Structured Error Context Extraction via Custom Unwrap Interfaces and errors.As

package main
 
import (
	"errors"
	"fmt"
)
 
type CodedError interface {
	error
	ErrorCode() string
}
 
type DomainError struct {
	Code    string
	Message string
	Err     error
}
 
func (e *DomainError) Error() string     { return fmt.Sprintf("[%s] %s", e.Code, e.Message) }
func (e *DomainError) ErrorCode() string { return e.Code }
func (e *DomainError) Unwrap() error     { return e.Err }
 
func ExtractCode(err error) string {
	var coded CodedError
	if errors.As(err, &coded) {
		return coded.ErrorCode()
	}
	return "UNKNOWN_ERROR"
}
 
func main() {
	base := errors.New("connection timeout")
	wrapped := &DomainError{Code: "NET_504", Message: "Gateway Timeout", Err: base}
	fmt.Println("Extracted Error Code:", ExtractCode(wrapped))
}