Lark.js

Generate LALR(1) parsers in Javascript

Lark is a popular parsing toolkit for Python.

This project is a port of the Lark standalone parser to Javascript.

Lark.js takes a .lark grammar, and from it generates a standalone Javascript parser.

The LALR(1) algorithm guarantees O(n) running time (proportional to input length), and minimal memory consumption.

Lark grammars

Lark grammars are written in an augmented EBNF (a textual format), and usually use the .lark extension.

Because they are purely declarative, and don't contain code, they are entirely portable between languages.

It is now possible to use Lark grammars in 3 languages: Python, Javascript, and Julia.

"Hello World" Grammar

The following Lark grammar will match the string "Hello, World!" --

// A bunch of words
start: word+                

// Allow optional punctuation after each word
word: WORD ["," | "!"]

// imports WORD from library
%import common.WORD   

// Disregard spaces in text
%ignore " "

Quick links

Install Lark.js

Install lark-js on Python:

    pip install lark-js --upgrade

Note: lark-js requires that the version for lark-python is <=1.1.5

Generate a Javascript LALR(1) parser

	lark-js my_grammar.lark -o my_parser.js

For help, run:

	lark-js --help

Features

  • LALR(1) parser - Fast and light
  • EBNF grammar
  • Builds a parse-tree (AST) automagically, based on the structure of the grammar
  • Usable in the browser and in Node.js
  • Interactive parsing (step-by-step)
  • Error handling
  • Tree utilities (including transformers & visitors)
  • Line & column tracking
  • Standard library of terminals (strings, numbers, names, etc.)
  • Import grammars from Nearley.js

Planned features:

  • Support for Earley

Syntax Highlighting

Lark provides syntax highlighting for its grammar files (*.lark):

Live Port

Usually, ports from one language to another are at risk of falling out of sync as time goes on.

But Lark.js wasn't translated by hand -- 98% of the lines were transpiled directly from Lark's Python code!

That means that futures updates to Lark-Python (fixes, features, etc.) will automatically sync to Lark.js.

License

Lark.js uses the MIT license.

Contribute

Lark.js is accepting pull-requests. If you would like to help, open an issue or find us on gitter.

Sponsoring

Lark.js was made possible with the help of a generous donation by Smore ❤️

If you like Lark, and want to see it grow, please consider sponsoring us!