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doc: update wiki tutorial templates, and template discussion

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Fixes #3384.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5915044
This commit is contained in:
Andrew Gerrand 2012-03-27 16:07:46 +11:00
parent 7a39654174
commit d98507f1c4
3 changed files with 58 additions and 1125 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
doc/articles/wiki/final.go
View File

@ -58,17 +58,10 @@ func saveHandler(w http.ResponseWriter, r *http.Request, title string) {
http.Redirect(w, r, "/view/"+title, http.StatusFound)
}
var templates = make(map[string]*template.Template)
func init() {
for _, tmpl := range []string{"edit", "view"} {
t := template.Must(template.ParseFiles(tmpl + ".html"))
templates[tmpl] = t
}
}
var templates = template.Must(template.ParseFiles("edit.html", "view.html"))
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
err := templates[tmpl].Execute(w, p)
err := templates.ExecuteTemplate(w, tmpl+".html", p)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}

393
doc/articles/wiki/index.html
View File

@ -1,5 +1,6 @@
<!--{
"Title": "Writing Web Applications"
"Title": "Writing Web Applications",
"Template": true
}-->
<h2>Introduction</h2>
@ -36,7 +37,7 @@ Install Go (see the <a href="/doc/install">Installation Instructions</a>).
</p>
<p>
Make a new directory for this tutorial and cd to it:
Make a new directory for this tutorial inside your <code>GOPATH</code> and cd to it:
</p>
<pre>
@ -73,12 +74,7 @@ Here, we define <code>Page</code> as a struct with two fields representing
the title and body.
</p>
<pre>
type Page struct {
Title string
Body []byte
}
</pre>
{{code "doc/articles/wiki/part1.go" `/^type Page/` `/}/`}}
<p>
The type <code>[]byte</code> means "a <code>byte</code> slice".
@ -95,12 +91,7 @@ But what about persistent storage? We can address that by creating a
<code>save</code> method on <code>Page</code>:
</p>
<pre>
func (p *Page) save() error {
filename := p.Title + &#34;.txt&#34;
return ioutil.WriteFile(filename, p.Body, 0600)
}
</pre>
{{code "doc/articles/wiki/part1.go" `/^func.*Page.*save/` `/}/`}}
<p>
This method's signature reads: "This is a method named <code>save</code> that
@ -134,13 +125,7 @@ read-write permissions for the current user only. (See the Unix man page
We will want to load pages, too:
</p>
<pre>
func loadPage(title string) *Page {
filename := title + &#34;.txt&#34;
body, _ := ioutil.ReadFile(filename)
return &amp;Page{Title: title, Body: body}
}
</pre>
{{code "doc/articles/wiki/part1-noerror.go" `/^func loadPage/` `/^}/`}}
<p>
The function <code>loadPage</code> constructs the file name from
@ -162,16 +147,7 @@ the file might not exist. We should not ignore such errors. Let's modify the
function to return <code>*Page</code> and <code>error</code>.
</p>
<pre>
func loadPage(title string) (*Page, error) {
filename := title + &#34;.txt&#34;
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &amp;Page{Title: title, Body: body}, nil
}
</pre>
{{code "doc/articles/wiki/part1.go" `/^func loadPage/` `/^}/`}}
<p>
Callers of this function can now check the second parameter; if it is
@ -186,14 +162,7 @@ load from a file. Let's write a <code>main</code> function to test what we've
written:
</p>
<pre>
func main() {
p1 := &amp;Page{Title: &#34;TestPage&#34;, Body: []byte(&#34;This is a sample Page.&#34;)}
p1.save()
p2, _ := loadPage(&#34;TestPage&#34;)
fmt.Println(string(p2.Body))
}
</pre>
{{code "doc/articles/wiki/part1.go" `/^func main/` `/^}/`}}
<p>
After compiling and executing this code, a file named <code>TestPage.txt</code>
@ -227,23 +196,7 @@ This is a sample page.
Here's a full working example of a simple web server:
</p>
<pre>
package main
import (
&#34;fmt&#34;
&#34;net/http&#34;
)
func handler(w http.ResponseWriter, r *http.Request) {
fmt.Fprintf(w, &#34;Hi there, I love %s!&#34;, r.URL.Path[1:])
}
func main() {
http.HandleFunc(&#34;/&#34;, handler)
http.ListenAndServe(&#34;:8080&#34;, nil)
}
</pre>
{{code "doc/articles/wiki/http-sample.go"}}
<p>
The <code>main</code> function begins with a call to
@ -305,15 +258,9 @@ import (
Let's create a handler to view a wiki page:
</p>
<pre>
const lenPath = len(&#34;/view/&#34;)
{{code "doc/articles/wiki/part2.go" `/^const lenPath/`}}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, _ := loadPage(title)
fmt.Fprintf(w, &#34;&lt;h1&gt;%s&lt;/h1&gt;&lt;div&gt;%s&lt;/div&gt;&#34;, p.Title, p.Body)
}
</pre>
{{code "doc/articles/wiki/part2.go" `/^func viewHandler/` `/^}/`}}
<p>
First, this function extracts the page title from <code>r.URL.Path</code>,
@ -342,12 +289,7 @@ initializes <code>http</code> using the <code>viewHandler</code> to handle
any requests under the path <code>/view/</code>.
