Crate parce[][src]

Expand description

Parce is a parser and lexer generator, where the grammar and the parse tree are the same data structure. It is similar to ANTLR, but the grammar is written in Rust code, not a special DSL.

Quick Links to Documentation

If you’re new to Parce, read the example below, then go to these links to learn more.

Basic Example

Parce is used in three steps:

  • Create a lexer
  • Create a parser that uses the lexer
  • Parse things with the parser!

Creating a lexer

A lexer is a single enum with the lexer attribute macro applied to it. Rules for each lexeme are written as a string literal discriminant which get turned into matching logic by the macro. The example below is just to give you a feel for the syntax; the real docs for lexer creation are under the lexer attribute macro.

use parce::prelude::*;

enum MyLexemes {
    Bool = " 'true' | 'false' ", // match string literals, use | for multiple possible patterns
    Digit = "[0-9]", // use Regex-like character classes
    And = '&', // can omit double quotes if pattern is a single character
    #[skip] Whitespace = "[ \n\r\t]" // skippable lexemes

// The macro generates an enum "MyLexer" that implements the Lexer trait.

Creating a parser

A parser is a collection of enums with the parser attribute macro applied to it. Each enum is a grammar rule, and the variants of the enum are productions. Again, the rules for each production are written as a string literal discriminant which is processed by the parser macro. The example below is just to give you a feel for the syntax; the real docs for parser creation are under the parser attribute macro.

TODO after left-recursion rewriting


Lexer Features

  • Regex-like repetition operators
    • The usual *, +, and ?
    • And also {n} (exactly n), {n,} (n or more), and {n,m} (between n and m inclusive) <- ANTLR doesn’t have those :)
  • Lexeme nesting
  • Regex-like character classes
  • Skipped lexemes
  • Fragment lexemes
  • Modal lexers
    • unlike ANTLR, lexemes can be active in multiple modes

Parser Features

Comparison to ANTLR

Since Parce and ANTLR serve very similar purposes, here are the pros and cons of using Parce over ANTLR:


  • Parce operates directly on the syntax tree enums that you create. ANTLR generates it’s own tree, and if you don’t want to use that one, you have to convert it to your own.
  • Rust’s enums make Parce’s usage more intuitive for people who are unfamiliar with ANTLR.
  • ANTLR currently doesn’t have a stable Rust target.


  • ANTLR’s runtime performance is faster.
    • I haven’t actually benchmarked it, but Parce is extremely unlikely to be faster, given how much smarter the ANTLR devs are than me ;)
  • ANTLR’s grammars are language-independent, as long as you don’t embed code in your grammars.
  • ANTLR has more features.
    • Mixed lexer/parse grammars.
    • (there are others, but I don’t know what they are off the top of my head.)

Future plans

  • Semantic predicates
    • left-recursive grammar re-writing like ANTLR, using semantic predicates
  • Data post-processors
  • multi-threaded lexing and parsing.


If you find a bug or want a new feature, please create an issue or pull request on GitHub!



Errors generated by the lexer and parser.


Contains reexported crates and types used by the code generated by the lexer and parser macros, as well as a prelude with some internal types.


Contains the Lexer trait and the Lexeme wrapper struct used by the generated lexers.


Contains traits for parsing, and the skeleton of the packrat parser algorithm used by the parsers.


All the usual imports that the user will need to use.