Automatic detection and replacement of syntactic constructs causing shift/reduce conflicts

A method is described for repairing some shift/reduce conflicts caused by limited lookahead in LALR(1) parsers such as those built by bison. Also, six types of Extended BNF (EBNF) construct are identified that cause a shift/reduce conflict when a Yet Another Compiler Compiler (YACC) file translated directly from EBNF is processed by bison. For each type, a replacement EBNF construct is described representing the same grammar and causing no shift/reduce conflict when its YACC equivalent is processed by bison. Algorithms are given for identifying instances of each type and transforming them into their replacements. The algorithms are implemented in an automatic parser builder that builds parsers for subsets of the DMIS language. The parser builder reads an EBNF file and writes C++ classes, a YACC file, and a Lex file, which are then processed to build a parser. The parsers build a parse tree using the C++ classes. The EBNF for DMIS is written in natural terms so that natural C++ classes are generated. However, if translated directly into YACC, the natural EBNF leads to 22 shift/reduce conflicts that fall into the six types. The parser builder recognizes the six constructs and replaces them automatically before generating YACC. The YACC that is generated parses in terms of unnatural constructs while building a parse tree using natural C++ classes. The six types of construct may occur in any statement‐based language that uses a minor separator, such as a comma; hence knowing how to recognize and replace them may be broadly useful. Published in 2010 by John Wiley & Sons, Ltd.