+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";
+ "http";
+ "io";
+ "log";
+ "strings";
+ "template";
+)
+
+var addr = flag.String("addr", ":1718", "http service address") // Q = 17, R = 18
+var fmap = template.FormatterMap{
+ "html": template.HtmlFormatter,
+ "url+html": UrlHtmlFormatter,
+}
+var templ = template.MustParse(templateStr, fmap)
+
+func main() {
+ flag.Parse();
+ http.Handle("/", http.HandlerFunc(QR));
+ err := http.ListenAndServe(*addr, nil);
+ if err != nil {
+ log.Exit("ListenAndServe:", err);
+ }
+}
+
+func QR(c *http.Conn, req *http.Request) {
+ templ.Execute(req.FormValue("s"), c);
+}
+
+func UrlHtmlFormatter(w io.Writer, v interface{}, fmt string) {
+ template.HtmlEscape(w, strings.Bytes(http.URLEscape(v.(string))));
+}
+
+
+const templateStr = `
+<html>
+<head>
+<title>QR Link Generator</title>
+</head>
+<body>
+{.section @}
+<img src="http://chart.apis.google.com/chart?chs=300x300&cht=qr&choe=UTF-8&chl={@|url+html}"
+/>
+<br>
+{@|html}
+<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 field named s.
+
+The template package, inspired by json-template, is
+powerful;
+this program just touches on its capabilities.
+In essence, it rewrites a piece of text on the fly by substituting elements derived
+from data items passed to templ.Execute, in this case the
+string in the form data.
+Within the template text (templateStr),
+brace-delimited pieces denote template actions.
+The piece from the {.section @}
+to {.end} executes with the value of the data item @,
+which is a shorthand for “the current item”, in this case the form data.
+(When the string is empty, this piece of the template is suppressed.)
+
+The snippet {@|url+html} says to run the data through the formatter
+installed in the formatter map (fmap)
+under the name "url+html".
+That is the function UrlHtmlFormatter, which sanitizes the string
+for safe display on the web page.
+
+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 webserver 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. +
+