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prettier-plugin-sieve

A Prettier plugin for formatting Sieve email filtering scripts (RFC 5228).

Features

• Formats .sieve / .siv files via Prettier
• Hand-rolled Peggy.js PEG grammar for RFC 5228 core language
• Supports: require, if/elsif/else, all standard tests (address, header, envelope, allof, anyof, not, size, exists, true, false), all standard actions (fileinto, redirect, keep, discard, stop, reject, vacation, addflag, setflag, removeflag)
• Idempotent — formatting twice gives the same result
• Works with Prettier 3.x
• IntelliJ IDEA + VS Code Prettier extension compatible

Installation

npm install --save-dev prettier prettier-plugin-sieve

Configuration

.prettierrc

{
  "plugins": ["prettier-plugin-sieve"]
}

VS Code (settings.json)

{
  "prettier.documentSelectors": ["**/*.sieve", "**/*.siv"],
  "[sieve]": {
    "editor.defaultFormatter": "esbenp.prettier-vscode",
    "editor.formatOnSave": true
  }
}

IntelliJ IDEA

Go to Settings → Languages & Frameworks → JavaScript → Prettier:

• Enable "Run on save"
• Add **/*.sieve to the "Run for files" glob

Usage

# Format all .sieve files
npx prettier --write "**/*.sieve"

# Check formatting without writing
npx prettier --check "**/*.sieve"

Example

Input (messy):

require ["fileinto","reject","vacation"];
if anyof(header :contains "Subject" "URGENT",address :is "From" "boss@example.com"){fileinto "Priority";}
elsif header:contains "Subject" "[SPAM]"{discard;}
else{keep;}

Output (formatted):

require ["fileinto", "reject", "vacation"];
if anyof (
  header :contains "Subject" "URGENT",
  address :is "From" "boss@example.com"
) {
  fileinto "Priority";
} elsif header :contains "Subject" "[SPAM]" {
  discard;
} else {
  keep;
}

Project Structure

prettier-plugin-sieve/
├── src/
│   ├── index.ts               # Plugin entry: languages, parsers, printers
│   ├── types.ts               # Full TypeScript AST node interfaces
│   ├── printer.ts             # Prettier Doc builder / printer
│   └── parser/
│       ├── index.ts           # parse(), locStart(), locEnd() exports
│       └── sieve.pegjs        # Peggy PEG grammar for RFC 5228
├── tests/
│   └── format.test.ts         # Jest end-to-end formatting tests
├── package.json
└── tsconfig.json

Architecture

1 · Grammar (src/parser/sieve.pegjs)

A Peggy PEG grammar that directly encodes RFC 5228 syntax. Each grammar rule returns a typed AST node with a loc (source location) property. The grammar is compiled at runtime the first time parse() is called (or can be pre-compiled via npm run generate-parser for production builds).

2 · Parser adapter (src/parser/index.ts)

Wraps Peggy's parse() to:

• Return a Script root node
• Expose locStart / locEnd so Prettier can attach comments
• Surface grammar errors with human-readable messages

3 · Printer (src/printer.ts)

Converts the Sieve AST into Prettier's Doc IR using the doc.builders API:

Doc primitive	Where used
hardline	Between top-level commands and block contents
softline	Inside string lists, test argument lists
group	All commands and tests — allows line-breaking when over printWidth
indent	Block bodies, multi-line list arguments
join	Comma-separated string lists and test lists

Building

# Compile TypeScript → dist/
npm run build

# Optionally pre-compile the Peggy grammar to JS for faster startup
npm run generate-parser

Testing

npm test

Extending

To add support for Sieve extensions (e.g. RFC 5232 sieve-imap-flags, RFC 5230 vacation):

1. Add new rules to src/parser/sieve.pegjs
2. Add new AST interfaces to src/types.ts
3. Add case branches to printAction / printTest in src/printer.ts

License

MIT