goldmark

A Markdown parser written in Go. Easy to extend, standards-compliant, well-structured.

goldmark is compliant with CommonMark 0.30.

Motivation

I needed a Markdown parser for Go that satisfies the following requirements:

• Easy to extend.
  • Markdown is poor in document expressions compared to other light markup languages such as reStructuredText.
  • We have extensions to the Markdown syntax, e.g. PHP Markdown Extra, GitHub Flavored Markdown.
• Standards-compliant.
  • Markdown has many dialects.
  • GitHub-Flavored Markdown is widely used and is based upon CommonMark, effectively mooting the question of whether or not CommonMark is an ideal specification.
    • CommonMark is complicated and hard to implement.
• Well-structured.
  • AST-based; preserves source position of nodes.
• Written in pure Go.

golang-commonmark may be a good choice, but it seems to be a copy of markdown-it.

blackfriday.v2 is a fast and widely-used implementation, but is not CommonMark-compliant and cannot be extended from outside of the package, since its AST uses structs instead of interfaces.

Furthermore, its behavior differs from other implementations in some cases, especially regarding lists: Deep nested lists don't output correctly #329, List block cannot have a second line #244, etc.

This behavior sometimes causes problems. If you migrate your Markdown text from GitHub to blackfriday-based wikis, many lists will immediately be broken.

As mentioned above, CommonMark is complicated and hard to implement, so Markdown parsers based on CommonMark are few and far between.

Features

• Standards-compliant. goldmark is fully compliant with the latest CommonMark specification.
• Extensible. Do you want to add a @username mention syntax to Markdown? You can easily do so in goldmark. You can add your AST nodes, parsers for block-level elements, parsers for inline-level elements, transformers for paragraphs, transformers for the whole AST structure, and renderers.
• Performance. goldmark's performance is on par with that of cmark, the CommonMark reference implementation written in C.
• Robust. goldmark is tested with go-fuzz, a fuzz testing tool.
• Built-in extensions. goldmark ships with common extensions like tables, strikethrough, task lists, and definition lists.
• Depends only on standard libraries.

Installation

$ go get github.com/yuin/goldmark

Usage

Import packages:

import (
    "bytes"
    "github.com/yuin/goldmark"
)

Convert Markdown documents with the CommonMark-compliant mode:

var buf bytes.Buffer
if err := goldmark.Convert(source, &buf); err != nil {
  panic(err)
}

With options

var buf bytes.Buffer
if err := goldmark.Convert(source, &buf, parser.WithContext(ctx)); err != nil {
  panic(err)
}

Functional option	Type	Description
parser.WithContext	A parser.Context	Context for the parsing phase.

Context options

Functional option	Type	Description
parser.WithIDs	A parser.IDs	IDs allows you to change logics that are related to element id(ex: Auto heading id generation).

Custom parser and renderer

import (
    "bytes"
    "github.com/yuin/goldmark"
    "github.com/yuin/goldmark/extension"
    "github.com/yuin/goldmark/parser"
    "github.com/yuin/goldmark/renderer/html"
)

md := goldmark.New(
          goldmark.WithExtensions(extension.GFM),
          goldmark.WithParserOptions(
              parser.WithAutoHeadingID(),
          ),
          goldmark.WithRendererOptions(
              html.WithHardWraps(),
              html.WithXHTML(),
          ),
      )
var buf bytes.Buffer
if err := md.Convert(source, &buf); err != nil {
    panic(err)
}

Functional option	Type	Description
goldmark.WithParser	parser.Parser	This option must be passed before goldmark.WithParserOptions and goldmark.WithExtensions
goldmark.WithRenderer	renderer.Renderer	This option must be passed before goldmark.WithRendererOptions and goldmark.WithExtensions
goldmark.WithParserOptions	...parser.Option
goldmark.WithRendererOptions	...renderer.Option
goldmark.WithExtensions	...goldmark.Extender

Parser and Renderer options

Parser options

Functional option	Type	Description
parser.WithBlockParsers	A util.PrioritizedSlice whose elements are parser.BlockParser	Parsers for parsing block level elements.
parser.WithInlineParsers	A util.PrioritizedSlice whose elements are parser.InlineParser	Parsers for parsing inline level elements.
parser.WithParagraphTransformers	A util.PrioritizedSlice whose elements are parser.ParagraphTransformer	Transformers for transforming paragraph nodes.
parser.WithASTTransformers	A util.PrioritizedSlice whose elements are parser.ASTTransformer	Transformers for transforming an AST.
parser.WithAutoHeadingID	-	Enables auto heading ids.
parser.WithAttribute	-	Enables custom attributes. Currently only headings supports attributes.

HTML Renderer options

Functional option	Type	Description
html.WithWriter	html.Writer	html.Writer for writing contents to an io.Writer.
html.WithHardWraps	-	Render newlines as <br>.
html.WithXHTML	-	Render as XHTML.
html.WithUnsafe	-	By default, goldmark does not render raw HTML or potentially dangerous links. With this option, goldmark renders such content as written.

