处理大猩猩mux空变量

I'm using gorilla mux to get pattern values. How do I handle an empty variable like so:

Go:

func ProductHandler (w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    a := vars["key"]
    if a = "" {       //does not seem to register empty string
       //do something
    } else 
       //do something
}

var r = mux.NewRouter()

func main() {
    r.HandleFunc("/products/{key}", ProductHandler)

    http.Handle("/", r)

    http.ListenAndServe(":8080", nil)
}

When I type the url www.example.com/products or www.example.com/products/ I get a 404 page not found error. How do i handle an empty variable in ProductHandler?

http://www.gorillatoolkit.org/pkg/mux

Simplest solution? Add:

r.HandleFunc("/products", ProductHandler)

I am pretty sure Gorilla will route the longest match in order of registration.

This is also the way the documentation's overview page suggest it be used:

Then register routes in the subrouter:

s.HandleFunc("/products/", ProductsHandler)
s.HandleFunc("/products/{key}", ProductHandler)
s.HandleFunc("/articles/{category}/{id:[0-9]+}"), ArticleHandler)