interface for fields in golang

Question

Let's say I have a struct which should be used as a result for an upload:

type uploadResult struct {
  Filename string `json:"filename"`
  Code string `json:"code"`
  Reason string `json:"reason"`
}

There will be other structs like this one, both having a field Code and another one called Reason. It would therefore be interesting to have something like a common interface (pseudo-go-code; this one doesn't work):

type apiResult interface {
  Code string `json:"code"`
  Reason string `json:"reason"`
}

Because I would like to call a function which extracts some common fields, but only those that are common:

func failExit(result apiResult) {
  fmt.Println(result.Reason)
}

So how would I rewrite it so that it does what I'm expecting?

Best regards

Answer 1

You should just be able to embed a struct with the common fields in the specific structs.

Live demo: http://play.golang.org/p/7Ju-r-yE1-

type apiResult struct {
  Code string `json:"code"`
  Reason string `json:"reason"`
}

func failExit(result apiResult) {
  fmt.Println(result.Reason)
}

type uploadResult struct {
  Filename string `json:"filename"`
  apiResult // <-- embedded
}

func main() {
  var ul uploadResult
  ul.Code = "...some code..."
  ul.Reason = "The description"
  ul.Filename = "foo.txt"

  failExit(ul.apiResult)
}

So there shouldn't really be any need for interfaces in this situation. Just embed the apiResult in whatever structs need it.

Answer 2

Long story short, Go interfaces don't allow fields to be declared because conceptually they deal with behavior not data. Fields are data.

There's a couple of ways to go about what you want to do:

Here's some examples on how to approach this:

This one uses an interface to expose the APIResult fields as part of the apiResponse interface: http://play.golang.org/p/ONLzvqlP5R

This one uses an interface to expose the APIResult as part of any Result struct that contains one: http://play.golang.org/p/NzxPHhDls_

On that note, you could solve this by using composition by having APIResponse be a struct that has the common fields and then any other struct that needs those fields exports the APIResponse struct.

type APIResult struct {
    Code   string `json:"code"`
    Reason string `json:"reason"`
}

type UploadResult struct {
  Filename string `json:"filename"`
  APIResult
}

func failExit(result APIResult) {
  fmt.Println(result.Reason)
}

http://play.golang.org/p/k85vTJoFRn