edit.html

<h1>Editing {{.Title}}</h1>

<form action="/save/{{.Title}}" method="POST">
  <div><textarea name="body" rows="20" cols="80">{{printf "%s" .Body}}</textarea></div>
  <div><input type="submit" value="Save"></div>
</form>

view.html

<h1>{{.Title}}</h1>

<p>[<a href="/edit/{{.Title}}">edit</a>]</p>

<div>{{.EscapedBody}}</div>

wiki.go

package main

import (
	"html/template"
	"io/ioutil"
	"net/http"
	"os"
	"path/filepath"
	"regexp"
)

const lenPath = len("/view/")

var templates = template.Must(template.ParseFiles("tmpl/edit.html", "tmpl/view.html"))
var titleValidator = regexp.MustCompile("^[a-zA-Z0-9]+$")
var linkPattern = regexp.MustCompile("\\[[a-zA-Z0-9]+\\]")

type Page struct {
	Title string
	Body  []byte
}

func (p *Page) save() error {
	filename := filepath.Join("data", p.Title+".txt")
	err := os.MkdirAll(filepath.Dir(filename), 0755)
	if err != nil {
		return err
	}
	return ioutil.WriteFile(filename, p.Body, 0600)
}

func loadPage(title string) (*Page, error) {
	filename := filepath.Join("data", title+".txt")
	body, err := ioutil.ReadFile(filename)
	if err != nil {
		return nil, err
	}
	return &Page{Title: title, Body: body}, nil
}

func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
	err := templates.ExecuteTemplate(w, tmpl+".html", p)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
}

func makeHandler(fn func(http.ResponseWriter, *http.Request, string)) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		title := r.URL.Path[lenPath:]
		if !titleValidator.MatchString(title) {
			http.NotFound(w, r)
			return
		}
		fn(w, r, title)
	}
}

func (page Page) EscapedBody() template.HTML {
	body := linkPattern.ReplaceAllFunc(page.Body, func(matched []byte) []byte {
		title := string(matched[1 : len(matched)-1])
		return []byte(`<a href="/view/` + title + `">` + title + `</a>`)
	})
	return template.HTML(body)
}

func viewHandler(w http.ResponseWriter, r *http.Request, title string) {
	p, err := loadPage(title)
	if err != nil {
		http.Redirect(w, r, "/edit/"+title, http.StatusFound)
		return
	}
	renderTemplate(w, "view", p)
}

func editHandler(w http.ResponseWriter, r *http.Request, title string) {
	p, err := loadPage(title)
	if err != nil {
		p = &Page{Title: title}
	}
	renderTemplate(w, "edit", p)
}

func saveHandler(w http.ResponseWriter, r *http.Request, title string) {
	body := r.FormValue("body")
	p := &Page{Title: title, Body: []byte(body)}
	err := p.save()
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	http.Redirect(w, r, "/view/"+title, http.StatusFound)
}

func redirectFrontPage(w http.ResponseWriter, r *http.Request) {
	http.Redirect(w, r, "/view/FrontPage", http.StatusFound)
}

func main() {
	http.HandleFunc("/", redirectFrontPage)
	http.HandleFunc("/view/", makeHandler(viewHandler))
	http.HandleFunc("/edit/", makeHandler(editHandler))
	http.HandleFunc("/save/", makeHandler(saveHandler))
	http.ListenAndServe(":8080", nil)
}