func main() {
    dbConnection := db.connect("...")
    defer dbConnection.Close() // this operation won't be executed!    
    entity := Entity{}
    err := dbConnection.Save(entity)
    if err != nil {
        os.Exit(1)
    }
}

package order
// package errors
var (
    UnableToShipToCountry = errors.New("unable to ship order to the requested country")
)
type Order struct {
    // ... order fields
}
type OrderRepo struct {
    DB db
    // ...
}
func newOrderFromRequest(o OrderRequest) (Order, error) {
    if o.ShippingAddress.Country != "DE" {
	return UnableToShipToCountry
    }    // ... the creation logic
    return Order{...}, nil
}
func (r *OrderRepo)PlaceOrder(o OrderRequest) error {
    order, err := newOrderFromRequest(o)
    if err != nil {
        // don't handle the error here, its handling may differ
        return err
    }    // ... db transaction may also return an error
    return r.db.Save(order)
}

package http
http.HandleFunc("/order", func (w http.ResponseWriter, r *http.Request) {
    orderRequest := createOrderRequest(r)
    err := orderRepo.PlaceOrder(orderRequest)
    if errors.Is(err, order.UnableToShipToCountry) {
        w.WriteHeader(http.StatusBadRequest)
        return
    }
    if err != nil {
        // this error in case of DB transaction failure
        w.WriteHeader(http.StatusInternalServerError)
        return
    }    // ...
    w.WriteHeader(http.StatusOK)
})

type AppErr struct {  
    msg string  
    code int  
    trace string  
}  
func (e AppErr) Error() string {  
    return fmt.Sprintf("Msg: %s, code: %d, trace:\n %s", e.msg, e.code, e.trace)  
}  
func NewAppErr(msg string, code int) AppErr {  
    stackSlice := make([]byte, 512)  
    s := runtime.Stack(stackSlice, false)  
    return AppErr{msg, code, fmt.Sprintf("\n%s", stackSlice[0:s])}  
}

package admin  
func A() error {  
    return b()  
}  
func b() error {  
    return NewAppErr("error from b function!", 3)  
}

func main() {  
    err := admin.A()
    fmt.Println(err)
}