How to have an optional query in GET request using Gorilla Mux?

Just a comment to the previous answer.

We can just add two routes there, I feel it is more readable, like below:

r.HandleFunc("/user", userByValueHandler).
    Queries(
        "username", "{username}",
        "email", "{email}",
    ).
    Methods("GET")
r.HandleFunc("/user", UserByValueHandler).Methods("GET")

So I found the solution to re-write my logic as:

r.HandleFunc("/user", UserByValueHandler).Methods("GET")

And in UserByValueHandler we can have something like:

func UserByValueHandler(w http.ResponseWriter, r *http.Request) {
       v := r.URL.Query()

       username := v.Get("username")
       email := v.Get("email")
       .....
}