Error recovery in parsing expression grammars through labeled failures and its implementation based on a parsing machine

Abstract

Parsing Expression Grammars (PEGs) are a formalism used to describe top-down parsers with backtracking. As PEGs do not provide a good error recovery mechanism, PEG-based parsers usually do not recover from syntax errors in the input, or recover from syntax errors using ad-hoc, implementation-specific features. The lack of proper error recovery makes PEG parsers unsuitable for use with Integrated Development Environments (IDEs), which need to build syntactic trees even for incomplete, syntactically invalid programs.

We discuss a conservative extension, based on PEGs with labeled failures, that adds a syntax error recovery mechanism for PEGs. This extension associates recovery expressionsto labels, where a label now not only reports a syntax error but also uses this recovery expression to reach a synchronization point in the input and resume parsing. We give an operational semantics of PEGs with this recovery mechanism, as well as an operational semantics for a parsing machinethat we can translate labeled PEGs with error recovery to, and prove the correctness of this translation. We use an implementation of labeled PEGs with error recovery via a parsing machine to build robust parsers, which use different recovery strategies, for the Lua language. We evaluate the effectiveness of these parsers, alone and in comparison with a Lua parser with automatic error recovery generated by ANTLR, a popular parser generator .