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Code Issues Packages Projects Releases Wiki Activity

doc/articles/wiki: numerous fixes 

Fixes #3733
Fixes #2149
Updated Syntax
Added part3.go example program
Added part3-errorhandling.go example program
Improved wording in some places

R=golang-dev, adg, minux.ma
CC=golang-dev
https://golang.org/cl/6636048
This commit is contained in:
Jimmy Zelinskie 2012-10-11 13:07:34 +11:00 committed by Andrew Gerrand
parent a3d116cf78
commit dad1228cc3
3 changed files with 243 additions and 99 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

208
doc/articles/wiki/index.html
View File

@ -46,7 +46,7 @@ $ cd gowiki
</pre>
<p>
Create a file named <code>wiki.go</code>, open it in your favorite editor, and
Create a file named <code>wiki.go</code>, open it in your favorite editor, and
add the following lines:
</p>
@ -60,8 +60,8 @@ import (
</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
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>
@ -77,7 +77,7 @@ the title and body.
{{code "doc/articles/wiki/part1.go" `/^type Page/` `/}/`}}
<p>
The type <code>[]byte</code> means "a <code>byte</code> slice".
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
@ -86,8 +86,8 @@ 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
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>
@ -96,11 +96,11 @@ But what about persistent storage? We can address that by creating a
<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>."
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
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>
@ -110,35 +110,37 @@ 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
<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
The octal integer literal <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:
In addition to saving pages, we will want to load pages, too:
</p>
{{code "doc/articles/wiki/part1-noerror.go" `/^func loadPage/` `/^}/`}}
<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>.
the title parameter, reads the file's contents into a new
variable <code>body</code>, and returns two values: a pointer to a
<code>Page</code> literal constructed with the proper title and body
values and <code>nil</code> for the error value.
</p>
<p>
Functions can return multiple values. The standard library function
<code>io.ReadFile</code> returns <code>[]byte</code> and <code>error</code>.
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).
error return value (in essence, assigning the value to nothing).
</p>
<p>
@ -152,7 +154,7 @@ function to return <code>*Page</code> and <code>error</code>.
<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
<code>error</code> that can be handled by the caller (see the
<a href="/ref/spec#Errors">language specification</a> for details).
</p>
@ -172,7 +174,7 @@ printed to the screen.
</p>
<p>
You can compile and run the program like this:
You can compile and run the program like this:
</p>
<pre>
@ -182,7 +184,7 @@ This is a sample page.
</pre>
<p>
(If you're using Windows you must type "<code>wiki</code>" without the
(If you're using Windows you must type "<code>wiki</code>" without the
"<code>./</code>" to run the program.)
</p>
@ -199,10 +201,10 @@ Here's a full working example of a simple web server:
{{code "doc/articles/wiki/http-sample.go"}}
<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>.
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>
@ -219,20 +221,20 @@ its arguments.
</p>
<p>
An <code>http.ResponseWriter</code> value assembles the HTTP server's response; by writing
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."
HTTP request. <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:
If you run this program and access the URL:
</p>
<pre>http://localhost:8080/monkeys</pre>
<p>
@ -249,13 +251,14 @@ To use the <code>net/http</code> package, it must be imported:
<pre>
import (
"fmt"
<b>"net/http"</b>
"io/ioutil"
<b>"net/http"</b>
)
</pre>
<p>
Let's create a handler to view a wiki page:
Let's create a handler, <code>viewHandler</code> that will allow users to
view a wiki page. It will handle URLs prefixed with "/view/".
</p>
{{code "doc/articles/wiki/part2.go" `/^const lenPath/`}}
@ -264,28 +267,28 @@ Let's create a handler to view a wiki page:
<p>
First, this function extracts the page title from <code>r.URL.Path</code>,
the path component of the request URL. The global constant
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.
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's 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>.
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>
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
To use this handler, we rewrite our <code>main</code> function to
initialize <code>http</code> using the <code>viewHandler</code> to handle
any requests under the path <code>/view/</code>.
</p>
@ -310,6 +313,11 @@ $ go build wiki.go
$ ./wiki
</pre>
<p>
(If you're using Windows you must type "<code>wiki</code>" without the
"<code>./</code>" to run the program.)
</p>
<p>
With this web server running, a visit to <code><a
href="http://localhost:8080/view/test">http://localhost:8080/view/test</a></code>
@ -326,14 +334,14 @@ form.
</p>
<p>
First, we add them to <code>main()</code>:
First, we add them to <code>main()</code>:
</p>
{{code "doc/articles/wiki/final-noclosure.go" `/^func main/` `/^}/`}}
<p>
The function <code>editHandler</code> loads the page
(or, if it doesn't exist, create an empty <code>Page</code> struct),
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>
@ -343,7 +351,7 @@ and displays an HTML form.
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>
@ -354,20 +362,20 @@ underlying Go code.
</p>
<p>
First, we must add <code>html/template</code> to the list of imports:
First, we must add <code>html/template</code> to the list of imports. We
also won't be using <code>fmt</code> anymore, so we have to remove that.
</p>
<pre>
import (
<b>"html/template"</b>
"http"
"io/ioutil"
"os"
"net/http"
)
</pre>
<p>
Let's create a template file containing the HTML form.
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>
@ -381,8 +389,8 @@ HTML:
{{code "doc/articles/wiki/final-noerror.go" `/^func editHandler/` `/^}/`}}
<p>
The function <code>template.ParseFiles</code> will read the contents of
<code>edit.html</code> and return a <code>*template.Template</code>.
The function <code>template.ParseFiles</code> will read the contents of
<code>edit.html</code> and return a <code>*template.Template</code>.
</p>
<p>
@ -405,12 +413,7 @@ 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
Since we're working with templates now, let's create a template for our
<code>viewHandler</code> called <code>view.html</code>:
</p>
@ -428,28 +431,31 @@ handlers. Let's remove this duplication by moving the templating code
to its own function:
</p>
{{code "doc/articles/wiki/final-template.go" `/^func renderTemplate/` `/^}/`}}
{{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.
If we comment out the registration of our unimplemented save handler in
<code>main</code>, we can once again build and test our program.
<a href="part3.go">Click here to view the code we've written so far.</a>
</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:
<code>/view/APageThatDoesntExist</code></a>? You'll see a page containing
HTML. This is because it ignores the error return value from
<code>loadPage</code> and continues to try and fill out the template
with no data. Instead, if the requested Page doesn't exist, it should
redirect the client to the edit Page so the content may be created:
</p>
{{code "doc/articles/wiki/final-noclosure.go" `/^func viewHandler/` `/^}/`}}
{{code "doc/articles/wiki/part3-errorhandling.go" `/^func viewHandler/` `/^}/`}}
<p>
The <code>http.Redirect</code> function adds an HTTP status code of
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>
@ -457,22 +463,24 @@ header to the HTTP response.
<h2>Saving Pages</h2>
<p>
The function <code>saveHandler</code> will handle the form submission.
The function <code>saveHandler</code> will handle the submission of forms
located on the edit pages. After uncommenting the related line in
<code>main</code>, let's implement the the handler:
</p>
{{code "doc/articles/wiki/final-template.go" `/^func saveHandler/` `/^}/`}}
<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 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
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>
@ -481,9 +489,9 @@ the conversion.
<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.
have unintended behavior. 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 function exactly how we want and the user can be notified.
</p>
<p>
@ -493,7 +501,7 @@ First, let's handle the errors in <code>renderTemplate</code>:
{{code "doc/articles/wiki/final-parsetemplate.go" `/^func renderTemplate/` `/^}/`}}
<p>
The <code>http.Error</code> function sends a specified HTTP response code
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>
@ -502,18 +510,18 @@ Already the decision to put this in a separate function is paying off.
Now let's fix up <code>saveHandler</code>:
</p>
{{code "doc/articles/wiki/final-noclosure.go" `/^func saveHandler/` `/^}/`}}
{{code "doc/articles/wiki/part3-errorhandling.go" `/^func saveHandler/` `/^}/`}}
<p>
Any errors that occur during <code>p.save()</code> will be reported
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.
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 at program
initialization, parsing all templates into a single <code>*Template</code>.
Then we can use the
@ -536,10 +544,10 @@ can't be loaded the only sensible thing to do is exit the program.
</p>
<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.
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>
@ -571,25 +579,27 @@ Then we can create a global variable to store our validation regexp:
{{code "doc/articles/wiki/final-noclosure.go" `/^var titleValidator/`}}
<p>
The function <code>regexp.MustCompile</code> will parse and compile the
regular expression, and return a <code>regexp.Regexp</code>.
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.
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:
Now, let's write a function, <code>getTitle</code>, that extracts the title
string from the request URL, and tests it against our
<code>TitleValidator</code> expression:
</p>
{{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>
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.
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. To create a new error, we have to import the <code>errors</code>
package.
</p>
<p>
@ -604,10 +614,10 @@ Let's put a call to <code>getTitle</code> in each of the handlers:
<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
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>
@ -654,19 +664,19 @@ Now we can take the code from <code>getTitle</code> and use it here
<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>).
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.
<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>
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>
@ -698,7 +708,7 @@ $ ./wiki
<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
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>
@ -710,11 +720,11 @@ Here are some simple tasks you might want to tackle on your own:
<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
<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
<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)

