===== Custom errors =====

==== What is it? ====
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.).

==== What is it used for? ====
  * Return structured error details.
  * Enable typed checks with `errors.As`.
  * Add machine-readable info (e.g., HTTP status mapping).

==== Example ====
<code go>
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)
}
</code>

==== Notes / best practice ====
  * Prefer pointers for custom error types when they have fields.
  * Provide helper constructors like `NewNotFound(resource, id)`.

==== Hard words (English) ====
  * **structured** /ˈstrʌktʃərd/: có cấu trúc
  * **machine-readable** /məˈʃiːn ˈriːdəbəl/: máy đọc được
  * **constructor** /kənˈstrʌktər/: hàm tạo