The parol Parser Generator

parol is a parser generator with some unique characteristics.

It is an installable command line tool that can generate complete parsers from a single grammar description file. parol is also a library that you can use in your own crates.

Using a builder API it is easy to integrate the code generation process into your crate's build process via a cargo build script (build.rs).

parol infers and generates all AST data types that you would otherwise have to design yourself. parol does this simply by analyzing your language's grammar description.

You can control the process of AST type generation in several ways. First, you can mark elements to be omitted from your AST. Second, you can specify your own types for language elements, which are then inserted into the resulting AST type at the correct position. Third, for each symbol on the right-hand side of a production, you can specify how its corresponding member should be named in the generated structure.

Language description and language implementation is strictly separated in parol. Thus, you can design your language's grammar without any need to process anything because generated parsers function by default as acceptors. This allows you to do real rapid prototyping of your grammar.

parol generates a trait as interface between the generated parser and your language processing. The trait contains functions for each non-terminal of your grammar which you can implement for non-terminals you need to process. In the simplest case you only implement the trait function for the start symbol of your grammar which is called after the whole input string is parsed. This function then is called with a parameter that comprises the complete structure of the parsed document.

The parser calls the interface trait's functions via a separately generated adapter automatically during the process of parsing.

parol now provides a whole ecosystem of tools including an Extension for Visual Studio Code and a Language Server.

As of version 0.24.0 generated parsers can recover from syntax errors automatically. This means that the parser usually doesn't stop parsing the input after the first syntax error occurs, and instead tries to synchronize with the input in order to continue the analysis accordingly.