Summary - Interface and error
- Go course
- Beginner
- Lesson 86/90
- 4 min
Combine interface and error to write a validator.
Interfaces and errors summary
An interface = a list of methods; types satisfy it implicitly. Small interfaces (io.Reader, fmt.Stringer) are powerful abstractions. error is an interface too: Error() string. Errors are created with errors.New and fmt.Errorf("...: %w", err), returned as the last value, and inspected with errors.Is (value) and errors.As (type).
Type switch
To branch on the real type of an interface value, use switch v := x.(type):
func tanimla(x any) string {
switch v := x.(type) {
case int:
return fmt.Sprintf("tam sayı %d", v)
case string:
return "metin " + v
case error:
return "hata: " + v.Error()
default:
return "bilinmeyen"
}
}Practice Describe it
Write the tanimla(x any) string function above with a type switch. Print the results for 42, "go", errors.New("x") and 3.5 on separate lines.
Starter code
package main
import (
"errors"
"fmt"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"errors"
"fmt"
)
func tanimla(x any) string {
switch v := x.(type) {
case int:
return fmt.Sprintf("tam sayı %d", v)
case string:
return "metin " + v
case error:
return "hata: " + v.Error()
default:
return "bilinmeyen"
}
}
func main() {
for _, x := range []any{42, "go", errors.New("x"), 3.5} {
fmt.Println(tanimla(x))
}
}Expected output
tam sayı 42 metin go hata: x bilinmeyen
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What can the empty interface <code>any</code> (<code>interface{}</code>) hold?
- Only structs
- Only nil
- Only numbers
- A value of any type Correct answer
Explanation: Every type satisfies an interface with no methods; using the value requires a type assertion or a type switch.
2 Type switch
switch v := x.(____) {
case int:
fmt.Println("int", v)
}switch v := x.(type) {
case int:
fmt.Println("int", v)
}Explanation: x.(type) is used only inside a switch and branches on the real type.
3 Group summary
| Term | Meaning |
|---|---|
interface |
Behavior contract |
any |
Any type |
x.(type) |
Branching by type |
errors.Is / As |
Error chain |