// Use %w to preserve error chain
return fmt.Errorf("fetching user %s: %w", userID, err)

// Check wrapped errors
if errors.Is(err, sql.ErrNoRows) { ... }

var paymentErr *PaymentError
if errors.As(err, &paymentErr) { ... }

// notification/sender.go (consumer defines interface)
type EmailSender interface {
    Send(ctx context.Context, to, subject, body string) error
}

// email/client.go (provider implements)
type Client struct { ... }
func (c *Client) Send(ctx context.Context, to, subject, body string) error { ... }

type Reader interface { Read(p []byte) (n int, err error) }
type Writer interface { Write(p []byte) (n int, err error) }
type ReadWriter interface { Reader; Writer }

var _ EmailSender = (*Client)(nil)

func FetchData(ctx context.Context) ([]byte, error) {
    select {
    case <-ctx.Done():
        return nil, ctx.Err()
    case result := <-dataChan:
        return result, nil
    }
}

sem := make(chan struct{}, 10)
for _, item := range items {
    sem <- struct{}{}
    go func(item Item) {
        defer func() { <-sem }()
        process(item)
    }(item)
}

internal/
  user/          # Domain: single purpose
    user.go
    service.go
    repository.go
  order/         # Another domain
  app/           # Dependency wiring
cmd/
  api/           # Entry points

// Constructor accepts interfaces
func NewUserService(repo UserRepository, mailer EmailSender) *UserService {
    return &UserService{repo: repo, mailer: mailer}
}

// Wire in app/ package
func NewApp() *App {
    repo := postgres.NewUserRepo(db)
    mailer := sendgrid.NewClient(apiKey)
    userSvc := user.NewUserService(repo, mailer)
    return &App{UserService: userSvc}
}

// Group dependencies into named struct
type ServiceDeps struct {
    Repo     Repository
    Mailer   EmailSender
    Logger   Logger
    Config   *Config
    Cache    Cache
}

// Panic on nil - catches programming errors at construction
func NewService(deps ServiceDeps) *Service {
    if deps.Repo == nil {
        panic("NewService: Repo cannot be nil")
    }
    if deps.Mailer == nil {
        panic("NewService: Mailer cannot be nil")
    }
    // ... check all required deps
    return &Service{...}
}

// BAD - Error suppression hides failures
defer func() { _ = os.Chdir(orig) }()  // SA4017 warning

// GOOD - Proper cleanup with error handling
t.Cleanup(func() {
    if err := os.Chdir(orig); err != nil {
        t.Errorf("cleanup failed: %v", err)
    }
})