Category Structures

This paper outlines a simple and general notion of syntactic category on a metatheoretical level, independent of the notations and substantive claims of any particular grammatical framework. We define a class of formal objects called "category structures" where each such object provides a constructive definition for a space of syntactic categories. A unification operation and subsumption and identity relations are defined for arbitrary syntactic categories. In addition, a formal language for the statement of constraints on categories is provided. By combining a category structure with a set of constraints, we show that one can define the category systems of several well-known grammatical frameworks: phrase structure grammar, tagmemics, augmented phrase structure grammar, relational grammar, transformational grammar, generalized phrase structure grammar, systemic grammar, categorial grammar, and indexed grammar. The problem of checking a category for conformity to constraints is shown to be solvable in linear time. This work provides in effect a unitary class of data structures for the representation of syntactic categories in a range of diverse grammatical frameworks. Using such data structures should make it possible for various pseudo-issues in natural language processing research to be avoided. We conclude by examining the questions posed by set-valued features and sharing of values between distinct feature specifications, both of which fall outside the scope of the formal system developed in this paper. The notion syntactic category is a central one in most grammatical frameworks. As Karttunen and Zwicky (1985) observe, traditional "parsing" as taught for languages like Latin involved little more than supplying a detailed description of the grammatical category of each word in the sentence to be parsed. Phrase structure grammars are entirely concerned with assigning terminal strings to categories and determining dominance and precedence between constituents on the basis of their categories. In a classical transformational grammar (TG), the objects transformations manipulate are primarily strings of syntactic categories (and, to a lesser extent, of terminal symbols). This is just as true of recent TG work. Although the use of syntactic categories is not a logical prerequisite of generative grammar (see Levy and Joshi (1978)), no linguistic approach known to us dispenses with them altogether. In view of this, it is perhaps surprising that linguists have not attempted to explicate the concept "syntactic category" in any gen