Unification-Based Semantic Interpretation

Abstract Unification-Based Semantic Interpretation Robert C. Moore Artificial Intelligence Center SRI International Menlo Park, CA 94025 We show how unification can be used to spec- ify the semantic interpretation of natural-language expressions, including problematical constructions involving long-distance dependencies. We also sketch a theoretical foundation for unification- based semantic interpretation, and compare the unification-based approach with more conven- tional techniques based on the lambda calculus. 1 Introduction Over the past several years, unification-based for- malisms (Shieber, 1986) have come to be widely used for specifying the syntax of natural lan- guages, particularly among computational lin- guists. It is less widely realized by computa- tional linguists that unification can also be a pow- erful tool for specifying the semantic interpreta- tion of natural languages. While many of the techniques described in this paper are fairly well known among natural-language researchers work- ing with logic grammars, they have not been ex- tensively discussed in the literature, perhaps the only systematic presentation being that of Pereira and Shieber (1987). This paper goes into many is- sues in greater detail than do Pereira and Shieber, however, and sketches what may be the first the- oretical analysis of unification-based semantic in- terpretation. We begin by reviewing the basic ideas behind unification-based grammar formalisms, which will also serve to introduce the style of notation to be used throughout the paper. The notation is that used in the Core Language Engine (CLE) devel- oped by SKI's Cambridge Computer Science Re- search Center in Cambridge, England, a system whose semantic-interpretation component makes use of many of the ideas presented here. Fundamentally, unification grammar is a gener- alization of context-free phrase structure grammar in which grammatical:category expressions are not simply atomic symbols, but have sets of features with constraints on their values. Such constraints are commonly specified using sets of equations. Our notation uses equations of a very simple format--just ~eal;ure=value--and permits only one equation per feature per constituent, but we can indicate constraints that would be expressed in other formalisms using more complex equations by letting the value of a feature contain a variable that appears in more than one equation. The CLE is written in Prolog, to take advantage of the effi- ciency of Prolog unification in implementing cate- gory unification, so our grammar rules are written as Prolog assertions, and we follow Prolog con- ventions in that constants, such as category and feature names, start with lowercase letters, and variables start with uppercase letters. As an ex- ample, a simplified version of the rule for the basic subject-predicate sentence form might be written in our notation as (1) syn(s_np_vp, [s: [type=tensed], np: [person=P, hUm=N] , vp: [~ype=~ens ed, person=P, hum=N] ] ). The predicate syn indicates that this is a syntax rule, and the first argument s_npovp is a rule iden- tifier that lets us key the semantic-interpretation rules to the syntax rules. The second argu- ment of syn is a list of category expressions that make up the content of the rule, the first speci- fying …

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