tailieunhanh - Báo cáo khoa học: "How to covera grammar"

A novel formalism is presented for Earley-like parsers. It accommodates the simulation of non-deterministic pushdown automata. In particular, the theory is applied to non-deterministlc LRoparsers for RTN grammars. A major problem of computational linguistics is the inefficiency of parsing natural language. The most popular parsing method for context-free natural language grammars, is the genera/ context-free parsing method of Earley [1]. It was noted by Lang [2], that Earley-like methods can be used for simulating a class of non-determlnistic pushdown antomata(NPDA). Recently, Tondta [3] presented an algorithm that simulates non-determlnistic LRoparsers, and claimed it to be a fast Mgorithm for. | How to cover a grammar René Leermakers Philips Research Laboratories . Box 5600 JA Eindhoven The Netherlands ABSTRACT A novel formalism is presented for Earley-like parsers. It accommodates the simulation of non-deterministic pushdown automata. In particular the theory is applied to non-deterministic LR-parsers for RTN grammars. 1 Introduction A major problem of computational linguistics is the inefficiency of parsing natural language. The most popular parsing method for context-free natural language grammars is the general context-free parsing method of Earley 1 . It was noted by Lang 2 that Earley-like methods can be used for simulating a class of non-deterministic pushdown automata NPDA . Recently Tomita 3 presented an algorithm that simulates non-deterministic LR-parsers and claimed it to be a fast algorithm for practical natural language processing systems. The purpose of the present paper is threefold 1 A novel formalism is presented for Earley-like parsers. A key role herein is played by the concept of bilinear grammars. These are defined as context-free grammars that satisfy the constraint that the right hand side of each grammar rule have at most two non-terminals. The construction of parse matrices for bilinear grammars can be accomplished in cubic time by an algorithm called C-parser. It includes an elegant way to represent the possibly infinite set of parse trees. A case in point is the use of predict functions which impose restrictions on the parse matrix if part of it is known. The exact form and effectiveness of predict functions depend on the bilinear grammar at hand. In order to parse a general context-free grammar G a possible strategy is to define a cover for G that satisfies the bilinear grammar constraint and subsequently parse it with C-parser using appropriate predict functions. The resulting parsers are named Earley-like and differ only in the precise description for deriving covers and predict functions. 2 We present the Lang .