</p>
<pre>
func main() {
http.HandleFunc(&#34;/view/&#34;, viewHandler)
http.ListenAndServe(&#34;:8080&#34;, nil)
}
</pre>
{{code "doc/articles/wiki/part2.go" `/^func main/` `/^}/`}}
<p>
<a href="part2.go">Click here to view the code we've written so far.</a>
@ -387,14 +329,7 @@ form.
First, we add them to <code>main()</code>:
</p>
<pre>
func main() {
http.HandleFunc(&#34;/view/&#34;, viewHandler)
http.HandleFunc(&#34;/edit/&#34;, editHandler)
http.HandleFunc(&#34;/save/&#34;, saveHandler)
http.ListenAndServe(&#34;:8080&#34;, nil)
}
</pre>
{{code "doc/articles/wiki/final-noclosure.go" `/^func main/` `/^}/`}}
<p>
The function <code>editHandler</code> loads the page
@ -402,21 +337,7 @@ The function <code>editHandler</code> loads the page
and displays an HTML form.
</p>
<pre>
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, err := loadPage(title)
if err != nil {
p = &amp;Page{Title: title}
}
fmt.Fprintf(w, &#34;&lt;h1&gt;Editing %s&lt;/h1&gt;&#34;+
&#34;&lt;form action=\&#34;/save/%s\&#34; method=\&#34;POST\&#34;&gt;&#34;+
&#34;&lt;textarea name=\&#34;body\&#34;&gt;%s&lt;/textarea&gt;&lt;br&gt;&#34;+
&#34;&lt;input type=\&#34;submit\&#34; value=\&#34;Save\&#34;&gt;&#34;+
&#34;&lt;/form&gt;&#34;,
p.Title, p.Title, p.Body)
}
</pre>
{{code "doc/articles/wiki/notemplate.go" `/^func editHandler/` `/^}/`}}
<p>
This function will work fine, but all that hard-coded HTML is ugly.
@ -450,31 +371,14 @@ Let's create a template file containing the HTML form.
Open a new file named <code>edit.html</code>, and add the following lines:
</p>
<pre>
&lt;h1&gt;Editing {{.Title |html}}&lt;/h1&gt;
&lt;form action=&#34;/save/{{.Title |html}}&#34; method=&#34;POST&#34;&gt;
&lt;div&gt;&lt;textarea name=&#34;body&#34; rows=&#34;20&#34; cols=&#34;80&#34;&gt;{{printf &#34;%s&#34; .Body |html}}&lt;/textarea&gt;&lt;/div&gt;
&lt;div&gt;&lt;input type=&#34;submit&#34; value=&#34;Save&#34;&gt;&lt;/div&gt;
&lt;/form&gt;
</pre>
{{code "doc/articles/wiki/edit.html"}}
<p>
Modify <code>editHandler</code> to use the template, instead of the hard-coded
HTML:
</p>
<pre>
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, err := loadPage(title)
if err != nil {
p = &amp;Page{Title: title}
}
t, _ := template.ParseFiles(&#34;edit.html&#34;)
t.Execute(w, p)
}
</pre>
{{code "doc/articles/wiki/final-noerror.go" `/^func editHandler/` `/^}/`}}
<p>
The function <code>template.ParseFiles</code> will read the contents of
@ -509,26 +413,13 @@ While we're working with templates, let's create a template for our
<code>viewHandler</code> called <code>view.html</code>:
</p>
<pre>
&lt;h1&gt;{{.Title |html}}&lt;/h1&gt;
&lt;p&gt;[&lt;a href=&#34;/edit/{{.Title |html}}&#34;&gt;edit&lt;/a&gt;]&lt;/p&gt;
&lt;div&gt;{{printf &#34;%s&#34; .Body |html}}&lt;/div&gt;
</pre>
{{code "doc/articles/wiki/view.html"}}
<p>
Modify <code>viewHandler</code> accordingly:
</p>
<pre>
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, _ := loadPage(title)
t, _ := template.ParseFiles(&#34;view.html&#34;)
t.Execute(w, p)
}
</pre>
{{code "doc/articles/wiki/final-noerror.go" `/^func viewHandler/` `/^}/`}}
<p>
Notice that we've used almost exactly the same templating code in both
@ -536,27 +427,9 @@ handlers. Let's remove this duplication by moving the templating code
to its own function:
</p>
<pre>
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, _ := loadPage(title)
renderTemplate(w, &#34;view&#34;, p)
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, err := loadPage(title)
if err != nil {
p = &amp;Page{Title: title}
}
renderTemplate(w, &#34;edit&#34;, p)
}
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, _ := template.ParseFiles(tmpl + &#34;.html&#34;)
t.Execute(w, p)
}
</pre>
{{code "doc/articles/wiki/final-template.go" `/^func viewHandler/` `/^}/`}}
{{code "doc/articles/wiki/final-template.go" `/^func editHandler/` `/^}/`}}
{{code "doc/articles/wiki/final-template.go" `/^func renderTemplate/` `/^}/`}}
<p>
The handlers are now shorter and simpler.
