我有以下函数给我“已声明变量且未使用变量”错误：

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)

  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)
}

它给了我以下错误：

dpcompare.go:171: m declared and not used
dpcompare.go:171: err declared and not used
dpcompare.go:173: m declared and not used
dpcompare.go:173: err declared and not used
dpcompare.go:178: undefined: m
dpcompare.go:185: key declared and not used

东西是m，，err并且key都在使用中。我无法解释为什么编译器认为您不是。