Partax (Parser + Syntax) is a library of tools for manipulating Lean syntax and parser definitions. It currently provides two major features:

  • Configurable commands to compile Lean syntax categories and parser definitions into something else.
  • A monadic parser (LParse) for parsing Lean syntax without needing a whole Lean Environment.

Essentially, Partax allows users to compile builtin or custom Lean grammars into simple, runnable parsers that can then be used in libraries or executables without needed Lean symbols and meta code.


Partax provides two configurable commands for compiling Lean syntax: compile_parser_category and compile_parser. The former compiles a top-level syntax category and the later compiles a specific parser definition (which may include nested categories). Each of these commands accept a CompileConfig configuration provided via an optional with clause term, which defaults to a configuration that produces LParse definitions. For example:

import Lean
import Partax

open scoped Partax

compile_parser Lean.Parser.Term.attributes => attrs -- with CompileConfig.lParse
#eval' "@[instance high, inline]" -- TSyntax `Lean.Parser.Term.attributes

compile_parser_category prio -- with CompileConfig.lParse
#eval' "default + default" -- TSyntax `prio

open Lean Elab Command in -- 2000
#eval liftMacroM (m := CommandElabM) do
  match' "default + default" with
  | .ok stx => evalPrio stx
  | .error e => Macro.throwError e

If something goes wrong while compiling, Partax provides two custom trace classes, trace.Partax.compile.step and trace.Partax.compile.result, to view, respectively, the parser-by-parser steps of the compilation and the final produced definitions.

Limitations & Future Work

There are a number of additional features planned for Partax, including:

  • Support for antiquotations (e.g. $(...) in `(...) syntax).
  • Better optimized LParse with caching, a syntax stack, and indexed category parsing.
  • Formally verified well-formed parser results (e.g., well-formed nodes).
  • More compilation customization and other test cases than LParse.
  • Improved compilation performance (compiling large categories like term can currently take a few minutes).
  • An extended syntax command (called grammar) that supports all standard Lean parser features.