// Correct: happy path flows down.
func ProcessUser(id string) (*User, error) {
    user, err := db.GetUser(id)
    if err != nil {
        return nil, fmt.Errorf("get user %s: %w", id, err)
    }

    if err := user.Validate(); err != nil {
        return nil, fmt.Errorf("validate user %s: %w", id, err)
    }

    return user, nil
}

// Wrong: main logic nested inside conditions.
func ProcessUser(id string) (*User, error) {
    user, err := db.GetUser(id)
    if err == nil {
        if err := user.Validate(); err == nil {
            return user, nil
        } else {
            return nil, err
        }
    }
    return nil, err
}

// Correct: wrapped with context.
if err != nil {
    return fmt.Errorf("create order for customer %s: %w", customerID, err)
}

// Wrong: no context.
if err != nil {
    return err
}

// Define at package level.
var (
    ErrNotFound     = errors.New("not found")
    ErrUnauthorized = errors.New("unauthorized")
    ErrInvalidInput = errors.New("invalid input")
)

// Return sentinel errors.
func GetUser(id string) (*User, error) {
    user := db.Find(id)
    if user == nil {
        return nil, ErrNotFound
    }
    return user, nil
}

// Check with errors.Is().
user, err := GetUser(id)
if errors.Is(err, ErrNotFound) {
    // Handle not found case.
}

// ProcessOrder handles order creation and validation.
func ProcessOrder(o *Order) error {
    // Validate the order before processing.
    if err := o.Validate(); err != nil {
        return err
    }
    // Continue with order processing.
    return nil
}

// Wrong: shadows built-in identifiers.
func process(new string, len int, make bool) error {
    copy := "data"
    return nil
}

// Correct: use descriptive names instead.
func process(name string, length int, shouldCreate bool) error {
    dataCopy := "data"
    return nil
}

// Correct: break at logical points.
func ProcessOrderWithValidation(
    ctx context.Context,
    order *Order,
    validator OrderValidator,
) (*Result, error) {
    return nil, fmt.Errorf(
        "process order %s for customer %s: %w",
        order.ID,
        order.CustomerID,
        err,
    )
}

go doc fmt.Errorf
go doc errors.Is
go doc context