Hongdown is a Markdown formatter designed to enforce consistent formatting based on Hong Minhee's style conventions. Built using Rust and the Comrak library, it ensures that Markdown output adheres to a specific and widely adopted style used in various projects like Fedify, LogTape, and Optique.
Key Features:
Customizable line width for wrapping text.
Sentence case conversion with support for defining proper nouns and common nouns to maintain consistency in headings.
HTML comment directives to disable or control formatting behavior within specific sections of a document.
Cascading configuration system that allows merging settings from multiple files, including global, user-specific, and project-level configurations.
Audience & Benefit:
Ideal for developers, technical writers, and project maintainers who need consistent Markdown styling across their documentation. Hongdown ensures uniformity in formatting, which is particularly valuable for projects requiring strict adherence to style guidelines. It can be installed via winget, making it accessible for Windows users.
README
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
Windows Package Manager
winget install HongMinhee.Hongdown
Scoop
scoop bucket add extras
scoop install hongdown
npm
npm install -g hongdown
mise
mise use -g aqua:dahlia/hongdown
Nix
nix run 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
# Format all Markdown files in a directory (recursive)
hongdown -w .
hongdown -w docs/
# 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 word wrapping entirely
hongdown --no-line-width input.md
# Enable MDX mode for non-.mdx input (.mdx files enable it automatically)
hongdown --mdx input.md
Hongdown supports special HTML comment directives to control formatting
behavior within documents.
Disable formatting
You can disable formatting for specific sections:
This entire file will not be formatted.
This line preserves its spacing.
The next line will be formatted normally.
Everything here is preserved as-is
until the next heading (h1 or h2).
Next heading
------------
This section will be formatted normally.
This section is not formatted.
This section is formatted again.
Sentence case customization
When sentence case is enabled, you can define document-specific proper nouns
and common nouns:
# Using Swift And Go For Python Development
This heading will be formatted as: "Using Swift and Go for python development"
`` – Defines proper nouns to
preserve (case-sensitive)
`` – Overrides built-in proper
nouns by treating them as common nouns
These directives are merged with configuration file settings.
Configuration file
Hongdown supports cascading configuration files from multiple locations.
Configuration files are loaded and merged in the following order (lowest to
highest priority):
Project: .hongdown.toml in the current directory or any parent
directory
Settings from higher-priority configurations override those from lower-priority
ones. This allows you to set global defaults at the user or system level while
overriding them for specific projects.
You can also specify a configuration file explicitly with the --config option,
which bypasses the cascading system and uses only that file.
Disabling configuration inheritance
To ignore all system and user configurations and use only your project config:
no_inherit = true
# Your project-specific settings
line_width = 100
When no_inherit = true, only the project config and Hongdown's defaults are
used. This is useful for projects that need strict formatting control
regardless of user preferences.
Configuration options
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)
git_aware = true # Respect .gitignore and skip .git directory (default: true)
# Formatting options
line_width = 80 # Maximum line width (min: 8, default: 80); set to false to disable wrapping
math = true # Preserve TeX math ($…$ and $$…$$) verbatim (default: true)
mdx = false # MDX mode: preserve embedded JS/JSX verbatim (default: false)
[heading]
setext_h1 = true # Use === underline for h1 (default: true)
setext_h2 = true # Use --- underline for h2 (default: true)
sentence_case = false # Convert headings to sentence case (default: false)
proper_nouns = [] # Additional proper nouns to preserve (default: [])
common_nouns = [] # Exclude built-in proper nouns (default: [])
anchor_align = 0 # Spaces before anchor {#id}: positive = exact count,
# 0/negative = right-align to line_width+anchor_align (default: 0)
[unordered_list]
unordered_marker = "-" # "-", "*", or "+" (default: "-")
leading_spaces = 1 # Spaces before marker (0–3, default: 1)
trailing_spaces = 2 # Spaces after marker (0–3, default: 2)
indent_width = 4 # Indentation for nested items (min: 1, 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 (min: 1, default: 4)
[code_block]
fence_char = "~" # "~" or "`" (default: "~")
min_fence_length = 4 # Minimum fence length (min: 3, default: 4)
space_after_fence = true # Space between fence and language (default: true)
default_language = "" # Default language for code blocks (default: "")
# External code formatters (see "External code formatters" section)
[code_block.formatters]
# javascript = ["deno", "fmt", "--ext=js", "-"]
[thematic_break]
# Must be valid CommonMark: at least 3 of *, -, or _ (with optional spaces)
style = "- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -"
leading_spaces = 3 # Leading spaces (0–3, default: 3)
[punctuation]
curly_double_quotes = true # "text" to "text" (default: true)
curly_single_quotes = true # 'text' to 'text' (default: true)
curly_apostrophes = false # it's to it's (default: false)
ellipsis = true # ... to ... (default: true)
en_dash = false # Disabled by default (use "--" to enable)
em_dash = "--" # -- to --- (default: "--", use false to disable)
Configuration values are validated at parse time. Invalid values will produce
descriptive error messages:
$ hongdown --config invalid.toml input.md
Error: failed to parse configuration file: line_width must be at least 8, got 5.
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
# Disable word wrapping (overrides config file)
hongdown --no-line-width 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.)
