返回匿名功能的mux路由器类型

I have this:

type RegisterHandler struct{}

func (h RegisterHandler) Mount(router *mux.Router, v interface{}) {
    router.HandleFunc("/register", h.MakeRegisterNewUser(v))
}

func (h RegisterHandler) MakeRegisterNewUser(v interface{}) func(http.ResponseWriter, *http.Request) {
    return func(w http.ResponseWriter, r *http.Request) {
      // ...
    }
}

the return type of the returned func is func(http.ResponseWriter, *http.Request), my question is - there is already a type for that return type, surely. mux.Router.Handler? or something like that? Anyone know what the existing type is for that?