Built-in extensions

• extension.Table
  • GitHub Flavored Markdown: Tables
• extension.Strikethrough
  • GitHub Flavored Markdown: Strikethrough
• extension.Linkify
  • GitHub Flavored Markdown: Autolinks
• extension.TaskList
  • GitHub Flavored Markdown: Task list items
• extension.GFM
  • This extension enables Table, Strikethrough, Linkify and TaskList.
  • This extension does not filter tags defined in 6.11: Disallowed Raw HTML (extension). If you need to filter HTML tags, see Security.
  • If you need to parse github emojis, you can use goldmark-emoji extension.
• extension.DefinitionList
  • PHP Markdown Extra: Definition lists
• extension.Footnote
  • PHP Markdown Extra: Footnotes
• extension.Typographer
  • This extension substitutes punctuations with typographic entities like smartypants.

Attributes

The parser.WithAttribute option allows you to define attributes on some elements.

Currently only headings support attributes.

Attributes are being discussed in the CommonMark forum. This syntax may possibly change in the future.

Headings

## heading ## {#id .className attrName=attrValue class="class1 class2"}

## heading {#id .className attrName=attrValue class="class1 class2"}

heading {#id .className attrName=attrValue}
============

Table extension

The Table extension implements Table(extension), as defined in GitHub Flavored Markdown Spec.

Specs are defined for XHTML, so specs use some deprecated attributes for HTML5.

You can override alignment rendering method via options.

Functional option	Type	Description
extension.WithTableCellAlignMethod	extension.TableCellAlignMethod	Option indicates how are table cells aligned.

Typographer extension

The Typographer extension translates plain ASCII punctuation characters into typographic-punctuation HTML entities.

Default substitutions are:

Punctuation	Default entity
'	‘, ’
"	“, ”
--	–
---	—
...	…
<<	&laquo;
>>	&raquo;

You can override the default substitutions via extensions.WithTypographicSubstitutions:

markdown := goldmark.New(
    goldmark.WithExtensions(
        extension.NewTypographer(
            extension.WithTypographicSubstitutions(extension.TypographicSubstitutions{
                extension.LeftSingleQuote:  []byte("‚"),
                extension.RightSingleQuote: nil, // nil disables a substitution
            }),
        ),
    ),
)

Linkify extension

The Linkify extension implements Autolinks(extension), as defined in GitHub Flavored Markdown Spec.

Since the spec does not define details about URLs, there are numerous ambiguous cases.

You can override autolinking patterns via options.

Functional option	Type	Description
extension.WithLinkifyAllowedProtocols	[][]byte	List of allowed protocols such as [][]byte{ []byte("http:") }
extension.WithLinkifyURLRegexp	*regexp.Regexp	Regexp that defines URLs, including protocols
extension.WithLinkifyWWWRegexp	*regexp.Regexp	Regexp that defines URL starting with www.. This pattern corresponds to the extended www autolink
extension.WithLinkifyEmailRegexp	*regexp.Regexp	Regexp that defines email addresses`

Example, using xurls:

import "mvdan.cc/xurls/v2"

markdown := goldmark.New(
    goldmark.WithRendererOptions(
        html.WithXHTML(),
        html.WithUnsafe(),
    ),
    goldmark.WithExtensions(
        extension.NewLinkify(
            extension.WithLinkifyAllowedProtocols([][]byte{
                []byte("http:"),
                []byte("https:"),
            }),
            extension.WithLinkifyURLRegexp(
                xurls.Strict,
            ),
        ),
    ),
)

Footnotes extension

The Footnote extension implements PHP Markdown Extra: Footnotes.

This extension has some options:

Functional option	Type	Description
extension.WithFootnoteIDPrefix	[]byte	a prefix for the id attributes.
extension.WithFootnoteIDPrefixFunction	func(gast.Node) []byte	a function that determines the id attribute for given Node.
extension.WithFootnoteLinkTitle	[]byte	an optional title attribute for footnote links.
extension.WithFootnoteBacklinkTitle	[]byte	an optional title attribute for footnote backlinks.
extension.WithFootnoteLinkClass	[]byte	a class for footnote links. This defaults to footnote-ref.
extension.WithFootnoteBacklinkClass	[]byte	a class for footnote backlinks. This defaults to footnote-backref.
extension.WithFootnoteBacklinkHTML	[]byte	a class for footnote backlinks. This defaults to ↩︎.