75
doc/articles/wiki/part3-errorhandling.go Normal file
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@ -0,0 +1,75 @@
// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"html/template"
"io/ioutil"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
const lenPath = len("/view/")
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, _ := template.ParseFiles(tmpl + ".html")
t.Execute(w, p)
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
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 := r.URL.Path[lenPath:]
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func saveHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
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 main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
http.HandleFunc("/save/", saveHandler)
http.ListenAndServe(":8080", nil)
}

59
doc/articles/wiki/part3.go Normal file
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@ -0,0 +1,59 @@
// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"html/template"
"io/ioutil"
"net/http"
)
type Page struct {
Title string
Body []byte
}
func (p *Page) save() error {
filename := p.Title + ".txt"
return ioutil.WriteFile(filename, p.Body, 0600)
}
func loadPage(title string) (*Page, error) {
filename := title + ".txt"
body, err := ioutil.ReadFile(filename)
if err != nil {
return nil, err
}
return &Page{Title: title, Body: body}, nil
}
const lenPath = len("/view/")
func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
t, _ := template.ParseFiles(tmpl + ".html")
t.Execute(w, p)
}
func viewHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, _ := loadPage(title)
renderTemplate(w, "view", p)
}
func editHandler(w http.ResponseWriter, r *http.Request) {
title := r.URL.Path[lenPath:]
p, err := loadPage(title)
if err != nil {
p = &Page{Title: title}
}
renderTemplate(w, "edit", p)
}
func main() {
http.HandleFunc("/view/", viewHandler)
http.HandleFunc("/edit/", editHandler)
//http.HandleFunc("/save/", saveHandler)
http.ListenAndServe(":8080", nil)
}