Syntactic and Semantic Filtering in a Chart Parser

This paper describes a method to enhance the performance of a unification-style bottom-up chart parser by means of top-down filtering techniques. The filter developed consists of a syntactic module which prevents the construction of redundant edges in the chart by ensuring that a proposed edge in the chart can really be syntactically combined with neighboring edges later, and a semantic module which ensures that the semantic information in a proposed edge in the chart is compatible with semantic information in other edges. 1 I n t r o d u c t i o n Chart parsers used in natural language processing parse an input sentence by building up a data structure called the chart. A chart is a network of vertices representing points in the sentence which are linked by edges which represent constituents of the sentence. In a unification-style chart parser [5] the chart is augmented step by step by adding pieces of description according to the grammar. Each partial description added remains in the chart and serves to constrain the possibilities for further augmentation. In LINK, a bottom-up unification-style chart parser described by Lytinen [3], a directed acyclic graph (DAG) is built to represent the analysis of a sentence. Edges built by LINK are labeled by DAGs incorporating syntactic as well as semantic information. New edges are added to the chart by applying unification rules. This guarantees that the new constituent added to the parse has the necessary syntactic and semantic features. We present a way of integrating bottom-up parsing with top-down parsing, both in terms of syntax and semantics. Our approach can be described as bottom-up parsing with top-down filtering. We modified LINK using this approach. Similar approaches have been variously called in the literature a "filter" [1] and an "oracle" [4]. 2 N e c e s s i t y F o r T o p d o w n F i l t e r i n g M e t h o d s In a top-down parser, parsing is rule-driven while in a bottom-up parser, parsing is data-driven. Thus in a top-down chart parser an active edge in the chart is sought to be expanded with all rules in the grammar which have the current symbol as the left-hand side, causing the parser to be over-productive in edge