Exception Handling for Error Reporting in Parsing Expression Grammars

Parsing Expression Grammars (PEGs) are a new formalism to describe a top-down parser of a language. However, error handling techniques that are often applied to top-down parsers are not directly applicable to PEGs. This problem is usually solved in PEGs using a heuristic that helps to simulate the error reporting technique from topdown parsers, but the error messages are generic. We propose the introduction of labeled failures to PEGs for error reporting, as labels help to produce more meaningful error messages. The labeled failures approach is close to that of generating and handling exceptions often used in programming languages, being useful to annotate and label grammar pieces that should not fail. Moreover, our approach is an extension to the PEGs formalism that is expressive enough to implement some previous work on parser combinators. Finally, labeled failures are also useful to compose grammars preserving the error messages of each separate grammar.