fmt.Errorf
- Go course
- Intermediate
- Lesson 56/80
- 4 min
Learn to create formatted errors with Errorf.
fmt.Errorf
fmt.Errorf creates a formatted error. The %w verb wraps the underlying error: context is added to the message but the original error isn't lost.
f, err := os.Open(yol)
if err != nil {
return fmt.Errorf("ayar okunamadı (%s): %w", yol, err)
}The caller can still query the root cause with errors.Is(err, fs.ErrNotExist). With %v only the text would be copied and the chain would break.
errors.As and custom errors
To decide by error type, errors.As searches the chain for a specific type:
type DogrulamaHatasi struct{ Alan string }
func (e *DogrulamaHatasi) Error() string { return e.Alan + " geçersiz" }
err := fmt.Errorf("kayıt: %w", &DogrulamaHatasi{"eposta"})
var dh *DogrulamaHatasi
if errors.As(err, &dh) {
fmt.Println(dh.Alan) // eposta
}Since Go 1.20 several %w verbs can be used in one Errorf.
Practice Wrapped error
Declare var ErrYok = errors.New("kayıt yok"). The function bul(id int) error returns fmt.Errorf("bul %d: %w", id, ErrYok). Print the error and errors.Is(err, ErrYok) on separate lines.
Starter code
package main
import (
"errors"
"fmt"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"errors"
"fmt"
)
var ErrYok = errors.New("kayıt yok")
func bul(id int) error {
return fmt.Errorf("bul %d: %w", id, ErrYok)
}
func main() {
err := bul(7)
fmt.Println(err)
fmt.Println(errors.Is(err, ErrYok))
}Expected output
bul 7: kayıt yok true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 With an error wrapped using <code>%v</code> instead of <code>%w</code>, what does <code>errors.Is</code> return?
- false; the chain is broken Correct answer
- true
- panic
- nil
Explanation: %v only turns the message into text; without an Unwrap method errors.Is can't reach the original error.
2 Wrap the error
return fmt.Errorf("bağlantı kurulamadı: ____", err)return fmt.Errorf("bağlantı kurulamadı: %w", err)Explanation: %w wraps the error; errors.Is and errors.As can reach it in the chain.
3 Function and role
| Term | Meaning |
|---|---|
fmt.Errorf + %w |
Wrap with context |
errors.Is |
Is it a specific error? |
errors.As |
Is it a specific type? |
errors.Unwrap |
Unwrap one level |