purehttp

Normally when you want to serve HTTP requests in Go you define a type which implements the http.Handler interface:

func (h handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {...}

purehttp provides an alternate interface, where you supply a pure function which returns a response object without requiring side-effects. Here is what it looks like:

func main() {
	srv := http.Server{
		Addr:    "localhost:8080",
		Handler: purehttp.NewHandler(MyHandler),
	}
	srv.ListenAndServe()
}

func MyHandler(req *http.Request) (*purehttp.Response, error) {
	if req.Method != http.MethodPost {
		return &purehttp.Response{StatusCode: http.StatusMethodNotAllowed}, nil
	}

	if err := req.ParseForm(); err != nil {
		return nil, err
	}

	if req.Form.Get("foo") == "" {
		return &purehttp.Response{StatusCode: http.StatusBadRequest}, nil
	}

	response := map[string]interface{}{
		"code":    420,
		"message": "hello world",
	}
	rspData, err := json.Marshal(response)
	if err != nil {
		return nil, err
	}

	return &purehttp.Response{Body: rspData, JSON: true}, nil
}

Why?

The http.Handler interface required mutating the http.ResponseWriter object, which has its own particular set of rules on the order you should call certain methods in. As a specific example, attempting to write any header fields after writing the main Status header will just silently fail. By comparison, returning a Response object is much more clear and unambiguous.

Using "pure" functions as opposed to functions which rely on side-effects also has a number of other advantages, such as being easier to test and being more composable with other functions.