A web server
Let’s finish with a complete Go program, a web server. This one is actually a kind of web re-server. Google provides a service at http://chart.apis.google.com that does automatic formatting of data into charts and graphs. It’s hard to use interactively, though, because you need to put the data into the URL as a query. The program here provides a nicer interface to one form of data: given a short piece of text, it calls on the chart server to produce a QR code, a matrix of boxes that encode the text. That image can be grabbed with your cell phone’s camera and interpreted as, for instance, a URL, saving you typing the URL into the phone’s tiny keyboard.
Here’s the complete program. An explanation follows.
package main
import (
"flag"
"html/template"
"log"
"net/http"
)
var addr = flag.String("addr", ":1718", "http service address") // Q=17, R=18
var templ = template.Must(template.New("qr").Parse(templateStr))
func main() {
flag.Parse()
http.Handle("/", http.HandlerFunc(QR))
err := http.ListenAndServe(*addr, nil)
if err != nil {
log.Fatal("ListenAndServe:", err)
}
}
func QR(w http.ResponseWriter, req *http.Request) {
templ.Execute(w, req.FormValue("s"))
}
const templateStr = `
<html>
<head>
<title>QR Link Generator</title>
</head>
<body>
{{if .}}
<img src="http://chart.apis.google.com/chart?chs=300x300&cht=qr&choe=UTF-8&chl={{.}}" />
<br>
{{.}}
<br>
<br>
{{end}}
<form action="/" name=f method="GET"><input maxLength=1024 size=70
name=s value="" title="Text to QR Encode"><input type=submit
value="Show QR" name=qr>
</form>
</body>
</html>
`
The pieces up to main should be easy to follow. The one flag sets a default HTTP port for our server. The template variable templ is where the fun happens. It builds an HTML template that will be executed by the server to display the page; more about that in a moment.
The main function parses the flags and, using the mechanism we talked about above, binds the function QR to the root path for the server. Then http.ListenAndServe is called to start the server; it blocks while the server runs.
QR just receives the request, which contains form data, and executes the template on the data in the form value named s .
The template package html/template is powerful; this program just touches on its capabilities. In essence, it rewrites a piece of HTML text on the fly by substituting elements derived from data items passed to templ.Execute , in this case the form value. Within the template text ( templateStr ), double-brace-delimited pieces denote template actions. The piece from \{\{if.\}\ to \{\{end\}\ executes only if the value of the current data item, called . (dot), is non-empty. That is, when the string is empty, this piece of the template is suppressed.
The two snippets \{\{.\}\} say to show the data presented to the template—the query string—on the web page. The HTML template package automatically provides appropriate escaping so the text is safe to display.
The rest of the template string is just the HTML to show when the page loads. If this is too quick an explanation, see the documentation for the template package for a more thorough discussion.
And there you have it: a useful web server in a few lines of code plus some data-driven HTML text. Go is powerful enough to make a lot happen in a few lines.
