The Lexicon as Bridge between Phrase Structure Components

Abstract

Context-free phrase structure grammar (henceforth PSG) is capable of describing infinite languages consisting of finite strings drawn from a finite vocabulary and associating with each string of the target language a division into immediate constituents that can be represented as a labeled tree. This much, it would seem, is the least that we can expect of any grammar that would pretend to adequacy as a scheme for describing languages of the kind spoken by human beings. PSG has several advantages over competing systems of grammatical description. In particular:

• PSG is a conceptually simple formalism. Because PSG describes only a single level of analysis, movement rules and the constraints on them and triggers for them are eliminated. The ramifications of a postulated PSG rule are ordinarily immediately obvious and immediately testable against fact.

• PSG is completely formalizable. A finite set of rules and a finite vocabulary fully defines a PSG. The formal properties of such systems can be studied, and in fact there is a considerable formal literature on the subject. (See, for example Partee, et al. 1990 and the references it contains.). There are numerous interesting results concerning the classes of string sets and the classes of tree sets that PSGs are capable of specifying.

• PSG is computationally tractable. Quite efficient algorithms exist for parsing expressions in terms of PSGs. Though the parsing time of the best of these is still an exponential function of the length of the string, it is less disastrously so than for other formal grammar types.