prettier-plugin-pegjs

A prettier plugin for formatting Pegjs grammars. You can try it out online in the playground

Intro

Prettier is an opinionated code formatter. It enforces a consistent style by parsing your code and re-printing it with its own rules that take the maximum line length into account, wrapping code when necessary.

This plugin adds support for the Pegjs language to Prettier.

Input

Expression    = head:Term tail:(_("+"/"-")_ Term) * {
return tail.reduce(function(result, element) {if (element[1] === "+") { return result + element[3]; }
        if (element[1] === "-") { return result - element[3]; }
      }, head)}

Output

Expression
  = head:Term tail:(_ "+" / "-" _ Term)* {
      return tail.reduce(function (result, element) {
        if (element[1] === "+") {
          return result + element[3];
        }
        if (element[1] === "-") {
          return result - element[3];
        }
      }, head);
    }

Install

yarn:

yarn add --dev prettier prettier-plugin-pegjs
# or globally
yarn global add prettier prettier-plugin-pegjs

npm:

npm install --save-dev prettier prettier-plugin-pegjs
# or globally
npm install --global prettier prettier-plugin-pegjs

Use

With Node.js

If you installed prettier as a local dependency, you can add prettier as a script in your package.json,

{
    "scripts": {
        "prettier": "prettier"
    }
}

and then run it via

yarn run prettier path/to/grammar.pegjs --write
# or
npm run prettier path/to/grammar.pegjs --write

If you installed globally, run

prettier path/to/grammar.pegjs --write

In the Browser

This package exposes a standalone.ts that wraps prettier and exports a printPrettier function that can be called as

printPrettier(YOUR_CODE, {
    // example option
    tabWidth: 2,
});

Options

The standard Prettier options (such as tabWidth) can be used. Additionally, you may set actionParser to specify how the code inside a Pegjs action is printed. actionParser can be the parser from any valid Prettier plugin. It defaults to "babel-ts" for Javascript and Typescript, but it could be set to a different parser if your actions are written in a different language/dialect.

Development

To make a production build, run

npm run build

To develop, run

npm run watch

You can then execute Prettier with

prettier --plugin-search-dir=./ ...

or

prettier --plugin=./build/prettier-plugin-pegjs.js ...

and the Pegjs plugin will load from the current directory.

Code structure

prettier-plugin-pegjs uses a Pegjs grammar (located in grammars/) to parse Pegjs grammars! This grammar is slightly modified from Pegjs's official grammar to include delimiters and strings as AST nodes. For example, the = in Rule = a / b is assigned an AST node. This is so that prettier-plugin-pegjs can use Prettier's automatic comment placement algorithm, which searches through the AST and places comments based on an AST node's start and end position.

prettier-plugin-pegjs uses webpack to dynamically compile imported Pegjs grammars, so they can be used like native ES6 imports, though of course they are not.

The plugin is organized as follows:

• prettier-plugin-pegjs.ts This file exports the objects required of a Prettier plugin.
• standalone.ts This file wraps the Prettier parser and pre-loads prettier-plugin-pegjs as a plugin.
• grammars/pegjs.peggy The Pegjs grammar that parsers Pegjs/Peggy grammars.
• libs/parser.ts The parser which loads a Pegjs-created parser and creates an AST from a string.
• libs/printer.ts Printers take an AST and produce a Doc (the intermediate format that Prettier uses). This is where most of the details of the plugin lie.