Types in Functional Unification Grammars

TYPES IN FUNCTIONAL UNIFICATION GRAMMARS Michael Elhadad Department of Computer Science Columbia University New York, NY 10027 Internet: Elhadad@cs.columbia.edu ABSTRACT Functional Unification Grammars (FUGs) are popular for natural language applications because the formalism uses very few primitives and is uniform and expressive. In our work on text generation, we have found that it also has annoying limitations: it is not suited for the expression of simple, yet very common, taxonomic relations and it does not allow the specification of completeness conditions. We have implemented an extension of traditional functional unification. This extension addresses these limitations while preserving the desirable properties of FUGs. It is based on the notions of typed features and typed constituents. We show the advantages of this exten- sion in the context of a grammar used for text genera- tion. 1 INTRODUCTION Unification-based formalisms are increasingly used in linguistic theories (Shieber, 1986) and com- putational linguistics. In particular, one type of unification formalism, functional unification grammar (FUG) is widely used for text generation (Kay, 1979, McKeown, 1985, Appelt, 1985, Paris, 1987, McKeown & Elhadad, 1990) and is beginning to be used for parsing (Kay, 1985, Kasper, 1987). FUG enjoys such popularity mainly because it allies expres- siveness with a simple economical formalism. It uses very few primitives, has a clean semantics (Pereira&Shieber, 1984, Kasper & Rounds, 1986, E1- hadad, 1990), is monotonic, and grants equal status to function and structure in the descriptions. We have implemented a functional unifier (EI- hadad, 1988) covering all the features described in (Kay, 1979) and (McKeown & Paris, 1987). Having used this implementation extensively, we have found all these properties very useful, but we also have met with limitations. The functional unification (FU) for- malism is not well suited for the expression of simple, yet very common, taxonomic relations. The tradi- tional way to implement such relations in FUG is ver- bose, inefficient and unreadable. It is also impossible to express completeness constraints on descriptions. In this paper, we present several extensions to the FU formalism that address these limitations. These extensions are based on the formal semantics presented in (Elhadad, 1990). They have been im- plemented and tested on several applications. 157 We first introduce the notion of typed features. R allows the definition of a structure over the primitive symbols used in the grammar. The unifier can take advantage of this structure in a manner similar to (Ait- Kaci, 1984). We then introduce the notion of typed constituents and the FSET construct. It allows the dec- laration of explicit constraints on the set of admissible paths in functional descriptions. Typing the primitive elements of the formalism and the constituents allows a more concise expression of grammars and better checking of the input descriptions. It also provides more readable and better documented grammars. Most work in computational linguistics using a unification-based formalism (e.g., (Sag & Pollard, 1987, Uszkoreit, 1986, Karttunen, 1986, Kay, 1979, Kaplan & Bresnan, 1982)) does not make use of ex- plicit typing. In (Ait-Kaci, 1984), Ait-Kaci introduced …

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