type model struct {
  cursor  int
  choices []string
  selected map[int]struct{}
}

func (m model) Init() tea.Cmd {
  return nil // no initial command
}

func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
  switch msg := msg.(type) {
  case tea.KeyMsg:
    switch msg.String() {
    case "q", "ctrl+c":
      return m, tea.Quit
    case "up", "k":
      if m.cursor > 0 { m.cursor-- }
    case "down", "j":
      if m.cursor < len(m.choices)-1 { m.cursor++ }
    case "enter", " ":
      if _, ok := m.selected[m.cursor]; ok {
        delete(m.selected, m.cursor)
      } else {
        m.selected[m.cursor] = struct{}{}
      }
    }
  }
  return m, nil
}

func (m model) View() string {
  s := "Pick items:\n\n"
  for i, choice := range m.choices {
    cursor := " "
    if m.cursor == i { cursor = ">" }
    checked := " "
    if _, ok := m.selected[i]; ok { checked = "x" }
    s += fmt.Sprintf("%s [%s] %s\n", cursor, checked, choice)
  }
  return s + "\nq to quit\n"
}

// ✅ Correct: define domain messages
type statusMsg string
type errMsg struct{ err error }

func fetchStatus() tea.Msg {
  resp, err := http.Get("https://api.example.com/status")
  if err != nil { return errMsg{err} }
  defer resp.Body.Close()
  body, _ := io.ReadAll(resp.Body)
  return statusMsg(body)
}

// In Update:
case tea.KeyMsg:
  if msg.String() == "r" {
    return m, fetchStatus  // fire command
  }
case statusMsg:
  m.status = string(msg)
case errMsg:
  m.err = msg.err

// ✅ Correct: parent delegates to child components
type parentModel struct {
  activeTab int
  tabs      []string
  list      listModel
  detail    detailModel
}

func (m parentModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
  switch m.activeTab {
  case 0:
    newList, cmd := m.list.Update(msg)
    m.list = newList.(listModel)
    return m, cmd
  case 1:
    newDetail, cmd := m.detail.Update(msg)
    m.detail = newDetail.(detailModel)
    return m, cmd
  }
  return m, nil
}

// ❌ Wrong: one giant Update with all logic mixed

var (
  titleStyle = lipgloss.NewStyle().
    Bold(true).
    Foreground(lipgloss.Color("205")).
    MarginBottom(1)

  selectedStyle = lipgloss.NewStyle().
    Foreground(lipgloss.Color("170")).
    Bold(true)
)

func (m model) View() string {
  title := titleStyle.Render("My App")
  item := selectedStyle.Render(m.choices[m.cursor])
  return lipgloss.JoinVertical(lipgloss.Left, title, item)
}