A General, Sound and Efficient Natural Language Parsing Algorithm based on Syntactic Constraints Propagation

This paper presents a new context-free parsing algorithm based on a bidirectional strictly horizontal strategy which incorporates strong top–down predictions (derivations and adjacencies). From a functional point of view, the parser is able to propagate syntactic constraints reducing parsing ambiguity. From a computational perspective, the algorithm includes different techniques aimed at the improvement of the manipulation and representation of the structures used. 1 Parsing Ambiguity and Parsing Efficiency In Formal Language Theory [Aho & Ullman 1972, Drobot 1989] a language is a set, and in Set Theory an element belongs or not to a set. That is to say, a set (and therefore a language) is an unambiguous structure. A grammar may be considered as an intensive definition of a language. Thus, the notion of grammaticality corresponds to the relation of membership over a language (set). But a grammar incorporates more information than the simple report of the elements of the language (the extensive specification). A grammar defines a structure: the parse tree or forest. The distance between grammaticality and grammatical structure is a first level of ambiguity: grammatical ambiguity. The next notion to take into account is the process of analysis of a string of words with a grammar, that is, the parser [Kay 1980, Bolc 1987, Sikkel & Nijholt 1997]. A parser must be able to determine the relation of grammaticality and to obtain the grammatical structure, by mean of a set of operations, that we will call the parsing structure. The distance between the grammatical structure and the parsing structure defines a second level of ambiguity: parsing ambiguity, usually referred as temporal ambiguity. Parsing ambiguity depends on two factors: the grammar and the parsing strategy. A very important design requirement of natural language parsers is to eliminate parsing ∗Jose F. Quesada: A General, Sound and Efficient Natural Language Parsing Algorithm based on Syntactic Constraints Propagation. Proceedings of CAEPIA’97, M/’alaga, Spain. 775–786