decorator.go

package main

import (
	"log"
	"net/http"
)

type handler func(w http.ResponseWriter, r *http.Request)

func decorator(ft handler) handler {
	return func(w http.ResponseWriter, r *http.Request) {
		ft(w, r)
		w.Write(([]byte)("Hello world."))
	}
}

func filter(w http.ResponseWriter, r *http.Request) {
	w.Write(([]byte)("add someting to response.\n"))
}

func main() {
	port := ":8003"
	http.HandleFunc("/", decorator(filter))
	log.Fatal(http.ListenAndServe(port, nil))
}