LR Parsers For Natural Languages

MLR, an extended LR parser, is introduced, and its application to natural language parsing is discussed. An LR parser is a ~;hift-reduce parser which is doterministically guided by a parsing table. A parsing table can be obtained automatically from a context-free phrase structure grammar. LR parsers cannot manage antl)iguous grammars such as natural language grammars, because their I)arsing tables would have multiply-defined entries, which precludes deterministic parsing. MLR, however, can handle mulliply-defined entries, using a dynamic programnting method. When an input sentence is ambiguous, the MI.R parser produces all possible parse trees witftoul parsing any part of the input sentenc:e more than once in the same way, despite the fact that the parser does not maintain a chart as in chart par~ing. Our method also prnvkles an elegant solution to the problem of multi-part-of-speech words such as "that". The MLR parser and its parsing table generator have been implemented at Carnegie-Mellon University.