@ -572,20 +445,7 @@ if the requested Page doesn't exist, it should redirect the client to the edit
Page so the content may be created:
</p>
<pre>
func viewHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
p, err := loadPage(title)
if err != nil {
http.Redirect(w, r, &#34;/edit/&#34;+title, http.StatusFound)
return
}
renderTemplate(w, &#34;view&#34;, p)
}
</pre>
{{code "doc/articles/wiki/final-noclosure.go" `/^func viewHandler/` `/^}/`}}
<p>
The <code>http.Redirect</code> function adds an HTTP status code of
@ -599,15 +459,7 @@ header to the HTTP response.
The function <code>saveHandler</code> will handle the form submission.
</p>
<pre>
func saveHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
body := r.FormValue(&#34;body&#34;)
p := &amp;Page{Title: title, Body: []byte(body)}
p.save()
http.Redirect(w, r, &#34;/view/&#34;+title, http.StatusFound)
}
</pre>
{{code "doc/articles/wiki/final-template.go" `/^func saveHandler/` `/^}/`}}
<p>
The page title (provided in the URL) and the form's only field,
@ -637,19 +489,7 @@ function and the user will be notified.
First, let's handle the errors in <code>renderTemplate</code>:
</p>
<pre>
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, err := template.ParseFiles(tmpl + &#34;.html&#34;)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
err = t.Execute(w, p)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
</pre>
{{code "doc/articles/wiki/final-parsetemplate.go" `/^func renderTemplate/` `/^}/`}}
<p>
The <code>http.Error</code> function sends a specified HTTP response code
@ -661,22 +501,7 @@ Already the decision to put this in a separate function is paying off.
Now let's fix up <code>saveHandler</code>:
</p>
<pre>
func saveHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
body := r.FormValue(&#34;body&#34;)
p := &amp;Page{Title: title, Body: []byte(body)}
err = p.save()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
http.Redirect(w, r, &#34;/view/&#34;+title, http.StatusFound)
}
</pre>
{{code "doc/articles/wiki/final-noclosure.go" `/^func saveHandler/` `/^}/`}}
<p>
Any errors that occur during <code>p.save()</code> will be reported
@ -687,40 +512,28 @@ to the user.
<p>
There is an inefficiency in this code: <code>renderTemplate</code> calls
<code>ParseFile</code> every time a page is rendered.
A better approach would be to call <code>ParseFile</code> once for each
template at program initialization, and store the resultant
<code>*Template</code> values in a data structure for later use.
<code>ParseFiles</code> every time a page is rendered.
A better approach would be to call <code>ParseFiles</code> once at program
initialization, parsing all templates into a single <code>*Template</code>.
Then we can use the
<a href="/pkg/html/template/#Template.ExecuteTemplate"><code>ExecuteTemplate</code></a>
method to render a specific template.
</p>
<p>
First we create a global map named <code>templates</code> in which to store
our <code>*Template</code> values, keyed by <code>string</code>
(the template name):
First we create a global variable named <code>templates</code>, and initialize
it with <code>ParseFiles</code>.
</p>
<pre>
var templates = make(map[string]*template.Template)
</pre>
{{code "doc/articles/wiki/final.go" `/var templates/`}}
<p>
Then we create an <code>init</code> function, which will be called before
<code>main</code> at program initialization. The function
<code>template.Must</code> is a convenience wrapper that panics when passed a
non-nil <code>error</code> value, and otherwise returns the
The function <code>template.Must</code> is a convenience wrapper that panics
when passed a non-nil <code>error</code> value, and otherwise returns the
<code>*Template</code> unaltered. A panic is appropriate here; if the templates
can't be loaded the only sensible thing to do is exit the program.
</p>
<pre>
func init() {
for _, tmpl := range []string{&#34;edit&#34;, &#34;view&#34;} {
t := template.Must(template.ParseFiles(tmpl + &#34;.html&#34;))
templates[tmpl] = t
}
}
</pre>
<p>
A <code>for</code> loop is used with a <code>range</code> statement to iterate
over an array constant containing the names of the templates we want parsed.
@ -729,18 +542,17 @@ that array.
</p>
<p>
We then modify our <code>renderTemplate</code> function to call
the <code>Execute</code> method on the appropriate <code>Template</code> from
<code>templates</code>:
We then modify the <code>renderTemplate</code> function to call the
<code>templates.ExecuteTemplate</code> method with the name of the appropriate
template:
</p>
<pre>
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
err := templates[tmpl].Execute(w, p)
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
}
}
</pre>
{{code "doc/articles/wiki/final.go" `/func renderTemplate/` `/^}/`}}
<p>
Note that the template name is the template file name, so we must
append <code>".html"</code> to the <code>tmpl</code> argument.
</p>
<h2>Validation</h2>
@ -755,9 +567,7 @@ First, add <code>"regexp"</code> to the <code>import</code> list.
