Lean 4 / Parser

A parser combinator library for Lean 4.

Source documentation is available at www.dorais.org/lean4-parser/doc/.

Usage

Add this dependency to your project's lakefile.toml:

[[require]]
name = "Parser"
git = "https://github.com/fgdorais/lean4-parser"
rev = "main"

Then add import Parser at the top of any Lean file where you plan to use this library. For example:

import Parser

open Parser Char

/--
Parses a list of sign-separated integers (no spaces) from an input string and returns the sum.
-/
def parseSum : SimpleParser String.Slice Char Int := do
  let mut sum : Int := 0
  -- parse until all input is consumed
  while ! (← test endOfInput) do
    -- parse an integer (decimal only) and add to sum
    sum := sum + (← ASCII.parseInt)
  return sum

-- returns 42
#eval match parseSum.run "11-1+2-3+33" with
  | .ok _ sum => sum
  | .error _ e => panic! (toString e)

The examples directory contains more elaborate sample parsers.

Acknowledgements

Original work for the Lean 4 Parser library was done by François G. Dorais, Kyrill Serdyuk, and Emma Shroyer. This work was partly supported by the CEMS REU program at The University of Vermont.

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• The Parser library is copyright © 2022-2025 François G. Dorais, Kyrill Serdyuk, Emma Shroyer. The library is released under the Apache 2.0 license. See the file LICENSE for additional details.