Computer Systems Laeoratory

This paper describes a new algorithm for top-down parsing of general context-free grammars. The algorithm does not require any changes to be made to the grammar, and can parse with respect to any grammar non-terminal as the start symbol. It is possible to generate all possible parse trees of the input string in the presence of ambiguous grammars. The algorithm reduces to recursive descent parsing on LL grammars. This algorithm is ideal for use in software development environments which include tools such as syntax-directed editors and incremental parsers, where the language syntax is an integral part of the user-interface. General context-free grammars can describe the language syntax more intuitively than, for example, LALR( 1) grammars. This algorithm is also applicable to batch-oriented language processors, especially during the development of new languages, where frequent changes are made to the language syntax and new prototype parsers need to be developed quickly. A prototype implementation of a parser generator that generates parsers based on this algorithm has been built. Parsing speeds of around 1000 lines per second have been achieved on a Sun SparcStation 2. This demonstrated performance is more than adequate for syntax-directed editors and incremental parsers, and in most cases, is perfectly acceptable for batch-oriented language processors.