如何扩展现有结构？

I want to extend the http.Request structure to include arguments present in the url. I know there are excellent libraries like gorilla that already do this, but I want to learn how its done on the nuts and bolts level.

Here's an example of what I'm trying to do:

// server.go

func Vars(r *http.Request) map[string]string {
    var args map[string]string
    args = make(map[string]string)
    s := r.URL.Path.split('/')[1:]
    for str := range s {
        _, arg, _ := regexp.MatchString("\{\w+\}", str)
        append(args, arg)
    }

    return args
}

httpMock.HandleFunc("/myurl/{arg1}/", MyHandler)

func MyHandler(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("Success! " + r.Vars()["arg1"]))
}

But I get an index out of range exception. Can anyone help me figure out how to do this?