HTTP Handler

Inside the HTTP package, there are two handler types, http.Handler and http.HandlerFunc. It is natural to ask why do we have two types of handler.

• http.Handler is an interface.

• http.HandlerFunc is a custom function type which takes two arguments, ResponseWriter and *Request.

Any data type that implements the method ServeHTTP will qualify as a HTTP handler. Thus, if we attach a ServeHTTP method to a data type, then that data type is automatically a http.Handler.

In Go, you can attach methods to any data type, even a string or an integer.

type HandlerString string

// ServeHTTP returns the string itself as a response
func (str HandlerString) ServeHTTP(w ResponseWriter, r *Request) {
  w.WriteHeader(http.StatusOK)
  w.Write([]byte(str))
}

Now it is much easier to explain what is a HandlerFunc. Essentially it is a function that implements ServeHTTP.

type HandlerFunc func(ResponseWriter, *Request)

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

If this is still confusing, don't worry. The concept will become clear once we work on calculator server.