Stand alone web application

---

examples/stand-alone/stand_alone.go

package main

import (
    "fmt"
    "log"
    "net/http"
    "os"
    "os/exec"
    "runtime"
    "strconv"
    "time"
)

func mainPage(w http.ResponseWriter, r *http.Request) {
    aStr := r.PostFormValue("a")
    bStr := r.PostFormValue("b")
    a, _ := strconv.Atoi(aStr)
    b, _ := strconv.Atoi(bStr)

    result := a + b

    html := fmt.Sprintf(`<h1>Calculator</h1>
    <form method="POST">
    <input name="a">
    <input name="b">
    <input type="submit" value="Add">
    </form>
    Result: %v
    <hr>
    <a href="/exit">exit</a>`, result)

    fmt.Fprintf(w, "%v", html)
}

func exitApp(w http.ResponseWriter, r *http.Request) {
    go func() {
        time.Sleep(1000)
        os.Exit(0)
    }()
    fmt.Fprintf(w, "goodbye")
}

func openBrowser(targetURL string) {
    var err error

    switch runtime.GOOS {
    case "linux":
        err = exec.Command("xdg-open", targetURL).Start()
        // TODO: "Windows Subsytem for Linux" is also recognized as "linux", but then we need
        // err = exec.Command("rundll32.exe", "url.dll,FileProtocolHandler", targetURL).Start()
    case "windows":
        err = exec.Command("rundll32.exe", "url.dll,FileProtocolHandler", targetURL).Start()
    case "darwin":
        err = exec.Command("open", targetURL).Start()
    default:
        err = fmt.Errorf("unsupported platform %v", runtime.GOOS)
    }
    if err != nil {
        log.Fatal(err)
    }

}

// TODO: Find usable port, but open the browser only to that
// TODO: is there a better way to stop the application than just exit?

func main() {
    http.HandleFunc("/", mainPage)
    http.HandleFunc("/exit", exitApp)

    port := 5000
    connection := fmt.Sprintf("127.0.0.1:%v", port)
    url := fmt.Sprintf("http://%v", connection)
    fmt.Printf("Going to listen on %v  Ctr-c to stop the server.\n", url)
    // TODO: only open once we know the server was started.
    go func() {
        time.Sleep(1000)
        openBrowser(url)
    }()
    err := http.ListenAndServe(connection, nil)
    if err != nil {
        //log.Fatal(err)
        panic(err)
    }
    fmt.Println("OK")
}