type Config struct {
  path: string
  id:   string
  key:  string
  addr: string
  size: uint64
}

func merge(default *Config, file *Config) (*Config) {
  b := reflect.ValueOf(default)
  o := reflect.ValueOf(file)

  for i := 0; i < b.NumField(); i++ {
    defaultField := b.Field(i)
    fileField := o.Field(i)
    if defaultField.Interface() != reflect.Zero(fileField.Type()).Interface() {
     defaultField.Set(reflect.ValueOf(fileField))
    }
  }

  return default
 }