ylfink@water.waterloo.edu (ylfink) (05/07/88)
DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF WATERLOO SEMINAR ACTIVITIES ARTIFICIAL INTELLIGENCE SEMINAR - Thursday, May 12, 1988 Dr. Remo Pareschi, University of Pennsylvania, will speak on ``A Definite Clause Version of Categorial Grammar''. TIME: 11:00 AM ROOM: DC 1302 ABSTRACT Categorial Grammar is a framework for natural language analysis where grammatical constituents are viewed either as functions or arguments, and are associated with corresponding syntactic types. We present a first-order version of such a framework, based on the idea of encoding syntactic types as definite clauses. Under this approach, there are no explicit requirements of adjacency between combinable constituents, and word-order constraints are obtained simply by allowing subformulae of complex types to share variables ranging over string positions. It is in this way possible to give a straightforward account of problematic syntactic constructions involving discontinuous constituents, which are particularly common in natural language. The second half of this enterprise consists of showing how, for this formalism, parsing can be efficiently implemented as theorem proving. Our approach to encoding types as definite clauses presupposes a modification of standard Horn logic syntax to allow internal implications in definite clauses. This modification is needed to account for the types of higher-order functions and, as a consequence, standard Prolog-like Horn logic theorem proving is not powerful enough. We tackle this problem by adopting an intuitionistic treatment of implication, which has been proposed elsewhere as a useful extension of Prolog for implementing hypothetical reasoning and modular logic programming. We then provide a parsing algorithm in terms of a decision procedure which tries to assign a type to a given input string, where the size of the logic program needed for a type assignment is always linear with respect to the length of the input.