Then we can create a global variable to store our validation regexp:
</p>
<pre>
var titleValidator = regexp.MustCompile(&#34;^[a-zA-Z0-9]+$&#34;)
</pre>
{{code "doc/articles/wiki/final-noclosure.go" `/^var titleValidator/`}}
<p>
The function <code>regexp.MustCompile</code> will parse and compile the
@ -772,16 +582,7 @@ Now, let's write a function that extracts the title string from the request
URL, and tests it against our <code>TitleValidator</code> expression:
</p>
<pre>
func getTitle(w http.ResponseWriter, r *http.Request) (title string, err error) {
title = r.URL.Path[lenPath:]
if !titleValidator.MatchString(title) {
http.NotFound(w, r)
err = errors.New(&#34;Invalid Page Title&#34;)
}
return
}
</pre>
{{code "doc/articles/wiki/final-noclosure.go" `/func getTitle/` `/^}/`}}
<p>
If the title is valid, it will be returned along with a <code>nil</code>
@ -794,47 +595,9 @@ handler.
Let's put a call to <code>getTitle</code> in each of the handlers:
</p>
<pre>
func viewHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
p, err := loadPage(title)
if err != nil {
http.Redirect(w, r, &#34;/edit/&#34;+title, http.StatusFound)
return
}
renderTemplate(w, &#34;view&#34;, p)
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
p, err := loadPage(title)
if err != nil {
p = &amp;Page{Title: title}
}
renderTemplate(w, &#34;edit&#34;, p)
}
func saveHandler(w http.ResponseWriter, r *http.Request) {
title, err := getTitle(w, r)
if err != nil {
return
}
body := r.FormValue(&#34;body&#34;)
p := &amp;Page{Title: title, Body: []byte(body)}
err = p.save()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
http.Redirect(w, r, &#34;/view/&#34;+title, http.StatusFound)
}
</pre>
{{code "doc/articles/wiki/final-noclosure.go" `/^func viewHandler/` `/^}/`}}
{{code "doc/articles/wiki/final-noclosure.go" `/^func editHandler/` `/^}/`}}
{{code "doc/articles/wiki/final-noclosure.go" `/^func saveHandler/` `/^}/`}}
<h2>Introducing Function Literals and Closures</h2>
@ -885,18 +648,7 @@ Now we can take the code from <code>getTitle</code> and use it here
(with some minor modifications):
</p>
<pre>
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)
}
}
</pre>
{{code "doc/articles/wiki/final.go" `/func makeHandler/` `/^}/`}}
<p>
The closure returned by <code>makeHandler</code> is a function that takes
@ -917,49 +669,16 @@ Now we can wrap the handler functions with <code>makeHandler</code> in
package:
</p>
<pre>
func main() {
http.HandleFunc(&#34;/view/&#34;, makeHandler(viewHandler))
http.HandleFunc(&#34;/edit/&#34;, makeHandler(editHandler))
http.HandleFunc(&#34;/save/&#34;, makeHandler(saveHandler))
http.ListenAndServe(&#34;:8080&#34;, nil)
}
</pre>
{{code "doc/articles/wiki/final.go" `/func main/` `/^}/`}}
<p>
Finally we remove the calls to <code>getTitle</code> from the handler functions,
making them much simpler:
</p>
<pre>
func viewHandler(w http.ResponseWriter, r *http.Request, title string) {
p, err := loadPage(title)
if err != nil {
http.Redirect(w, r, &#34;/edit/&#34;+title, http.StatusFound)
return
}
renderTemplate(w, &#34;view&#34;, p)
}
func editHandler(w http.ResponseWriter, r *http.Request, title string) {
p, err := loadPage(title)
if err != nil {
p = &amp;Page{Title: title}
}
renderTemplate(w, &#34;edit&#34;, p)
}
func saveHandler(w http.ResponseWriter, r *http.Request, title string) {
body := r.FormValue(&#34;body&#34;)
p := &amp;Page{Title: title, Body: []byte(body)}
err := p.save()
if err != nil {
http.Error(w, err.Error(), http.StatusInternalServerError)
return
}
http.Redirect(w, r, &#34;/view/&#34;+title, http.StatusFound)
}
</pre>
{{code "doc/articles/wiki/final.go" `/^func viewHandler/` `/^}/`}}
{{code "doc/articles/wiki/final.go" `/^func editHandler/` `/^}/`}}
{{code "doc/articles/wiki/final.go" `/^func saveHandler/` `/^}/`}}
<h2>Try it out!</h2>

779
doc/articles/wiki/wiki.html
View File

@ -1,779 +0,0 @@
<!--{
"Title": "Writing Web Applications"
}-->
<h2>Introduction</h2>
<p>
Covered in this tutorial:
</p>
<ul>
<li>Creating a data structure with load and save methods</li>
<li>Using the <code>net/http</code> package to build web applications
<li>Using the <code>html/template</code> package to process HTML templates</li>
<li>Using the <code>regexp</code> package to validate user input</li>
<li>Using closures</li>
</ul>
<p>
Assumed knowledge:
</p>
<ul>
<li>Programming experience</li>
<li>Understanding of basic web technologies (HTTP, HTML)</li>
<li>Some UNIX/DOS command-line knowledge</li>
</ul>
<h2>Getting Started</h2>
<p>
At present, you need to have a FreeBSD, Linux, OS X, or Windows machine to run Go.
