Expressing generalizations in unification-based grammar formalisms

Expressing generalizations in unification-based grammar formalisms * Marc Moens, Jo Calder Ewan Klein, Mike Reape, Henk Zeevat Centre for Cognitive Science, University of Edinburgh 2, Buccleuch Place, Edinburgh EH8 9LW Scotland, UK Abstract This paper shows how higher levels of general- ization can be introduced into unification gram- mars by exploiting methods for typing grammati- cal objects. We discuss the strategy of using global declarations to limit possible linguistic structures, and sketch a few unusual aspects of our type- checking algorithm. We also describe the sort system we use in our semantic representation lan- guage and illustrate the expressive power gained by being able to state global constraints over these sorts. Finally, we briefly illustrate the sort system by applying it to some agreement phenomena and to problems of adjunct resolution. 1 Introduction Since Kay's seminal work (Kay 1979), the util- ity of unification as a general tool in computa- tional linguistics has gained widespread recogni- tion. One major point on which the methodology of unification grammars differs radically from that assumed by linguistic theories lies in the way they deal with generalizations that hold over the do- main of description. In unification-based theories, such generalizations are typically implicit, or ex- tremely limited in their import. The reasons for this are easy to pinpoint. First, in such theories one has to be explicit about the feature structures that the grammar manipulates, and these struc- tures have to be described more or less directly. In PATR-II for example (Shieber et al 1983) the only means of expressing a generalization is via the no- tion of template, a structure which merely repre- sents recurring information--i.e, information that *The work reported here was carried out ae part of ES- PRIT project P393 ACORD. A longer version of this paper can be found in Calder et a! (1988a). recurs in different lexical items, combination rules, lexical rules or other templates. A second reason why unification-based theories do not lend them- selves easily to the expression of general state- ments is that there is no explicit quantification in unification formalisms. In fact, every statement in these formalisms represents a simple existential constraint, never a universal generalization. The work reported here is an attempt to intro- duce higher levels of organization into unification grammars. The notions we employ to do this come from sorted logics and from strong data typing in programming language theory. We will show that the typing of grammatical objects offers a way of stating structural constraints on, or equivalently universal properties of, the objects that constitute the grammar. The grammatical framework in which these ideas have been implemented is Uaificatioa Cat- egorial Grammar (UCG) and its semantic repre- sentation language InL, both developed as part of the ESPRIT-funded project ACORD. Introduc- tions to UCG and InL can be found in Calder et al (1988b) and Zeevat (1988). For present purposes it is sufficient to note that UCG uses a sorted logic which requires being able to express complex con- straints over clusters …

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