Go - passing generic struct to function

Question

Considering the following code, which is responding to GET '/venues/:id':

func venueShow(w http.ResponseWriter, req *http.Request) {

  // get ID from params
  vars := mux.Vars(req)
  id := vars["id"]

  // initialise new struct
  var venue Venue

  // select by id and scan into struct
  db.First(&venue, id).Scan(&venue)

  // turn it to json
  response := structToJSON(&venue)

  // write headers and provide response
  w.Header().Set("Content-Type", "application/json")
  w.Write(response)
}

and:

func structToJSON (s interface{}) (response []byte) {
  // turn it into pretty-ish json
  response, err := json.MarshalIndent(&s, "", "  ")
  if err != nil {
   return []byte("Venue does not exist")
  }
  // return the json as the reponse
  return response
}

My structToJSON function is taking an empty interface as the argument, because I want to pass various different structs to the function and have them spewed out as JSON.

However, it doesn't strike me as very safe. If anything satisfies an empty interface, I could pass whatever I wanted into that function, and all sorts of errors might happen when json.Marshal tries to do it's business. This (I suppose) would be caught by the compiler rather than at runtime, but is there a safer way?

I could duplicate the structToJSON method for each different type of Struct/Model that I pass to it, but that's not very DRY.

Thanks

Solution

The Marshal function also receives its parameters as interface{} therefore there's no way to detect if you are passing something invalid at compile time, it's all caught at runtime.

One thing you can do to check if an invalid type was passed to Marshal is to check the error type, Marshal returns an UnsupportedTypeError when you try to Marshal an invalid type (like chan or func) so you can check for that error when Marshaling.

So you could try something like that:

if err != nil {
    _, ok := err.(*json.UnsupportedTypeError)
    if ok {
        return []byte("Tried to Marshal Invalid Type")
    } else {
        return []byte("Venue does not exist")
    }
}

Comments

[Ralph King]

Ah ok, didn't think of that. So there's no real problem with using an empty interface as an argument, as stuff in the standard library does it loads I suppose. Thanks. I'll mark this as correct!

[hbejgel]

Yeah, sice go doesn't have generics and doesn't allow method overload we're pretty stuck passing interface{} as parameters.