We will use <code>$</code> to represent the command prompt.
</p>
<p>
Install Go (see the <a href="/doc/install">Installation Instructions</a>).
</p>
<p>
Make a new directory for this tutorial inside your <code>GOPATH</code> and cd to it:
</p>
<pre>
$ mkdir gowiki
$ cd gowiki
</pre>
<p>
Create a file named <code>wiki.go</code>, open it in your favorite editor, and
add the following lines:
</p>
<pre>
package main
import (
"fmt"
"io/ioutil"
)
</pre>
<p>
We import the <code>fmt</code> and <code>ioutil</code> packages from the Go
standard library. Later, as we implement additional functionality, we will
add more packages to this <code>import</code> declaration.
</p>
<h2>Data Structures</h2>
<p>
Let's start by defining the data structures. A wiki consists of a series of
interconnected pages, each of which has a title and a body (the page content).
Here, we define <code>Page</code> as a struct with two fields representing
the title and body.
</p>
<pre>
!srcextract.bin -src=part1.go -name=Page
</pre>
<p>
The type <code>[]byte</code> means "a <code>byte</code> slice".
(See <a href="/doc/articles/slices_usage_and_internals.html">Slices: usage and
internals</a> for more on slices.)
The <code>Body</code> element is a <code>[]byte</code> rather than
<code>string</code> because that is the type expected by the <code>io</code>
libraries we will use, as you'll see below.
</p>
<p>
The <code>Page</code> struct describes how page data will be stored in memory.
But what about persistent storage? We can address that by creating a
<code>save</code> method on <code>Page</code>:
</p>
<pre>
!srcextract.bin -src=part1.go -name=save
</pre>
<p>
This method's signature reads: "This is a method named <code>save</code> that
takes as its receiver <code>p</code>, a pointer to <code>Page</code> . It takes
no parameters, and returns a value of type <code>error</code>."
</p>
<p>
This method will save the <code>Page</code>'s <code>Body</code> to a text
file. For simplicity, we will use the <code>Title</code> as the file name.
</p>
<p>
The <code>save</code> method returns an <code>error</code> value because
that is the return type of <code>WriteFile</code> (a standard library function
that writes a byte slice to a file). The <code>save</code> method returns the
error value, to let the application handle it should anything go wrong while
writing the file. If all goes well, <code>Page.save()</code> will return
<code>nil</code> (the zero-value for pointers, interfaces, and some other
types).
</p>
<p>
The octal integer constant <code>0600</code>, passed as the third parameter to
<code>WriteFile</code>, indicates that the file should be created with
read-write permissions for the current user only. (See the Unix man page
<code>open(2)</code> for details.)
</p>
<p>
We will want to load pages, too:
</p>
<pre>
!srcextract.bin -src=part1-noerror.go -name=loadPage
</pre>
<p>
The function <code>loadPage</code> constructs the file name from
<code>Title</code>, reads the file's contents into a new
<code>Page</code>, and returns a pointer to that new <code>page</code>.
</p>
<p>
Functions can return multiple values. The standard library function
<code>io.ReadFile</code> returns <code>[]byte</code> and <code>error</code>.
In <code>loadPage</code>, error isn't being handled yet; the "blank identifier"
represented by the underscore (<code>_</code>) symbol is used to throw away the
error return value (in essence, assigning the value to nothing).
</p>
<p>
But what happens if <code>ReadFile</code> encounters an error? For example,
the file might not exist. We should not ignore such errors. Let's modify the
function to return <code>*Page</code> and <code>error</code>.
</p>
<pre>
!srcextract.bin -src=part1.go -name=loadPage
</pre>
<p>
Callers of this function can now check the second parameter; if it is
<code>nil</code> then it has successfully loaded a Page. If not, it will be an
<code>error</code> that can be handled by the caller (see the
<a href="/ref/spec#Errors">language specification</a> for details).
</p>
<p>
At this point we have a simple data structure and the ability to save to and
load from a file. Let's write a <code>main</code> function to test what we've
written:
</p>
<pre>
!srcextract.bin -src=part1.go -name=main
</pre>
<p>
After compiling and executing this code, a file named <code>TestPage.txt</code>
would be created, containing the contents of <code>p1</code>. The file would
then be read into the struct <code>p2</code>, and its <code>Body</code> element
printed to the screen.
</p>
<p>
You can compile and run the program like this:
</p>
<pre>
$ go build wiki.go
$ ./wiki
This is a sample page.
</pre>
<p>
(If you're using Windows you must type "<code>wiki</code>" without the
"<code>./</code>" to run the program.)
</p>
<p>
<a href="part1.go">Click here to view the code we've written so far.</a>
</p>
<h2>Introducing the <code>net/http</code> package (an interlude)</h2>
<p>
Here's a full working example of a simple web server:
</p>
<pre>
!htmlify.bin < http-sample.go
</pre>
<p>
The <code>main</code> function begins with a call to
<code>http.HandleFunc</code>, which tells the <code>http</code> package to
handle all requests to the web root (<code>"/"</code>) with
<code>handler</code>.
