tailieunhanh - Báo cáo khoa học: "THE LIMITS OF UNIFICATION"

Current complex-feature based grammars use a single procedure--unification--for a multitude of purposes, among them, enforcing formal agreement between purely syntactic features. This paper presents evidence from several natural languages that unification--variable-matching combined with variable substitution--is the wrong mechanism for effecting agreement. The view of grammar developed here is one in which unification is used for semantic interpretation, while purely formal agreement involves only a check for , variable-matching without variable substitution. . | THE LIMITS OF UNIFICATION Robert J. p. Ingria BBN Systems and Technologies Corporation 10 Moulton Street Mailstop 6 4C Cambridge MA 02138 Internet ingria@ ABSTRACT Current complex-feature based grammars use a single procedure unification for a multitude of purposes among them enforcing formal agreement between purely syntactic features. This paper presents evidence from several natural languages that unification variable-matching combined with variable substitution is the wrong mechanism for effecting agreement. The view of grammar developed here is one in which unification is used for semantic interpretation while purely formal agreement involves only a check for non-distinctness . variable-matching without variable substitution. 1 Introduction In recent years a great deal of attention has been devoted to complex-feature based grammar formalisms . grammar formalisms in which syntactic elements are not atomic symbols but rather complex elements such as value-attribute or term structures see Shieber 1986 for an overview. Typically such formalisms use a single mechanism variable substitution for all purposes and the most widely used variable substitution mechanism is Such complex-feature based grammars then are viewed as systems in which partial feature structures are built up by the process of unification into successively more specified structures. While it is formally elegant to use a single mechanism for a number of purposes this theoretical elegance is realized in practice only if the mechanism does not require the other modules of the system to be complicated to achieve this elegance . Currently unification is used for at least four purposes 11n the rest of this paper for convenience I win use the tertn unification instead of variable substitution since it is the most commonly used type of variable substitution but it should be bome in mind that the point being made here bolds for variable substitution in general. to enforce formal .

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