Optional sentence case conversion (disabled by default)
Document Title
==============
Section
-------
### Subsection
Sentence case (optional)
When sentence_case = true is set in the configuration, Hongdown automatically
converts headings to sentence case using intelligent heuristics:
# Development Commands → Development commands
# Working With JSON APIs → Working with JSON APIs
# Using `MyClass` In Code → Using `MyClass` in code
You can also exclude built-in proper nouns by treating them as common nouns.
This is useful for words like “Go” which can be either a programming language
or a common verb:
[heading]
sentence_case = true
common_nouns = ["Go", "Swift"] # Treat these as common nouns, not proper nouns
Built-in proper nouns include ~450 entries: programming languages (JavaScript,
TypeScript, Python, Rust, Go), technologies (GitHub, Docker, React, Node.js),
databases (PostgreSQL, MySQL, MongoDB), countries (United States, Republic of
Korea), natural languages (English, Korean, Japanese), and more.
You can also use HTML comment directives to define document-specific proper
nouns and common nouns. See the “HTML comment directives” section for details.
You can configure external formatters for code blocks in your .hongdown.toml.
This allows automatic formatting of embedded code using language-specific tools.
The hongdown-no-format keyword is preserved in the output, ensuring the code
block remains unformatted on subsequent runs.
For WASM builds, use the formatWithCodeFormatter function with a callback:
import { formatWithCodeFormatter } from "@hongdown/wasm";
import * as prettier from "prettier";
const { output, warnings } = await formatWithCodeFormatter(markdown, {
codeFormatter: (language, code) => {
if (language === "javascript" || language === "typescript") {
return prettier.format(code, { parser: "babel" });
}
return null; // Keep original for other languages
},
});
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
Math
Inline ($…$) and display ($$…$$) TeX/LaTeX math is preserved verbatim,
never escaped or punctuation-transformed (like code spans)
Inline math is kept on a single line when wrapping
Dollar-math parsing follows GitHub's rules, so a lone $, $ command, or
$5 stays literal text
Disable with math = false in .hongdown.toml to treat every $ as text
The complexity is $O(n \log n)$.
$$
x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}
$$
MDX
MDX documents mix Markdown with JavaScript and JSX. Because the underlying
CommonMark parser does not understand those constructs, formatting an MDX
document normally corrupts the embedded code. MDX mode (“first, do no harm”)
detects the JS/JSX constructs and passes them through verbatim, while the
surrounding Markdown prose is still formatted.
Within paragraph text, the constructs Hongdown would otherwise corrupt are
preserved verbatim — never escaped or punctuation-transformed:
ESM import/export statements, JSX elements and fragments (, , <>…</>), and {…} expressions (including {/* … */}
comments)
The embedded JavaScript/JSX is preserved, not reformatted
Files with the .mdx extension enable MDX mode automatically; for stdin or
.md input, enable it with --mdx or mdx = true in .hongdown.toml
import { Chart } from "./chart.js";
export const meta = { author: "Hong Minhee" };
A paragraph of prose is still wrapped and punctuation-transformed, while the
JSX and imports above are left exactly as written.
A JSX tag that is already valid inline HTML (for example a single {value}
attribute) is left to the underlying parser, which keeps the tag while still
formatting the element's Markdown children; only tags that break the HTML
grammar (a {{…}} attribute, embedded quotes or spaces, or a multi-line opener)
are protected as a whole.
Some constructs are intentionally left unprotected: anything inside a heading
(an explicit {#id} anchor stays a heading anchor), braces inside inline code
(`{x}`), math ($\frac{1}{2}$), or link/image syntax, an expression
containing a regex literal whose context is ambiguous (a / immediately after
}), and an ESM object whose body contains a blank line.
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.
By default, the extension looks for .hongdown.toml in the workspace root. For
a standalone file outside any workspace, it falls back to the file's containing
directory. Set hongdown.config.path to use a different configuration file.
Zed
Add the following to your Zed settings to use Hongdown as the Markdown
formatter (contributed by Lee Dogeon):
use hongdown::{format, Options};
let input = "# Hello World\nThis is a paragraph.";
let options = Options::default();
let output = format(input, &options).unwrap();
println!("{}", output);
JavaScript/TypeScript
Hongdown is available as a WebAssembly-based library for JavaScript and
TypeScript:
npm install @hongdown/wasm
import { format, formatWithWarnings, loadConfigFromToml } from "@hongdown/wasm";
// Basic usage
const markdown = "# Hello\nWorld";
const formatted = await format(markdown);
// With options
const result = await format(markdown, {
lineWidth: 100,
setextH1: false,
fenceChar: "`",
});
// Get warnings along with formatted output
const { output, warnings } = await formatWithWarnings(markdown);
if (warnings.length > 0) {
for (const warning of warnings) {
console.warn(`Line ${warning.line}: ${warning.message}`);
}
}
// Parse .hongdown.toml for WASM formatting
const { options: configOptions } = await loadConfigFromToml(configToml);
const configured = await format(markdown, configOptions);
External code block formatters configured in code_block.formatters are
reported as warnings because the WASM runtime cannot execute external
commands. Use the CLI for those settings.
The library works in Node.js, Bun, Deno, and web browsers. See the
TypeScript type definitions for all available options.
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.”