Parsing Without (Much) Pharse Structure

Approaches to NL syntax conform in varying degrees to the older relational/dependency model, (essentially that assumed in traditional grammar), which treats a sentence as a group of words united by various relations, and the newer constituent model. Some modern approaches have nonetheless involved shifts away from essentially constituent-based models of the sort associated with Bloomfield and Chomsky to more relation-based ones (e.g. case grammar, relational grammar, daughter-dependency and word grammar, corepresentational grammar) while some others, notably lexical-functional grammar, have nonetheless continued to rely crucially on certain techniques inherited from constituency-based grammar, particularly context-free grammar. In computational linguistics there is a strong (if not universal) reliance on phrase structure as the medium via which to represent syntactic structure; call this the CONSENSUS VIEW. A significant amount of effort has accordingly been invested in techniques by which to build such a representation efficiently, which has in turn led to considerable work on the formal and computational properties of context-free gramamrs (or natural extensions of them) and of the associated languages. In its strongest form, the consensus view says that the recovery of a fully specified parse tree is an essential step in computational language processing, and would, if correct, provide important support for the constituent model. In this paper, we shall critically examine the rationale for this view, and will sketch (informally) an alternative view which we find more defensible. The actual position we shall take for this discussion, however, is conservative in that we will not argue that there is no place whatever for constituent analysis in parsing or in syntactic analysis generally. What we WILL argue is that phrase structure is at least partly redundant in that a direct leap to the composition of some semantic units is possible from a relatively underspecified syntactic representation (as opposed to a complete parse tree). However, see Rindflesch forthcoming for an approach to.parsing which entails a much stronger denial of the consensus view.