A Logic of Semantic Representations for Shallow Parsing

Proceedings of the 12th Conference of the European Chapter of the ACL, pages 451–459, Athens, Greece, 30 March – 3 April 2009. c⃝2009 Association for Computational Linguistics A Logic of Semantic Representations for Shallow Parsing Alexander Koller Saarland University Saarbr¨ucken, Germany koller@mmci.uni-saarland.de Alex Lascarides University of Edinburgh Edinburgh, UK alex@inf.ed.ac.uk Abstract One way to construct semantic represen- tations in a robust manner is to enhance shallow language processors with seman- tic components. Here, we provide a model theory for a semantic formalism that is de- signed for this, namely Robust Minimal Recursion Semantics (RMRS). We show that RMRS supports a notion of entailment that allows it to form the basis for compar- ing the semantic output of different parses of varying depth. 1 Introduction Representing semantics as a logical form that sup- ports automated inference and model construc- tion is vital for deeper language engineering tasks, such as dialogue systems. Logical forms can be obtained from hand-crafted deep grammars (Butt et al., 1999; Copestake and Flickinger, 2000) but this lacks robustness: not all words and con- structions are covered and by design ill-formed phrases fail to parse. There has thus been a trend recently towards robust wide-coverage semantic construction (e.g., (Bos et al., 2004; Zettlemoyer and Collins, 2007)). But there are certain seman- tic phenomena that these robust approaches don’t capture reliably, including quantifier scope, op- tional arguments, and long-distance dependencies (for instance, Clark et al. (2004) report that the parser used by Bos et al. (2004) yields 63% ac- curacy on object extraction; e.g., the man that I met. . .). Forcing a robust parser to make a de- cision about these phenomena can therefore be error-prone. Depending on the application, it may be preferable to give the parser the option to leave a semantic decision open when it’s not sufficiently informed—i.e., to compute a partial semantic rep- resentation and to complete it later, using informa- tion extraneous to the parser. In this paper, we focus on an approach to se- mantic representation that supports this strategy: Robust Minimal Recursion Semantics (RMRS, Copestake (2007a)). RMRS is designed to support underspecification of lexical information, scope, and predicate-argument structure. It is an emerg- ing standard for representing partial semantics, and has been applied in several implemented sys- tems. For instance, Copestake (2003) and Frank (2004) use it to specify semantic components to shallow parsers ranging in depth from POS tag- gers to chunk parsers and intermediate parsers such as RASP (Briscoe et al., 2006). MRS anal- yses (Copestake et al., 2005) derived from deep grammars, such as the English Resource Grammar (ERG, (Copestake and Flickinger, 2000)) are spe- cial cases of RMRS. But RMRS, unlike MRS and re- lated formalisms like dominance constraints (Egg et al., 2001), is able to express semantic infor- mation in the absence of full predicate argument structure and lexical subcategorisation. The key contribution we make is to cast RMRS, for the first time, as a logic with a well-defined model theory. Previously, no such model theory …

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