The Specification and Implementation of Constraint-Based Unification Grammars
Robert L. Carpenter, Carl Jesse Pollard, Alex Franz · 1991
this paper. The research of Pollard and Franz was supported by a grant from the National Science Foundation (IRI-8806913). (Empty Category Principle and Subjacency) and so forth. Patterns of cross-linguistic variation are accounted for by means of the parametrization of these principles. The methodological distinction between these two approaches is widely supposed to be that rules enumerate possibilities, while principles eliminate possibilities. But it is quite difficult to distinguish formally between a parametrized disjunctive principle and a collection of schematic rules only one of which can apply to a given structure. Consider, for example, the distinction between categorial grammar application schemata, basic ID rules of GPSG, and the C-structure constraints of LFG, on the one hand, and the disjunctive clauses of ¯ X Theory or the Empty Category Principle on the other. It should also be borne in mind that so-called rule-based approaches often employ not only rules but also global constraints on representations which behave similarly to principles, such as the Head Feature Convention and the Control Agreement Principle of GPSG or the Completeness and Function-Argument Biuniqueness Conditions of LFG. HPSG belongs to the "unification-based" family of linguistic theories, but differs from LFG and GPSG in that grammars are formulated entirely in terms of universal and language-specific principles expressed as constraints on feature structures, which in turn are taken to represent possible linguistic objects. As shown by Pollard and Sag (1987), constraints on feature structures can be used to do the same duty as many of the principles and rules of GPSG, LFG and GB. Unlike rule-based theories, in HPSG, immediate dominance and linear precedence conditions (traditional...