</p>
<p>
It then calls <code>http.ListenAndServe</code>, specifying that it should
listen on port 8080 on any interface (<code>":8080"</code>). (Don't
worry about its second parameter, <code>nil</code>, for now.)
This function will block until the program is terminated.
</p>
<p>
The function <code>handler</code> is of the type <code>http.HandlerFunc</code>.
It takes an <code>http.ResponseWriter</code> and an <code>http.Request</code> as
its arguments.
</p>
<p>
An <code>http.ResponseWriter</code> value assembles the HTTP server's response; by writing
to it, we send data to the HTTP client.
</p>
<p>
An <code>http.Request</code> is a data structure that represents the client
HTTP request. The string <code>r.URL.Path</code> is the path component
of the request URL. The trailing <code>[1:]</code> means
"create a sub-slice of <code>Path</code> from the 1st character to the end."
This drops the leading "/" from the path name.
</p>
<p>
If you run this program and access the URL:
</p>
<pre>http://localhost:8080/monkeys</pre>
<p>
the program would present a page containing:
</p>
<pre>Hi there, I love monkeys!</pre>
<h2>Using <code>net/http</code> to serve wiki pages</h2>
<p>
To use the <code>net/http</code> package, it must be imported:
</p>
<pre>
import (
"fmt"
<b>"net/http"</b>
"io/ioutil"
)
</pre>
<p>
Let's create a handler to view a wiki page:
</p>
<pre>
!srcextract.bin -src=part2.go -name=lenPath
!srcextract.bin -src=part2.go -name=viewHandler
</pre>
<p>
First, this function extracts the page title from <code>r.URL.Path</code>,
the path component of the request URL. The global constant
<code>lenPath</code> is the length of the leading <code>"/view/"</code>
component of the request path.
The <code>Path</code> is re-sliced with <code>[lenPath:]</code> to drop the
first 6 characters of the string. This is because the path will invariably
begin with <code>"/view/"</code>, which is not part of the page title.
</p>
<p>
The function then loads the page data, formats the page with a string of simple
HTML, and writes it to <code>w</code>, the <code>http.ResponseWriter</code>.
</p>
<p>
Again, note the use of <code>_</code> to ignore the <code>error</code>
return value from <code>loadPage</code>. This is done here for simplicity
and generally considered bad practice. We will attend to this later.
</p>
<p>
To use this handler, we create a <code>main</code> function that
initializes <code>http</code> using the <code>viewHandler</code> to handle
any requests under the path <code>/view/</code>.
</p>
<pre>
!srcextract.bin -src=part2.go -name=main
</pre>
<p>
<a href="part2.go">Click here to view the code we've written so far.</a>
</p>
<p>
Let's create some page data (as <code>test.txt</code>), compile our code, and
try serving a wiki page.
</p>
<p>
Open <code>test.txt</code> file in your editor, and save the string "Hello world" (without quotes)
in it.
</p>
<pre>
$ go build wiki.go
$ ./wiki
</pre>
<p>
With this web server running, a visit to <code><a
href="http://localhost:8080/view/test">http://localhost:8080/view/test</a></code>
should show a page titled "test" containing the words "Hello world".
</p>
<h2>Editing Pages</h2>
<p>
A wiki is not a wiki without the ability to edit pages. Let's create two new
handlers: one named <code>editHandler</code> to display an 'edit page' form,
and the other named <code>saveHandler</code> to save the data entered via the
form.
</p>
<p>
First, we add them to <code>main()</code>:
</p>
<pre>
!srcextract.bin -src=final-noclosure.go -name=main
</pre>
<p>
The function <code>editHandler</code> loads the page
(or, if it doesn't exist, create an empty <code>Page</code> struct),
and displays an HTML form.
</p>
<pre>
!srcextract.bin -src=notemplate.go -name=editHandler
</pre>
<p>
This function will work fine, but all that hard-coded HTML is ugly.
Of course, there is a better way.
</p>
<h2>The <code>html/template</code> package</h2>
<p>
The <code>html/template</code> package is part of the Go standard library.
We can use <code>html/template</code> to keep the HTML in a separate file,
allowing us to change the layout of our edit page without modifying the
underlying Go code.
</p>
<p>
First, we must add <code>html/template</code> to the list of imports:
</p>
<pre>
import (
"http"
"io/ioutil"
"os"
<b>"html/template"</b>
)
</pre>
<p>
Let's create a template file containing the HTML form.
Open a new file named <code>edit.html</code>, and add the following lines:
</p>
<pre>
!htmlify.bin < edit.html
</pre>
<p>
Modify <code>editHandler</code> to use the template, instead of the hard-coded
HTML:
</p>
<pre>
!srcextract.bin -src=final-noerror.go -name=editHandler
</pre>
<p>
The function <code>template.ParseFiles</code> will read the contents of
<code>edit.html</code> and return a <code>*template.Template</code>.
</p>
<p>
The method <code>t.Execute</code> executes the template, writing the
generated HTML to the <code>http.ResponseWriter</code>.
The <code>.Title</code> and <code>.Body</code> dotted identifiers refer to
<code>p.Title</code> and <code>p.Body</code>.
</p>
<p>
Template directives are enclosed in double curly braces.
