Svetofor faqat UCH RANGDA gapiradi (qizil, sariq, yashil), lekin HAR BIR haydovchi ularni BIR XIL tushunadi — chunki ma'no QAT'IY belgilangan. HTTP status kodlari ham xuddi shunday "universal til": 404 HAR DOIM "topilmadi", 409 HAR DOIM "ziddiyat" degani. Agar server "topilmadi" holatida 200 qaytarsa ("muvaffaqiyatli" degan ma'noda), bu — svetoforning QIZIL chirog'ida "yur" deyishga o'xshaydi: mijoz kutubxonalari (HTTP client'lar), monitoring tizimlari — BARCHASI adashadi.

package main

import (
	"errors"
	"fmt"
	"net/http"
	"net/http/httptest"
)

var (
	ErrNotFound   = errors.New("resurs topilmadi")
	ErrValidation = errors.New("validatsiya xatosi")
	ErrConflict   = errors.New("bunday resurs allaqachon mavjud")
)

// statusFor — xato TURIGA qarab, TO'G'RI HTTP status kodini tanlaydi
func statusFor(err error) int {
	switch {
	case errors.Is(err, ErrNotFound):
		return http.StatusNotFound
	case errors.Is(err, ErrValidation):
		return http.StatusUnprocessableEntity
	case errors.Is(err, ErrConflict):
		return http.StatusConflict
	default:
		return http.StatusInternalServerError
	}
}

func handle(w http.ResponseWriter, err error) {
	if err == nil {
		w.WriteHeader(http.StatusOK)
		fmt.Fprint(w, "ok")
		return
	}
	w.WriteHeader(statusFor(err))
	fmt.Fprint(w, err.Error())
}

func main() {
	for _, err := range []error{nil, ErrNotFound, ErrValidation, ErrConflict} {
		rec := httptest.NewRecorder()
		handle(rec, err)
		fmt.Println(rec.Code, rec.Body.String())
	}
}

Sentinel xatolar (ErrNotFound, ErrValidation, ErrConflict — "Sentinel Errors" darsini eslang) va errors.Is — biznes mantiq ("bu resurs topilmadimi?") bilan HTTP QATLAMI ("qaysi status kod?") ni ALOHIDA saqlaydi: biznes kod HECH QACHON http paketini bilmaydi, FAQAT error qaytaradi; statusFor esa BITTA joyda, xatoni HTTP tiliga TARJIMA qiladi.

Har bir status — ANIQ ma'noga ega: 404 Not Found — so'ralgan narsa YO'Q; 409 Conflict — so'rov O'ZI to'g'ri, lekin joriy HOLAT bilan ZID (masalan bir xil ISBN'li kitob YARATISHGA urinish); 422 Unprocessable Entity — so'rov TUZILISHI to'g'ri, lekin QIYMATLAR noto'g'ri; 500 Internal Server Error — bu ODDIY holatlar EMAS, dasturdagi KUTILMAGAN nosozlik.