Go
Returning errors
Learn to return errors with errors.New and fmt.Errorf.
Returning errors
Use errors.New for simple errors and fmt.Errorf for formatted messages and context. Errors the caller can compare against are defined as package-level variables (sentinel errors):
var ErrBulunamadi = errors.New("kayıt bulunamadı")
func bul(id int) (string, error) {
if id != 1 {
return "", fmt.Errorf("kullanıcı %d: %w", id, ErrBulunamadi)
}
return "Ada", nil
}
Error messages start with a lower-case letter and don't end with a period, because they are used by appending to other messages.
Adding context to errors
As an error is passed upward, each layer adds what it was doing; the result is a readable chain:
// repository: kayıt bulunamadı
// service: kullanıcı 7: kayıt bulunamadı
// handler: profil yüklenemedi: kullanıcı 7: kayıt bulunamadı
Both logging and returning the same error leads to duplicate log lines; either handle it or add context and pass it on.
Exercise Find a user
Write ErrBulunamadi and the bul(id int) (string, error) function above. Print the results of bul(1) and bul(7) on separate lines (fmt.Println(bul(...))).
Show example solution
package main
import (
"errors"
"fmt"
)
var ErrBulunamadi = errors.New("kayıt bulunamadı")
func bul(id int) (string, error) {
if id != 1 {
return "", fmt.Errorf("kullanıcı %d: %w", id, ErrBulunamadi)
}
return "Ada", nil
}
func main() {
fmt.Println(bul(1))
fmt.Println(bul(7))
}
Test yourself
-
When there is an error, what should the other return values be?
- Anything
- Their zero values ("", 0, nil)
- The last valid value
- -1
Show answer
Their zero values ("", 0, nil) — Convention: when there is an error the other values aren't trusted and zero values are returned; the caller checks the error first.
-
Wrap with context
return "", fmt.Errorf("kullanıcı %d: ____", id, ErrBulunamadi)
Show answer
return "", fmt.Errorf("kullanıcı %d: %w", id, ErrBulunamadi)
-
Tool and use
- errors.New
- A simple error
- fmt.Errorf + %w
- An error with context
- var ErrX = ...
- A comparable error
- return "", err
- Zero value + error
Next lesson: errors.Is and errors.As
