code: func (m *Map) Refresh(
  	info *terraform.InstanceInfo,
  	s *terraform.InstanceState,
  	meta interface{}) (*terraform.InstanceState, error) {
  	// If the resource isn't created, don't refresh.
  	if s.ID == "" {
  		return s, nil
  	}
  
  	r, ok := m.Mapping[info.Type]
  	if !ok {
  		return nil, fmt.Errorf("Unknown resource type: %s", info.Type)
  	}
  
  	return r.Refresh(s, meta)
  }
docs: // Refresh performs a Refresh on the proper resource type.
  //
  // Refresh on the Resource won't be called if the state represents a
  // non-created resource (ID is blank).
  //
  // An error is returned if the resource isn't registered.