A custom error is your own error type (usually a struct) that implements `Error() string`. It may include extra fields (code, operation, resource ID, etc.).

• Return structured error details.
• Enable typed checks with `errors.As`.
• Add machine-readable info (e.g., HTTP status mapping).

package main
 
import "fmt"
 
type NotFoundError struct {
    Resource string
    ID       string
}
 
func (e *NotFoundError) Error() string {
    return fmt.Sprintf("%s not found: %s", e.Resource, e.ID)
}

• Prefer pointers for custom error types when they have fields.
• Provide helper constructors like `NewNotFound(resource, id)`.

• structured /ˈstrʌktʃərd/: có cấu trúc
• machine-readable /məˈʃiːn ˈriːdəbəl/: máy đọc được
• constructor /kənˈstrʌktər/: hàm tạo