Defaults in Unification Grammar

DEFAULTS IN UNIFICATION GRAMMAR Gosse Bouma Research Institute for Knowledge Systems Postbus 463, 6200 AL Maa.qtrlcht. The Netherlands e-mall : gosse@riksnl.uucp ABSTRACT Incorporation of defaults in grammar formalisms is important for reasons of linguistic adequacy and grammar organization. In this paper we present an algorithm for handling default information in unification grammar. The algorithm specifies a logical operation on feature structures, merging with the non-default structure only those parts of the default feature structure which are not constrained by the non-default structure. We present various linguistic applications of default unification. L INTRODUCTION MOTIVATION. There a two, not quite unrelated, reasons for incorporating defaults mechanisms into a linguistic formalism. First, linguists have often argued that certain phenomena are described most naturally with the use of rules or other formal devices that make use of a notion of default (see, for instance, Gazdar 1987). The second reason is that the use of defaults simplifies the development of large and complex grammars, in particular, the development of lexicons for such grammars (Evans & Gazdar 1988). The latter suggests that the use of defaults Is of particular relevance for those brands of Unification Grammar (UG) that are lexicalist, that is, in which the lexicon is the main source of grammatical information (such as Categorial Unification Grammar (Uskorelt 1986, Calder et al. 1988) and Head-driven Phrase Structure Grammar (Pollard & Sag 1987)). We propose a method for incorporating defaults Into UG, in such a way that It both extends the linguistic adequacy of UG and supports the formulation of rules, templates and lexical entries for many unification-based theories. In the next section, we define default unification, a logical operation on feature structures. It is defined for a language. FM/, ~, which is in many respects identical to the language FML as defined in Kasper & Rounds (1986). Next, we come to linguistic applications of default unification. A linguistic notation is introduced, which can be used to describe a number of linguistically interesting phenomena, such as feature percolation. coordination, and many aspects of inflectional morphology. Furthermore. it can be used in the sense of Fllcglnger et al. (1985) to define exceptions to rules, non-monotonic specialization of templates or irregular lexlcal entries. BACKGROUND. There are several proposals which hint at the possibility of adding default mechanisms to the linguistic formalisms and theories Just mentioned. The fact that GPSG (Gazdar et al., 1985) makes heavy use of defaults, has led to some research concerning the compatibility of GPSG with a formalism such PATR-II (Shieber 1986a) and concerning the logical nature of the mechanisms used in GPSG (Evans 1987). Shieber (1986a) proposes an operation .rid conservatively, which adds information of a feature structure A to a feature structure B, in as far as this information is not in conflict with information In B. Suggestions for similar operations can be found in Shivber (1986b:59-61) (the overwrite option of PATR-II) and Kaplan (1987) (priority union). Fllckinger et al. (1985) argue for the incorporation of default inheritance mechanisms In UG as an …

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