PonyFmt - Experimental Pony Code Formatter

An experimental code formatter for the Pony programming language written in Rust. PonyFmt uses tree-sitter with the tree-sitter-pony grammar to parse source code into an AST and apply consistent formatting rules.

Status

Early prototype - This formatter implements basic indentation and whitespace *normalization but is not yet idempotent or specification-complete. Use with *caution on production code and always backup your files.

Current Limitations

• Formatting is not idempotent (running multiple times may produce different results)

• Comment positioning may not be preserved perfectly

• Limited error recovery for malformed source code

• Not all Pony language constructs are optimally formatted

Features

• Tree-sitter parsing - Reliable syntax-aware formatting

• Parallel processing - Fast formatting of multiple files with Rayon

• Multiple modes - Print to stdout, write files in-place, or check formatting

• ️Configurable indentation - Customize indentation width

• Directory support - Format entire directories recursively

Installation

From Source

git clone https://github.com/yourusername/ponyfmt.git

cd ponyfmt

cargo install --path .

Usage

Command Line Interface

PonyFmt provides a command-line interface with several modes of operation:

Basic Formatting

Format all .pony files in the current directory and print to stdout:

ponyfmt fmt

Format specific files or directories:

ponyfmt fmt src/main.pony
ponyfmt fmt src/ examples/

Check Mode (CI-Friendly)

Check if files are properly formatted without making changes. Returns non-zero exit code if formatting changes would be made:

ponyfmt fmt --check .
ponyfmt fmt --check src/

This is useful for CI/CD pipelines to ensure code is properly formatted.

Write Mode

Write formatted changes back to files in-place:

ponyfmt fmt --write src/
ponyfmt fmt --write --indent 4 src/

⚠️ Warning: Always backup your files before using --write mode, as *formatting is not yet idempotent.

Custom Indentation

Specify the number of spaces for indentation:

ponyfmt fmt --indent 2 src/        # 2 spaces (default)
ponyfmt fmt --indent 4 --write src/ # 4 spaces, write changes

Debug Mode

Inspect the AST structure of Pony files:

ponyfmt debug src/main.pony

This prints the tree-sitter AST for debugging formatter behavior.

Library Usage

PonyFmt can also be used as a Rust library in your own projects.

Add to your Cargo.toml:

[dependencies]
ponyfmt = "0.1"

Basic Library Usage

use ponyfmt::formatter::{FormatOptions, Mode, format_source};

let unformatted_code = r#"
actor Main
new create(env:Env)=>
env.out.print("Hello, World!")
"#;

let options = FormatOptions {
    indent_width: 2,
    mode: Mode::Stdout,
};

match format_source(unformatted_code, &options) {
    Ok(formatted) => println!("{}", formatted),
    Err(e) => eprintln!("Formatting error: {}", e),
}

Advanced Library Usage

use ponyfmt::formatter::{FormatOptions, Mode, format_source};
use std::fs;

// Read a Pony file
let source = fs::read_to_string("src/main.pony")?;

// Configure formatting options
let options = FormatOptions {
    indent_width: 4,
    mode: Mode::Write,  // Although mode doesn't affect format_source output
};

// Format the code
let formatted = format_source(&source, &options)?;

// Write back to file (if desired)
fs::write("src/main.pony", formatted)?;

Integration Examples

CI/CD Pipeline (GitHub Actions)

name: Check Formatting
on: [push, pull_request]

jobs:
  format-check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: actions-rs/toolchain@v1
        with:
          toolchain: stable
      - name: Install ponyfmt
        run: cargo install ponyfmt
      - name: Check formatting
        run: ponyfmt fmt --check .

Pre-commit Hook

#!/bin/sh
# .git/hooks/pre-commit
ponyfmt fmt --check . || {
    echo "Code is not formatted. Run 'ponyfmt fmt --write .' to fix."
    exit 1
}

Makefile Integration

.PHONY: format format-check

format:
    ponyfmt fmt --write src/

format-check:
    ponyfmt fmt --check src/

test: format-check
    # run your tests here

Command Reference

ponyfmt fmt [OPTIONS] [PATHS]...

ARGUMENTS:
    [PATHS]...    Files or directories to format (default: current directory)

OPTIONS:
    --write       Write formatted output back to files
    --check       Check if files are formatted (exit 1 if not)
    --indent <N>  Number of spaces for indentation (default: 2)
    -h, --help    Print help information

ponyfmt debug <FILE>

ARGUMENTS:
    <FILE>        Pony file to debug

OPTIONS:
    -h, --help    Print help information

License

This project is licensed under the MIT License - see the LICENSE file for details.

Acknowledgments

• Pony Language
• tree-sitter
• tree-sitter-pony