http.Handle(Handler or HandlerFunc) - http

Question

How is the following function implemented?
func handle(pattern string, handler interface{}) {
// ... what goes here? ...
http.Handle(pattern, ?)
}
handle("/foo", func(w http.ResponseWriter, r http.Request) { io.WriteString(w, "foo") }
handle("/bar", BarHandler{})
handle() is passed either a function which matches the type of http.HandlerFunc or a type which implements the http.Handler interface.

Answer 1

Instead of resorting to reflection, I would do it this way:
func handle(pattern string, handler interface{}) {
var h http.Handler
switch handler := handler.(type) {
case http.Handler:
h = handler
case func(http.ResponseWriter, *http.Request):
h = http.HandlerFunc(handler)
default:
// error
}
http.Handle(pattern, h)
}

Answer 2

First we need to introduce the term "reflections" in Java/C#'s terminology, RTTI in C++'s terminology. It's quite simple actually. The compiler keeps data to find out what is the type of an instance var i SomeType during runtime. Go supports reflection, and that's how it finds out what's the type of handler during runtime.
The handle function uses the reflection. A crude example
package main
import ("reflect";"http")
type fakeHandler struct{}
func (frw *fakeHandler) ServeHTTP(http.ResponseWriter, *http.Request) {}
func handle(pattern string, handler interface{}) {
handlerInterface := reflect.TypeOf(new(http.Handler)).Elem()
handlerFunction := reflect.TypeOf(new(http.HandlerFunc)).Elem()
t := reflect.TypeOf(handler)
if t.Implements(handlerInterface) {fmt.Println("http.Handler")}
//http.HandlerFunc is a different type than
// func(http.ResponseWriter, *http.Request), but we can do
// var hf HandlerFunc = func(http.ResponseWriter, *http.Request){}
if t.AssignableTo(handlerFunction) {fmt.Println("http.HandleFunc")}
}
func f(http.ResponseWriter, *http.Request) {}
func main() {
handle("",&fakeHandler{})
handle("",f)
}