Towards Automatic Error Recovery in Parsing Expression

TL;DR

This paper introduces an algorithm that automatically annotates Parsing Expression Grammars with error recovery labels and expressions, improving parser robustness in IDEs with minimal manual effort.

Contribution

It presents a novel algorithm for automatic annotation of PEGs with error recovery mechanisms, reducing manual effort in parser error handling.

Findings

Successfully added error recovery to Titan language parser

Minimal manual intervention needed for effective recovery

Most grammar alternatives are disjoint, aiding recovery process

Abstract

Error recovery is an essential feature for a parser that should be plugged in Integrated Development Environments (IDEs), which must build Abstract Syntax Trees (ASTs) even for syntactically invalid programs in order to offer features such as automated refactoring and code completion. Parsing Expressions Grammars (PEGs) are a formalism that naturally describes recursive top-down parsers using a restricted form of backtracking. Labeled failures are a conservative extension of PEGs that adds an error reporting mechanism for PEG parsers, and these labels can also be associated with recovery expressions to also be an error recovery mechanism. These expressions can use the full expressivity of PEGs to recover from syntactic errors. Manually annotating a large grammar with labels and recovery expressions can be difficult. In this work, we present an algorithm that automatically annotates a PEG with labels, and builds their corresponding recovery expressions. We evaluate this algorithm by adding error recovery to the parser of the Titan programming language. The results shown that with a small amount of manual intervention our algorithm can be used to produce error recovering parsers for PEGs where most of the alternatives are disjoint.