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Motivation for including relational constraints other than equality within grammatical formalisms has come from discontinuous constituency and partially free word order for natural languages as well as from the need to define combinatory operations at the most basic level for languages with a two-dimensional syntax (e.g., mathematical notation, chemical equations, and various diagramming languages). This paper presents F-PATR, a generalization of the PATR-II unification-based formalism, which incorporates relational constraints expressed as user-defined functions. . | F-PATR FUNCTIONAL CONSTRAINTS FOR UNIFICATION-BASED GRAMMARS Kent Wittenburg Bellcore 445 South St. MRE 2A-347 Morristown NJ 07962-1910 USA Internet kentw@bellcore.com Abstract Motivation for including relational constraints other than equality within grammatical formalisms has come from discontinuous constituency and partially free word order for natural languages as well as from the need to define combinatory operations at the most basic level for languages with a two-dimensional syntax e.g. mathematical notation chemical equations and various diagramming languages . This paper presents F-PATR a generalization of the PATR-n unification-based formalism which incorporates relational constraints expressed as user-defined functions. An operational semantics is given for unification that is an adaptation and extension of the approach taken by Ait-Kaci and Nasr 1989 . It is designed particularly for unificationbased formalisms implemented in functional programming environments such as Lisp. The application of unification in a chart parser for relational set languages is discussed briefly. 1. INTRODUCTION For the most part unification-based grammar formalisms e.g. Kaplan and Bresnan 1982 Pereira and Warren 1980 Shieber 1984 have adopted string rewriting conventions from context-free grammar rules assuming string concatenation as the basic combining operator external to the unification process itself. Kay s Functional Unification Grammar Kay 1979 while not borrowing the conventions of CFG rewriting rules still assumed concatenation of strings as the underlying combining operation. However recent work in HPSG e.g. Pollard and Sag 1987 Reape 1990 Carpenter et al. 1991 and elsewhere has sought to incorporate constraints for combining operations into the unificationbased representation directly. Part of the motivation for doing so is to accommodate partially free word order and discontinuous constituency without the complication of passing along intermediate threading .
