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](https://gitlab.com/golang-commonmark/markdown) may be a good choice, but it seems to be a copy of [markdown-it](https://github.com/markdown-it).
[blackfriday.v2](https://github.com/russross/blackfriday/tree/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](https://github.com/russross/blackfriday/issues/329), [List block cannot have a second line #244](https://github.com/russross/blackfriday/issues/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](https://commonmark.org/) 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](https://github.com/dvyukov/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
----------------------
```bash
$ go get github.com/yuin/goldmark
```
Usage
----------------------
Import packages:
```go
import (
"bytes"
"github.com/yuin/goldmark"
)
```
Convert Markdown documents with the CommonMark-compliant mode:
```go
var buf bytes.Buffer
if err := goldmark.Convert(source, &buf); err != nil {
panic(err)
}
```
With options
------------------------------
```go
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
--------------------------
```go
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` |
| `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. |
- [GitHub Flavored Markdown: Task list items](https://github.github.com/gfm/#task-list-items-extension-)
-`extension.GFM`
- This extension enables Table, Strikethrough, Linkify and TaskList.
- This extension does not filter tags defined in [6.11: Disallowed Raw HTML (extension)](https://github.github.com/gfm/#disallowed-raw-html-extension-).
If you need to filter HTML tags, see [Security](#security).
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:
```go
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:
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](https://github.com/microcosm-cc/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.
