Functions implementing interfaces in go

In Go, it is possible for functions to implement interfaces. This is neat feature allows you to easily create implementations to satisfy “one-function interfaces” without creating a struct. An example for this is HandlerFunc from the net/http package in go. If you look at the source code for Handler, you’ll see that it is an interface that requires a single function:

type Handler interface {
	ServeHTTP(ResponseWriter, *Request)
}

Now, scroll a bit down, you’ll see how HandlerFunc is defined:

type HandlerFunc func(ResponseWriter, *Request)

// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
	f(w, r)
}  

What this effectively does, is allow the programmer to pass a HandlerFunc into any function that accepts a Handler.

So, you can do:

func PongHandler(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("pong"))
}

func main() {
    handler := http.HandlerFunc(PongHandler)
    http.Handle("/ping", handler)
    http.ListenAndServe(":8080", nil)
}

(But don’t do this, use http.HandleFunc instead, which would be much cleaner in this case.)

What’s more interesting is that any type in go can implement interfaces. This example has a Counter, which is an unsigned integer, satisfy the Handler interface:

type Counter uint64

func (i *Counter) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	*i++
	response := fmt.Sprintf("%v", *i)
	w.Write([]byte(response))
}

func main() {
	var c Counter
	http.Handle("/count", &c)
	http.ListenAndServe(":8080", nil)
}