functions) or '~," (for leftward). Must then gets the following type-assignment: (I) must : (SkNP)/VP In pure categorial grammar, the only other element is a single "combinatory" rule of Functional Application. which gives rise to the following two instances: 1 1 All combinatory roles are written as productions in the present paper, in contrast with the reduction rule notation used in the earlier papers. The change is intended to aid comparison with other tmification-based grammars, and has no theoretical significance. ~) a. Rightward Application: X --> X/Y Y b. Leftward Application: X --> Y X\Y These rules allow functions to combine with inunediam~ adjacent a~uments in the obv~us way, to ~dd the obv~ surface su'ucmres and interpretations, as in:

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@inproceedings{Pareschi1987ALW, title={A Lazy way to Chart-Parse with Categorial Grammars}, author={Remo Pareschi and Mark Steedman}, booktitle={ACL}, year={1987} }