Left-corner Unification-based Natural Language Processing

In this paper, we present an efficient algorithm for parsing natural language using unification grammars. The algorithm is an extension of left-corner parsing, a bottom-up algorithm which utilizes top-down expectations. The extension exploits unification grammar's uniform representation of syntactic, semantic, and domain knowledge, by incorporating all types of grammatical knowledge into parser expectations. In particular, we extend the notion of the reachability table, which provides information as to whether or not a top-down expectation can be realized by a potential subconstituent, by including all types of grammatical information in table entries, rather than just phrase structure information. While our algorithm's worstcase computational complexity is no better than that of many other algorithms, we present empirical testing in which average-case linear time performance is achieved. Our testing indicates this to be much improved average-case performance over pre..