Syntactic Preferences for Robust Parsing With Semantic Preferences

SYNTACTIC PREFERENCES FOR ROBUST I)ARSING WIT[t SEMANTIC PREFERENCES JIN WANG Computing Research Laboratory New Mexico State University Las Cruces, NM 88003 E-mail: wjin@nmsu.edu Abstract Using constraints in robust parsing seems to have what we call "robust parsing paradox". Preference Sem,'ultics and Coimecfionisln both offered a promis- ing approach to fins problem. However, Prefelence Semantics has not addressed the problem of how to make flfll use of syntactic constraints, and Connec- tiouism has some inherent difficulties of its OWll which prevent it producing a practical system. In this paper we are proposing a method to add syntactic preferences to the Preferealce Semantics paradigm while nmiutaining its fundamental philosophy. It will be shown ttmt syntactic preferences can be coded as a set of weights associated with the set of symboli- cally manipulatable rules of a new grammar formal- ism. ~121e syntactic preferences such codezt can lye easily used to compute with semantic preferences. With the help of some tectmiques borrowed from Connectioeisln, these weights can be adjusted through tralrting. 1. Introduction Robust parsing faces what seems to be a "para- dox". On one hand, it needs to tolerate input sentences which more or less break syntactic and semantic constraints; that is, given m~ ill- formed sentence, the parser should attempt to interpret it somehow rather thml simply reject it because it violates some constraints. In order to do this it allows syntactic and semantic con- stralnts to be relaxed. On the other hand, robust parsing still need to be able to disambiguate. For the sake of disambiguation, stronger syntactic and ,semantic coilstraints are required, since the stronger these constraints are, the greater the number of unlikely readings that can be rejected, and the more likely the correct reading is selected. A lot of work related to this problem has been done. The most commonly used snategy is to use the strongest constraints first to parse the input. If the parser fails, the violated constraints are relaxed in order to recover the failed process m~d arrive at a successful parse (CarboneU & Hayes, 1983) (Huang, 1988) (Kwasny & Son- dheimer, 1981) (Weischedel & Sondheimer, 1983). The major problem with this approach is that it is difficult to tell which constraint is actu- ally being violated and, therefore, needs to be relaxed when the parser fails. Tiffs l)roblem is more serious with the parser using backtracking. Another strategy is based on Preference Seman- tics (Wilks, 1975, 1978) (Fass & Wilks, 1983). The idea is that all the possible sentence read- ings can (though not necessarily) be produced. All the "readings are scored according to how mm~y preference satisfactions they contmn", and "the best reading (that is, the one with the most preference satisfactions) is taken, even if it con- tains preference violations." Selfridge (1986) and Slator (1988) 'also use this strategy. One important advantage of this approach is that within a uniform mechanism, the semantic con- straints can both be used maximally for the sake of disambiguation m~d be gracefully relaxed when nece,ssmy for …

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