go - Where does the serveHTTP utility come from on supposedly naked func

Question

I have this utility:

type Handler struct{}

func (h Handler) Mount(router *mux.Router, v PeopleInjection) {
    router.HandleFunc("/api/v1/people", h.makeGetMany(v)).Methods("GET")
}

the above calls this:

func (h Handler) makeGetMany(v PeopleInjection) http.HandlerFunc {

    type RespBody struct {}

    type ReqBody struct {
        Handle string
    }

    return tc.ExtractType(
        tc.TypeList{ReqBody{},RespBody{}},
        func(w http.ResponseWriter, r *http.Request) {
         // ...
    })
}

and then tc.ExtractType is like so:

func ExtractType(s TypeList, h http.HandlerFunc) http.HandlerFunc {

    return func(w http.ResponseWriter, r *http.Request) {

        h.ServeHTTP(w, r)  // <<< h is just a func right? so where does ServeHTTP come from?
    }
}

my question is - where does the serveHTTP method/func come from??

isn't the h parameter just a func with this signature:

func(w http.ResponseWriter, r *http.Request) { ... }

so then how does that func have the ServeHTTP func attached to it?

In other words, why I am I calling

h.ServeHTTP(w,r)

instead of

h(w,r)

?

Answer 1

Waitting for answers