What can you do in 30 lines of Go? Can you create a useful, complete program that demonstrates its features?

Question

So, the big buzz in the last few days is Go, the new language from Google. Assuming you're all obsessive programming language geeks like me, you've all downloaded it, built it, and run your "Hello, 世界" program (isn't it nice using a language written by the inventors of UTF-8?). You've all read the tutorial, Effective Go, and some of the other docs.

Now, what are you going to do with it?

I'd like to see some demos that show off the power of Go. What can you do in a brief program? Show off your best example code. While the true measure of a language can't really be taken until you've written and maintained a large codebase with a team of many programmers over the course of a project with changing requirements, seeing how much you can do in a limited amount of code does help to demonstrate the expressive power of a language. I'd like to see short, complete programs that truly exercise the unique new features of Go; not just snippets or "Hello, World".

So, post some cool code you've written with Go. Take advantage of its unique features, like its goroutines and channels for concurrency, or its interface based type system. Can you write a primitive chat server, or cool IRC bot? Implement a parallel Mandelbrot set that scales to many cores? Write an interpreter for some tiny language? And can you do it all in 30 lines?

I chose 30 arbitrarily as about as much as you can fit into a Stack Overflow code block without it overflowing and getting a scroll bar; it should be enough to do something interesting without golfing too much, but short enough to keep everyone's attention for a quick demo. For instance, with just a bit of reformatting, the example web server should be able to fit (not counting the data).

Show us your Go code!

Answer 1

This is a web proxy I wrote to provide unauthenticated access to a web service that required HTTP basic auth. I needed it for an internal thingy (and still use it):

package main

import (
    "flag"
    "log"
    "net/http"
    "net/url"
)

var target = flag.String("target", "http://www.google.com/", "Where to go.")
var addr = flag.String("listen", ":12345", "Address/port on which to listen.")
var auth = flag.String("auth", "", "Authorization header to add (optional).")

func main() {
    flag.Parse()

    targetUrl, uerr := url.Parse(*target)
    if uerr != nil {
        log.Fatalf("Error parsing target ``%s'': ", target, uerr.String())
    }

    proxy := http.ReverseProxy{Director: func(req *http.Request) {
        req.URL.Scheme = targetUrl.Scheme
        req.URL.Host = targetUrl.Host
        req.Host = targetUrl.Host
        if *auth != "" {
            req.Header.Set("Authorization", *auth)
        }
    }}

    log.Fatal(http.ListenAndServe(*addr, &proxy))
}