Syntax highlight fenced code blocks in Markdown

chroma.go

package main

import (
	"io"

	"github.com/alecthomas/chroma/formatters/html"
	"github.com/alecthomas/chroma/lexers"
	"github.com/alecthomas/chroma/styles"
	"github.com/russross/blackfriday"
)

type Renderer struct {
	html *blackfriday.HTMLRenderer
}

func (r *Renderer) RenderHeader(w io.Writer, ast *blackfriday.Node) {}
func (r *Renderer) RenderFooter(w io.Writer, ast *blackfriday.Node) {}
func (r *Renderer) RenderNode(w io.Writer, node *blackfriday.Node, entering bool) blackfriday.WalkStatus {
	switch node.Type {
	case blackfriday.CodeBlock:
		lexer := lexers.Analyse(string(node.Literal))
		if lexer == nil {
			lexer = lexers.Fallback
		}
		style := styles.Get("manni")
		if style == nil {
			style = styles.Fallback
		}
		iterator, err := lexer.Tokenise(nil, string(node.Literal))
		if err != nil {
			panic(err)
		}
		formatter := html.New()
		err = formatter.Format(w, style, iterator)
		if err != nil {
			panic(err)
		}
		return blackfriday.GoToNext
	}
	return r.html.RenderNode(w, node, entering)
}

const markdown = `
package main

func main() {
	fmt.Println("Hello world!")
}
`

func main() {
	r := Renderer{blackfriday.NewHTMLRenderer(blackfriday.HTMLRendererParameters{})}
	blackfriday.Run([]byte(markdown), blackfriday.WithRenderer(&r))
}