DSL in Rust — Part 6

Grammar-Driven Parsing: pest and ANTLR

Parser combinators let you build parsers compositionally in code. You define small parsing functions and combine them into larger ones. The result lives in your Rust files, compiled alongside your business logic, debugged with your standard tools. This approach works well, but it has a limitation: the grammar exists implicitly in code structure rather than explicitly in a readable specification.

Grammar-driven parsing flips this relationship. Instead of encoding syntax rules in function composition, you write a declarative grammar specification in its own file. A tool reads this specification and generates the parser for you. The grammar becomes documentation. A colleague can open your .pest or .g4 file and understand the language’s syntax without tracing through combinator chains.

This separation of concerns brings practical benefits. Grammars are easier to review because they express intent directly. Tools can analyze grammars for ambiguity, unreachable rules, or conflicts. IDEs can provide syntax highlighting and validation for grammar files. Error messages improve because the parser generator understands the grammar’s structure and can report what it expected at any given position.

The tradeoff is control. With combinators, you decide exactly how parsing proceeds…