The <code>printf "%s" .Body</code> instruction is a function call
that outputs <code>.Body</code> as a string instead of a stream of bytes,
the same as a call to <code>fmt.Printf</code>.
The <code>|html</code> part of each directive pipes the value through the
<code>html</code> formatter before outputting it, which escapes HTML
characters (such as replacing <code>&gt;</code> with <code>&amp;gt;</code>),
preventing user data from corrupting the form HTML.
</p>
<p>
Now that we've removed the <code>fmt.Fprintf</code> statement, we can remove
<code>"fmt"</code> from the <code>import</code> list.
</p>
<p>
While we're working with templates, let's create a template for our
<code>viewHandler</code> called <code>view.html</code>:
</p>
<pre>
!htmlify.bin < view.html
</pre>
<p>
Modify <code>viewHandler</code> accordingly:
</p>
<pre>
!srcextract.bin -src=final-noerror.go -name=viewHandler
</pre>
<p>
Notice that we've used almost exactly the same templating code in both
handlers. Let's remove this duplication by moving the templating code
to its own function:
</p>
<pre>
!srcextract.bin -src=final-template.go -name=viewHandler
!srcextract.bin -src=final-template.go -name=editHandler
!srcextract.bin -src=final-template.go -name=renderTemplate
</pre>
<p>
The handlers are now shorter and simpler.
</p>
<h2>Handling non-existent pages</h2>
<p>
What if you visit <a href="http://localhost:8080/view/APageThatDoesntExist">
<code>/view/APageThatDoesntExist</code></a>? The program will crash. This is
because it ignores the error return value from <code>loadPage</code>. Instead,
if the requested Page doesn't exist, it should redirect the client to the edit
Page so the content may be created:
</p>
<pre>
!srcextract.bin -src=final-noclosure.go -name=viewHandler
</pre>
<p>
The <code>http.Redirect</code> function adds an HTTP status code of
<code>http.StatusFound</code> (302) and a <code>Location</code>
header to the HTTP response.
</p>
<h2>Saving Pages</h2>
<p>
The function <code>saveHandler</code> will handle the form submission.
</p>
<pre>
!srcextract.bin -src=final-template.go -name=saveHandler
</pre>
<p>
The page title (provided in the URL) and the form's only field,
<code>Body</code>, are stored in a new <code>Page</code>.
The <code>save()</code> method is then called to write the data to a file,
and the client is redirected to the <code>/view/</code> page.
</p>
<p>
The value returned by <code>FormValue</code> is of type <code>string</code>.
We must convert that value to <code>[]byte</code> before it will fit into
the <code>Page</code> struct. We use <code>[]byte(body)</code> to perform
the conversion.
</p>
<h2>Error handling</h2>
<p>
There are several places in our program where errors are being ignored. This
is bad practice, not least because when an error does occur the program will
crash. A better solution is to handle the errors and return an error message
to the user. That way if something does go wrong, the server will continue to
function and the user will be notified.
</p>
<p>
First, let's handle the errors in <code>renderTemplate</code>:
</p>
<pre>
!srcextract.bin -src=final-parsetemplate.go -name=renderTemplate
</pre>
<p>
The <code>http.Error</code> function sends a specified HTTP response code
(in this case "Internal Server Error") and error message.
Already the decision to put this in a separate function is paying off.
</p>
<p>
Now let's fix up <code>saveHandler</code>:
</p>
<pre>
!srcextract.bin -src=final-noclosure.go -name=saveHandler
</pre>
<p>
Any errors that occur during <code>p.save()</code> will be reported
to the user.
</p>
<h2>Template caching</h2>
<p>
There is an inefficiency in this code: <code>renderTemplate</code> calls
<code>ParseFiles</code> every time a page is rendered.
A better approach would be to call <code>ParseFiles</code> once for each
template at program initialization, and store the resultant
<code>*Template</code> values in a data structure for later use.
</p>
<p>
First we create a global map named <code>templates</code> in which to store
our <code>*Template</code> values, keyed by <code>string</code>
(the template name):
</p>
<pre>
!srcextract.bin -src=final.go -name=templates
</pre>
<p>
Then we create an <code>init</code> function, which will be called before
<code>main</code> at program initialization. The function
<code>template.Must</code> is a convenience wrapper that panics when passed a
non-nil <code>error</code> value, and otherwise returns the
<code>*Template</code> unaltered. A panic is appropriate here; if the templates
can't be loaded the only sensible thing to do is exit the program.
</p>
<pre>
!srcextract.bin -src=final.go -name=init
</pre>
<p>
A <code>for</code> loop is used with a <code>range</code> statement to iterate
over an array constant containing the names of the templates we want parsed.
If we were to add more templates to our program, we would add their names to
that array.
</p>
<p>
We then modify our <code>renderTemplate</code> function to call
the <code>Execute</code> method on the appropriate <code>Template</code> from
<code>templates</code>:
<pre>
!srcextract.bin -src=final.go -name=renderTemplate
</pre>
<h2>Validation</h2>
<p>
As you may have observed, this program has a serious security flaw: a user
can supply an arbitrary path to be read/written on the server. To mitigate
this, we can write a function to validate the title with a regular expression.
