.retextrc.mjs

import retextSpell from "retext-spell";

async function loadDictionary(dictionary) {
  const baseUrl = new URL(`https://unpkg.com/dictionary-${dictionary}@latest/`);

  const [aff, dic] = await Promise.all([
    fetch(new URL("index.aff", baseUrl)),
    fetch(new URL("index.dic", baseUrl)),
  ]);

  if (!(aff.ok && dic.ok)) {
    throw new Error(`Couldn't load dictionary files from ${baseUrl}`);
  }
  return {
    aff: Buffer.from(await aff.arrayBuffer()),
    dic: Buffer.from(await dic.arrayBuffer()),
  };
}

function plugin(options) {
  return async (tree, file, next) => {
    // The nascent beginnings of a configurable dictionary
    const locale = "en-gb";
    let dictionary;
    try {
      dictionary = await loadDictionary(locale);
    } catch (error) {
      file.fail(String(error));
    }
    const impl = retextSpell({ dictionary });
    return impl(tree, file, next);
  };
}

export default {
  plugins: [[plugin, {}]],
};

README.md

1. Install the NodeJS packages
2. Install the Python dependencies

jupyter_server_config.py

import shutil

key = "retext-language-server"
c.LanguageServerManager.language_servers = {
    key: dict(
        version=2,
        display_name=key,
        mime_types=["text/x-gfm", "text/x-ipythongfm", "text/x-markdown"],
        argv=[
            shutil.which("node"),
            "/tmp/tmp.O8NDAzyhVI/server.mjs",
            "--stdio",
        ],
        languages=["markdown", "ipythongfm", "gfm"],
        install=dict(
            npm="npm install --save-dev {}".format(key),
            yarn="yarn add --dev {}".format(key),
            jlpm="jlpm add --dev {}".format(key),
        ),
    )
}

package.json

{
  "dependencies": {
    "remark": "^15.0.1",
    "retext": "^9.0.0",
    "retext-spell": "^6.1.0",
    "unified-language-server": "^4.0.1"
  }
}

requirements.txt

jupyterlab-lsp 
python-lsp-server[all] 
jupyterlab

server.mjs

import { retext } from "retext";
import { createUnifiedLanguageServer } from "unified-language-server";

process.title = "retext-language-server";

createUnifiedLanguageServer({
  ignoreName: ".retextignore",
  pluginPrefix: "retext",
  rcName: ".retextrc",
  processorName: "retext",
  processorSpecifier: "retext",
  defaultProcessor: retext,
});