Conditional Descriptions in Functional Unification Grammar

Conditional Descriptions in Functional Unification Grammar Robert T. Kasper USC/Information Sciences Institute 4676 Admiralty Way, Suite 1001 Marina del Rey, CA 90292 U.S.A. Abstract A grammatical description often applies to a linguistic object only when that object has certain features. Such conditional descriptions can be indirectly modeled in Kay's Functional Unification Grammar (FUG) using functional descriptions that are embedded within disjunctive alternatives. An ex- tension to FUG is proposed that allows for a direct represen- tation of conditional descriptions. This extension has been used to model the input conditions on the systems of systemic grammar. Conditional descriptions are formally defined in terms of logical implication and negation. This formal defi- nition enables the use of conditional descriptions as a general notational extension to any of the unification-based gram° mar representation systems currently used in computational linguistics. 1 Introduction Functional Unification Grammar [Kay79] (FUG) and other grammatical formalisms that use feature structures and uni- fication provide a general basis for the declarative representa- tion of natural language grammars. In order to utilize some of the computational tools available with unification gram- mars, we have developed a mapping from sVstelnic ¢ram- mars [Hall76] into FUG notation. This mapping has been used as the first step in creating a general parsing method for systemic grammars [Kas87a]. The experience of trans- lating systemic grammars into FUG has shown several ways in which the notational resources of FUG may be improved. In particular, FUG has limited notational resources for ex- pressing conditional information. In this paper we describe how FUG has been enhanced by the addition of conditional descriptions, building on research that has already been re- ported [Kas87a,Kas86,Kas87b]. Conditional information is stated explicitly in systemic grammars by the input conditions of systems that specify when a system must be used. Consider, for example, the two systems (MoodType and Indicatlve'l~ype) x shown in Figure 1. The input condition for the MoodType system is the feature IThis example is extracted from Nigel [Mann83], a large sys- temic grammar of English that has been developed in text gener- ation research at USC/ISI. C/auae, and the input condition for the IndicatlveType sys- tem ls the feature Indicative. Because the features of a sys- temic grammar are normally introduced by a unique system, these input conditions actually express a bidirectional type of logical implication: I. If a constituent has the feature(s) specified by a sys- tem's input condition, then exactly one of the alterna- tives described by that system must also be yard for the constituent; 2. If a constituent has one of the feature alternatives de- scribed by a system, then it must also have the fea- ture(s) specified by that system's input condition. Thus the input condition of the Irtd/cative~pe system ex- presses the following implications: 1. If a clause ha~s the feature Indic,~tive, then it must also have exactly one of the alternatives from the Zndica- tive23,/pe system (either Declarative or Interrogative). 2. If a clause has one of the feature alternatives described by the Indicativs~3ype …

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