</p>
<p>
First, add <code>"regexp"</code> to the <code>import</code> list.
Then we can create a global variable to store our validation regexp:
</p>
<pre>
!srcextract.bin -src=final-noclosure.go -name=titleValidator
</pre>
<p>
The function <code>regexp.MustCompile</code> will parse and compile the
regular expression, and return a <code>regexp.Regexp</code>.
<code>MustCompile</code> is distinct from <code>Compile</code> in that it will
panic if the expression compilation fails, while <code>Compile</code> returns
an <code>error</code> as a second parameter.
</p>
<p>
Now, let's write a function that extracts the title string from the request
URL, and tests it against our <code>TitleValidator</code> expression:
</p>
<pre>
!srcextract.bin -src=final-noclosure.go -name=getTitle
</pre>
<p>
If the title is valid, it will be returned along with a <code>nil</code>
error value. If the title is invalid, the function will write a
"404 Not Found" error to the HTTP connection, and return an error to the
handler.
</p>
<p>
Let's put a call to <code>getTitle</code> in each of the handlers:
</p>
<pre>
!srcextract.bin -src=final-noclosure.go -name=viewHandler
!srcextract.bin -src=final-noclosure.go -name=editHandler
!srcextract.bin -src=final-noclosure.go -name=saveHandler
</pre>
<h2>Introducing Function Literals and Closures</h2>
<p>
Catching the error condition in each handler introduces a lot of repeated code.
What if we could wrap each of the handlers in a function that does this
validation and error checking? Go's
<a href="/ref/spec#Function_declarations">function
literals</a> provide a powerful means of abstracting functionality
that can help us here.
</p>
<p>
First, we re-write the function definition of each of the handlers to accept
a title string:
</p>
<pre>
func viewHandler(w http.ResponseWriter, r *http.Request, title string)
func editHandler(w http.ResponseWriter, r *http.Request, title string)
func saveHandler(w http.ResponseWriter, r *http.Request, title string)
</pre>
<p>
Now let's define a wrapper function that <i>takes a function of the above
type</i>, and returns a function of type <code>http.HandlerFunc</code>
(suitable to be passed to the function <code>http.HandleFunc</code>):
</p>
<pre>
func makeHandler(fn func (http.ResponseWriter, *http.Request, string)) http.HandlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
// Here we will extract the page title from the Request,
// and call the provided handler 'fn'
}
}
</pre>
<p>
The returned function is called a closure because it encloses values defined
outside of it. In this case, the variable <code>fn</code> (the single argument
to <code>makeHandler</code>) is enclosed by the closure. The variable
<code>fn</code> will be one of our save, edit, or view handlers.
</p>
<p>
Now we can take the code from <code>getTitle</code> and use it here
(with some minor modifications):
</p>
<pre>
!srcextract.bin -src=final.go -name=makeHandler
</pre>
<p>
The closure returned by <code>makeHandler</code> is a function that takes
an <code>http.ResponseWriter</code> and <code>http.Request</code> (in other
words, an <code>http.HandlerFunc</code>).
The closure extracts the <code>title</code> from the request path, and
validates it with the <code>TitleValidator</code> regexp. If the
<code>title</code> is invalid, an error will be written to the
<code>ResponseWriter</code> using the <code>http.NotFound</code> function.
If the <code>title</code> is valid, the enclosed handler function
<code>fn</code> will be called with the <code>ResponseWriter</code>,
<code>Request</code>, and <code>title</code> as arguments.
</p>
<p>
Now we can wrap the handler functions with <code>makeHandler</code> in
<code>main</code>, before they are registered with the <code>http</code>
package:
</p>
<pre>
!srcextract.bin -src=final.go -name=main
</pre>
<p>
Finally we remove the calls to <code>getTitle</code> from the handler functions,
making them much simpler:
</p>
<pre>
!srcextract.bin -src=final.go -name=viewHandler
!srcextract.bin -src=final.go -name=editHandler
!srcextract.bin -src=final.go -name=saveHandler
</pre>
<h2>Try it out!</h2>
<p>
<a href="final.go">Click here to view the final code listing.</a>
</p>
<p>
Recompile the code, and run the app:
</p>
<pre>
$ go build wiki.go
$ ./wiki
</pre>
<p>
Visiting <a href="http://localhost:8080/view/ANewPage">http://localhost:8080/view/ANewPage</a>
should present you with the page edit form. You should then be able to
enter some text, click 'Save', and be redirected to the newly created page.
</p>
<h2>Other tasks</h2>
<p>
Here are some simple tasks you might want to tackle on your own:
</p>
<ul>
<li>Store templates in <code>tmpl/</code> and page data in <code>data/</code>.
<li>Add a handler to make the web root redirect to
<code>/view/FrontPage</code>.</li>
<li>Spruce up the page templates by making them valid HTML and adding some
CSS rules.</li>
<li>Implement inter-page linking by converting instances of
<code>[PageName]</code> to <br>
<code>&lt;a href="/view/PageName"&gt;PageName&lt;/a&gt;</code>.
(hint: you could use <code>regexp.ReplaceAllFunc</code> to do this)
</li>
</ul>