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, Hollo, and Vertana.

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."

Installation

From source

cargo install hongdown

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
echo "# Hello" | hongdown
hongdown --stdin < 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.

line_width = 80

[heading]
setext_h1 = true          # Use === underline for h1
setext_h2 = true          # Use --- underline for h2

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

[ordered_list]
odd_level_marker = "."    # "1." at odd nesting levels
even_level_marker = ")"   # "1)" at even nesting levels

[code_block]
fence_char = "~"          # "~" or "`"
min_fence_length = 4      # Minimum fence length
space_after_fence = true  # Space between fence and language

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 SPEC.md for the complete style specification.

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);

TODO

Phase 1: Core formatting

• Basic CLI with file input/output
• Front matter preservation (YAML/TOML)
• Headings (setext and ATX)
• Paragraphs with line wrapping
• Lists (ordered and unordered)
• Code blocks (fenced)
• Block quotes
• Inline formatting (emphasis, strong, code, links)
• Basic test suite

Phase 2: Extended syntax

• Tables with alignment
• Definition lists
• GitHub alerts/admonitions
• Footnotes
• Reference link collection and placement

Phase 3: Polish

• Configuration file support
  • Config file parsing and discovery (.hongdown.toml)
  • line_width option
  • [heading] section
    • setext_h1 option
    • setext_h2 option
  • [list] section
    • unordered_marker option
    • leading_spaces option
    • trailing_spaces option
    • indent_width option
  • [ordered_list] section
    • odd_level_marker option
    • even_level_marker option
  • [code_block] section
    • fence_char option
    • min_fence_length option
    • space_after_fence option
• Check mode for CI integration
• Diff output mode
• Disable directives
• Edge case handling
• Performance optimization

License

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