Hongdown

Hongdown is a Markdown formatter that enforces Hong Minhee's Markdown style conventions. The formatter is implemented in Rust using the Comrak library for parsing. It produces consistently formatted Markdown output following a distinctive style used across multiple projects including Fedify, LogTape, and Optique.

Installation

npm

npm install -g hongdown

mise

mise use -g github:dahlia/hongdown

Cargo

cargo install hongdown

Pre-built binaries

Pre-built binaries for Linux, macOS, and Windows are available on the GitHub Releases page.

Usage

Basic usage

# Format a file and print to stdout
hongdown input.md

# Format a file in place
hongdown --write input.md
hongdown -w input.md

# Format multiple files
hongdown -w *.md

# Check if files are formatted (exit 1 if not)
hongdown --check input.md
hongdown -c input.md

# Show diff of formatting changes
hongdown --diff input.md
hongdown -d input.md

# Read from stdin (use --stdin flag or - as filename)
echo "# Hello" | hongdown --stdin
echo "# Hello" | hongdown -
hongdown --stdin < input.md
hongdown - < input.md

# Custom line width
hongdown --line-width 100 input.md

Disable directives

Hongdown supports special HTML comments to disable formatting for specific sections of your document:

<!-- hongdown-disable-file -->
This entire file will not be formatted.

<!-- hongdown-disable-next-line -->
This   line   preserves   its   spacing.

The next line will be formatted normally.

<!-- hongdown-disable-next-section -->
Everything here is preserved as-is
until the next heading (h1 or h2).

Next heading
------------

This section will be formatted normally.

<!-- hongdown-disable -->
This section is not formatted.
<!-- hongdown-enable -->
This section is formatted again.

Configuration file

Hongdown looks for a .hongdown.toml file in the current directory and parent directories. You can also specify a configuration file explicitly with the --config option.

Below is an example configuration with all available options and their default values:

# File patterns (glob syntax)
include = []              # Files to format (default: none, specify on CLI)
exclude = []              # Files to skip (default: none)

# Formatting options
line_width = 80           # Maximum line width (default: 80)

[heading]
setext_h1 = true          # Use === underline for h1 (default: true)
setext_h2 = true          # Use --- underline for h2 (default: true)

[list]
unordered_marker = "-"    # "-", "*", or "+" (default: "-")
leading_spaces = 1        # Spaces before marker (default: 1)
trailing_spaces = 2       # Spaces after marker (default: 2)
indent_width = 4          # Indentation for nested items (default: 4)

[ordered_list]
odd_level_marker = "."    # "." or ")" at odd nesting levels (default: ".")
even_level_marker = ")"   # "." or ")" at even nesting levels (default: ")")
pad = "start"             # "start" or "end" for number alignment (default: "start")
indent_width = 4          # Indentation for nested items (default: 4)

[code_block]
fence_char = "~"          # "~" or "`" (default: "~")
min_fence_length = 4      # Minimum fence length (default: 4)
space_after_fence = true  # Space between fence and language (default: true)
default_language = ""     # Default language for code blocks (default: "")

[thematic_break]
style = "- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -"
leading_spaces = 3        # Leading spaces (0-3, default: 3)

When include patterns are configured, you can run Hongdown without specifying files:

# Format all files matching include patterns
hongdown --write

# Check all files matching include patterns
hongdown --check

CLI options override configuration file settings:

# Use config file but override line width
hongdown --line-width 100 input.md

# Use specific config file
hongdown --config /path/to/.hongdown.toml input.md

Style rules

Hongdown enforces the following conventions:

Headings

• Level 1 and 2 use Setext-style (underlined with = or -)
• Level 3+ use ATX-style (###, ####, etc.)

Document Title
==============

Section
-------

### Subsection

Lists

• Unordered lists use - (space-hyphen-two spaces)
• Ordered lists use 1. format
• 4-space indentation for nested items

 -  First item
 -  Second item
     -  Nested item

Code blocks

• Fenced with four tildes (~~~~)
• Language identifier on the opening fence

~~~~ rust
fn main() {
    println!("Hello, world!");
}
~~~~

Line wrapping

• Lines wrap at approximately 80 display columns
• East Asian wide characters are counted as 2 columns
• Long words that cannot be broken are preserved

Links

• External URLs are converted to reference-style links
• References are placed at the end of each section
• Relative/local URLs remain inline

See the [documentation] for more details.

[documentation]: https://example.com/docs

Tables

• Pipes are aligned accounting for East Asian wide characters
• Minimum column width is maintained

See STYLE.md for the complete style specification, including the philosophy behind these conventions and detailed formatting rules.

Library usage

Hongdown can also be used as a Rust library:

use hongdown::{format, Options};

let input = "# Hello World\nThis is a paragraph.";
let options = Options::default();
let output = format(input, &options).unwrap();
println!("{}", output);

Development

This project uses mise for task management.

Initial setup

After cloning the repository, set up the Git pre-commit hook to automatically run quality checks before each commit:

mise generate git-pre-commit --task=check --write

Quality checks

The following tasks are available:

# Run all quality checks
mise run check

# Individual checks
mise run check:clippy     # Run clippy linter
mise run check:fmt        # Check code formatting
mise run check:type       # Run Rust type checking
mise run check:markdown   # Check Markdown formatting

See AGENTS.md for detailed development guidelines including TDD practices, code style conventions, and commit message guidelines.

Etymology

The name Hongdown is a portmanteau of Hong (from Hong Minhee, the author) and Markdown. It also sounds like the Korean word hongdapda (홍답다), meaning “befitting of Hong” or “Hong-like.”

License

Distributed under the GPL-3.0-or-later. See LICENSE for more information.