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gitmirror/docs/nearley.md

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Importing grammars from Nearley

Lark comes with a tool to convert grammars from Nearley, a popular Earley library for Javascript. It uses Js2Py to convert and run the Javascript postprocessing code segments.

Requirements

  1. Install Lark with the nearley component:
pip install lark-parser[nearley]
  1. Acquire a copy of the nearley codebase. This can be done using:
git clone https://github.com/Hardmath123/nearley

Usage

Here’s an example of how to import nearley’s calculator example into Lark:

git clone https://github.com/Hardmath123/nearley
python -m lark.tools.nearley nearley/examples/calculator/arithmetic.ne main nearley > ncalc.py

You can use the output as a regular python module:

>>> import ncalc
>>> ncalc.parse('sin(pi/4) ^ e')
0.38981434460254655

The Nearley converter also supports an experimental converter for newer JavaScript (ES6+), using the --es6 flag:

git clone https://github.com/Hardmath123/nearley
python -m lark.tools.nearley nearley/examples/calculator/arithmetic.ne main nearley --es6 > ncalc.py

Notes

  • Lark currently cannot import templates from Nearley

  • Lark currently cannot export grammars to Nearley

These might get added in the future, if enough users ask for them.