Error reporting in Parsing Expression Grammars

Abstract

Parsing Expression Grammars (PEGs) describe top-down parsers. Unfortunately, the errorreporting techniques used in conventional top-down parsers do not directly apply to parsers based on Parsing Expression Grammars (PEGs), so they have to be somehow simulated. While the PEG formalism has no account of semantic actions, actual PEG implementations add them, and we show how to simulate an error-reporting heuristic through these semantic actions.We also propose a complementary error reporting strategy that may lead to better error messages: labeled failures. This approach is inspired by exception handling of programming languages, and lets a PEG define different kinds of failure, with each ordered choice operator specifying which kinds it catches. Labeled failures give a way to annotate grammars for better error reporting, to express some of the error reporting strategies used by deterministic parser combinators, and to encode predictive top-down parsing in a PEG.

Automatic syntax error reporting and recovery in parsing expression grammars

“…We have presented a mechanism for partially automating the process of adding error reporting and error recovery to parsers based on Parsing Expression Grammars. To achieve this, we proposed algorithms that automatically annotate the LL(1) parts of a PEG with error labels [2,3] and associates recovery expressions for these labels [4].…”

“…A naive interpretation of PEGs is problematic when dealing with inputs with syntactic errors, as a failure during parsing an input is not necessarily an error, but can be just an indication that the parser should backtrack and try another alternative. Labeled failures [2,3] are a conservative extension of PEGs that address this problem of error reporting in PEGs by using explicit error labels, which are distinct from a regular failure. We throw a label to signal an error during parsing, and each label can then be tied to a specific error message.…”

Syntax error recovery in parsing expression grammars

“…In this section, we revisit the problem of error handling in PEGs, and show how labeled failures [10,11] combined with the farthest failure heuristic [6] can improve the error messages of a PEG-based parser. en we show how labeled PEGs can be the basis of an error recovery mechanism for PEGs, and show an extension of previous semantics for labeled PEGs that adds recovery expressions.…”

“…e great advantage of using the farthest failure is that the grammar writer does not need to do anything to get a parser with be er error reporting, as the error messages can be generated automatically. However, although this approach gives us error messages with a fine approximation of the error location, these messages may not give a good clue about how to fix the error, and may contain a long list of expected tokens [11].…”

“…A naive interpretation of PEGs is problematic when dealing with inputs with syntactic errors, as a failure during parsing an input is not necessarily an error, but can be just an indication that the parser should backtrack and try another alternative. While PEGs cannot use error handling techniques that are o en applied to predictive top-down parsers, because these techniques assume the parser reads the input without backtracking [6,24], some techniques for correctly reporting syntactic errors in PEG parsers have been proposed, such as tracking the position of the farthest failure [6] and labeled failures [10,11].…”