A Structure-Sharing Representation for Unification-Based Grammar Formalisms

This paper describes a structure-sharing method for the representation of complex phrase types in a parser for PATR-[I, a unification-based grammar formalism. In parsers for unification-based grammar formalisms, complex phrase types are derived by incremental refinement of rite phrase types defined in grammar rules and lexical entries. In a naive implementation, a new phrase type is built by copying older ones and then combining the copies according to the constraints s tated in a grammar rule. The structure-sharing method was designed to eliminate most such copying; indeed, practical tests suggest that the use of this technique reduces parsing t ime by as much as 60%. The present work is inspired by the structure-sharing method for theorem proving introduced by Boyer and Moore and on the variant of it that is used in some Prolog implementations.