The Limits of Unification

THE LIMITS OF UNIFICATION Robert J. P. Ingria BBN Systems and Technologies Corporation 10 Moulton Street, Mailstop 6/4C Cambridge, MA 02138 Intemet: ingria@BBN.COM ABSTRACT Current complex-feature based grammars use a sin- gle procedure--unification--for a multitude of pur- poses, 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 interpre- tation, while purely formal agreement involves only a check for non-distinctness---i.e, variable-matching without variable substitution. 1 Introduction In recent years, a great deal of attention has been de- voted to complex-feature based grammar formalisms-- i.e. grammar formalisms in which syntactic elements are not atomic symbols, but rather complex elements, such as value-attribute or term structu~s; see Shieber (1986) for an overview. Typically such formalisms use a single mechanism--variable substitution--for all pur- poses, and the most widely used variable substitution mechanism is unification) Such complex-feature based grammars, then, axe viewed as systems in which partial feature structures are built up, by the process of unifica- tion, into successively more specified structures. While it is formally elegant to use a single mechanism for a number of purposes, this theoretical elegance is real- ized 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 puq3oses: 1 In the rest of this paper, for convenience I will use the term "unification" instead of "variable substitution", since it is the most commonly used type of variable substitution, but it should be borne in mind that the point being made here holds for variable substitution, in general. • to enforce formal agreement between purely syn- tactic features • to "percolate" features between a pre-terminal cat- egory and the phrase which it heads • to pass features between a dislocated element-- such as a WH-phrase--and its trace • to build up semantic representations This paper will focus on the use of unification to enforce agreement and will present evidence from sev- eral natural languages which argues against its use in the case of purely formal syntactic features: when such features are lexically or morphologically underspecified, they remain so, even under agreement, contrary to the predictions of a system using unification for agreement. Moreover, it is worthwhile stressing at the outset that the main argument of this paper is not that there are certain constructions that present a problem for unifica- lion, and, hence, require some technical solution. The point is much stronger:, even if some elaborate analy- sis can be devised that allows unification to be used to effect agreement, this would be the wrong tack to take. Rather, the argument will go, using unification to effect agreement is incorrect both for theoretical reasons--it presents a view of language which is contradicted by the facts--and for practical reasons---using unification to effect agreement can impede …

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