GUARDED ATTRIBUTE GRAMMARS

Contrary to a widely-held belief, it is possible to construct executab le specifications of language processors that use a top-down parsing s trategy and which have structures that directly reflect the structure of grammars containing left-recursive productions. A novel technique h as been discovered by which the non-termination that would otherwise o ccur is avoided by 'guarding' top-down left-recursive language process ors by non-left-recursive recognizers. The use of a top-down parsing s trategy increases modularity and the use of left-recursive productions facilitates specification of semantic equations. A combination of the two is of significant practical value because it results in modular a nd expressively clear executable specifications of language processors . The new approach has been tested in an attribute grammar programming environment that has been used in a number of projects including the development of natural language interfaces, SQL processors and circuit design transformers within a VLSI design package.