The previous two chapters discussed techniques for finding errors within yacc grammars. In this chapter, we turn our attention to the other side of error correction and detection—how the parser and lexical analyzer detect errors. This chapter presents some techniques to incorporate error detection and reporting into the grammar. To ground the discussion in a complete example, we will refer to the menu generation language defined in Chapter 4, A Menu Generation Language .
Yacc provides the error token and the yyerror routine, which are typically sufficient for early versions of a tool. However, as any program begins to mature, especially a programming tool, it becomes important to provide better error recovery, which allows for detection of errors in the later portions of the file, and better error reporting.
Error reporting should give as much detail about the error as is possible. The default yacc error only declares that a syntax error exists and to stop parsing. In our examples, we typically added a mechanism for reporting the line number. This provides the location of the error but does not report any other errors within the file or where in the specified line the error occurs.
It is best to categorize the possible errors, perhaps building an array of error types and defining symbolic constants to identify the errors. For example, in the MGL a possible error is to fail to terminate a string. Another error might be using the ...