Go compiler says “declared and not used” but they are being used

Question

I have the following function that is giving me \"variable declared and not used\" errors:

type Comparison struct {
        Left []byte
        Right []byte
         


        

Answer 1

I think the variables you declare in those if branches are local to the code blocks of these branches. This is not JavaScript (luckily). So just declare your variables somewhere above if and use = instead of := to assign to them.

Answer 2

As @kostix said, m is local to the scope of the if. Try this code

type Comparison struct {
        Left []byte
        Right []byte
        Name string
}

func img(w http.ResponseWriter, r *http.Request, c appengine.Context, u *user.User) {
  key := datastore.NewKey("Comparison", r.FormValue("id"), 0, nil)
  side := r.FormValue("side")
  comparison := new(Comparison)
  err := datastore.Get(c, key, comparison)
  check(err)

  // NOTE! now m is in the function's scope
  var m Image    
  if( side == "left"){
    m, _, err = image.Decode(bytes.NewBuffer(comparison.Left))
  } else {
    m, _, err = image.Decode(bytes.NewBuffer(comparison.Right))
  }
  check(err)

  w.Header().Set("Content-type", "image/jpeg")
  jpeg.Encode(w, m, nil)
}