如何正确重构错误处理？

Question

I'm just starting with Go so I'm still not used to its patterns.

I have a web server that serves as a proxy to other remote services. I'm using mux to map routes to handlers, the code it's using App Engine.

// imports ommited.

func init() {
  m = mux.NewRouter()
  m.HandleFunc("/ponies", listPonies)
  m.HandleFunc("/rainbows", listRainbows)
  http.Handle("/", m)
}

func listPonies(w http.ResponseWriter, r *http.Request) {
  ponies, err := ponyService.getAll()

  if err != nil {
      w.write(err.Error())
      return;
  }

  w.write(string(ponies))
}

func listRainbows(w http.ResponseWriter, r *http.Request) {
  rainbows, err := rainbowService.getAll()

  if err != nil {
      w.write(err.Error())
      return;
  }

  w.write(string(rainbows))
}

I would like to refactor the common code (error handling, converting to string and writing the response) into a single function.

My first attempt was simply defining a common function to call:

func handleErrorAndWriteResponse(w http.ResponseWriter, obj Stringer, err error) {
  if err != nil {
      w.write(err.Error())
      return;
  }

  w.write(string(obj))
}

And call it like this

func listPonies(w http.ResponseWriter, r *http.Request) {
  handleErrorAndWriteResponse(w, ponyService.getAll())       
}

func listRainbows(w http.ResponseWriter, r *http.Request) {
  handleErrorAndWriteResponse(w, rainbowService.getAll())
}

But

1. It doesn't work. I get an insufficient arguments error. It probably has to do with mixing the multiple response values from the services that don't translate directly into arguments of the function called.
2. I don't like the idea of passing the error arguments around. Maybe that's fine, it just looks dirty to me. Then again, I don't know much about Go yet.

What's the "right way" (or the Go way) to do this?

Answer 1

There is an blog post on golang.org about error handling that specifically talks about error handling in an AppEngine application. Specifically, check out the "Simplifying repetitive error handling" section.

The way to do error handling is basically to let your handle functions return an error in case they fail and then wrap the call to them with the common error handler:

type appHandler func(http.ResponseWriter, *http.Request) error

func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    if err := fn(w, r); err != nil {
        http.Error(w, err.Error(), 500)
    }
}

func listPonies(w http.ResponseWriter, r *http.Request) error {
    ponies, err := ponyService.getAll()

    if err != nil {
        return err;
    }

    w.write(string(ponies))
}

You would also need to register your handlers differently:

func init() {
    http.Handle("/view", appHandler(listPonies))
}