Some options can have special substitutions. Occurrences of “^^” in the string will be replaced by the corresponding footnote number in the HTML output. Occurrences of “%%” will be replaced by a number for the reference (footnotes can have multiple references).

extension.WithFootnoteIDPrefix and extension.WithFootnoteIDPrefixFunction are useful if you have multiple Markdown documents displayed inside one HTML document to avoid footnote ids to clash each other.

extension.WithFootnoteIDPrefix sets fixed id prefix, so you may write codes like the following:

for _, path := range files {
    source := readAll(path)
    prefix := getPrefix(path)

    markdown := goldmark.New(
        goldmark.WithExtensions(
            NewFootnote(
                WithFootnoteIDPrefix([]byte(path)),
            ),
        ),
    )
    var b bytes.Buffer
    err := markdown.Convert(source, &b)
    if err != nil {
        t.Error(err.Error())
    }
}

extension.WithFootnoteIDPrefixFunction determines an id prefix by calling given function, so you may write codes like the following:

markdown := goldmark.New(
    goldmark.WithExtensions(
        NewFootnote(
                WithFootnoteIDPrefixFunction(func(n gast.Node) []byte {
                    v, ok := n.OwnerDocument().Meta()["footnote-prefix"]
                    if ok {
                        return util.StringToReadOnlyBytes(v.(string))
                    }
                    return nil
                }),
        ),
    ),
)

for _, path := range files {
    source := readAll(path)
    var b bytes.Buffer

    doc := markdown.Parser().Parse(text.NewReader(source))
    doc.Meta()["footnote-prefix"] = getPrefix(path)
    err := markdown.Renderer().Render(&b, source, doc)
}

You can use goldmark-meta to define a id prefix in the markdown document:

---
title: document title
slug: article1
footnote-prefix: article1
---

# My article

Security

By default, goldmark does not render raw HTML or potentially-dangerous URLs. If you need to gain more control over untrusted contents, it is recommended that you use an HTML sanitizer such as bluemonday.

Benchmark

You can run this benchmark in the _benchmark directory.

against other golang libraries

blackfriday v2 seems to be the fastest, but as it is not CommonMark compliant, its performance cannot be directly compared to that of the CommonMark-compliant libraries.

goldmark, meanwhile, builds a clean, extensible AST structure, achieves full compliance with CommonMark, and consumes less memory, all while being reasonably fast.

goos: darwin
goarch: amd64
BenchmarkMarkdown/Blackfriday-v2-12                  326           3465240 ns/op         3298861 B/op      20047 allocs/op
BenchmarkMarkdown/GoldMark-12                        303           3927494 ns/op         2574809 B/op      13853 allocs/op
BenchmarkMarkdown/CommonMark-12                      244           4900853 ns/op         2753851 B/op      20527 allocs/op
BenchmarkMarkdown/Lute-12                            130           9195245 ns/op         9175030 B/op     123534 allocs/op
BenchmarkMarkdown/GoMarkdown-12                        9         113541994 ns/op         2187472 B/op      22173 allocs/op

against cmark (CommonMark reference implementation written in C)

----------- cmark -----------
file: _data.md
iteration: 50
average: 0.0037760639 sec
go run ./goldmark_benchmark.go
------- goldmark -------
file: _data.md
iteration: 50
average: 0.0040964230 sec

As you can see, goldmark's performance is on par with cmark's.

Extensions

• goldmark-meta: A YAML metadata extension for the goldmark Markdown parser.
• goldmark-highlighting: A syntax-highlighting extension for the goldmark markdown parser.
• goldmark-emoji: An emoji extension for the goldmark Markdown parser.
• goldmark-mathjax: Mathjax support for the goldmark markdown parser
• goldmark-pdf: A PDF renderer that can be passed to goldmark.WithRenderer().

goldmark internal(for extension developers)

Overview

goldmark's Markdown processing is outlined in the diagram below.

            <Markdown in []byte, parser.Context>
                           |
                           V
            +-------- parser.Parser ---------------------------
            | 1. Parse block elements into AST
            |   1. If a parsed block is a paragraph, apply 
            |      ast.ParagraphTransformer
            | 2. Traverse AST and parse blocks.
            |   1. Process delimiters(emphasis) at the end of
            |      block parsing
            | 3. Apply parser.ASTTransformers to AST
                           |
                           V
                      <ast.Node>
                           |
                           V
            +------- renderer.Renderer ------------------------
            | 1. Traverse AST and apply renderer.NodeRenderer
            |    corespond to the node type

                           |
                           V
                        <Output>

Parsing

Markdown documents are read through text.Reader interface.

AST nodes do not have concrete text. AST nodes have segment information of the documents, represented by text.Segment .

text.Segment has 3 attributes: Start, End, Padding .

(TBC)

TODO

See extension directory for examples of extensions.

Summary:

1. Define AST Node as a struct in which ast.BaseBlock or ast.BaseInline is embedded.
2. Write a parser that implements parser.BlockParser or parser.InlineParser.
3. Write a renderer that implements renderer.NodeRenderer.
4. Define your goldmark extension that implements goldmark.Extender.

Donation

BTC: 1NEDSyUmo4SMTDP83JJQSWi1MvQUGGNMZB

License

MIT

Author

Yusuke Inuzuka