Go interface inheritance - go

Question

I am new to Go and don't understand one thing. Let's take one code which works:
package main
import "fmt"
type User struct {
Name string
Email string
}
type Admin struct {
User
Level string
}
type Notifier interface {
notify()
}
func (u *User) notify() {
fmt.Println("Notified", u.Name)
}
func SendNotification(notify Notifier) {
notify.notify()
}
func main() {
admin := Admin{
User: User{
Name: "john smith",
Email: "john#email.com",
},
Level: "super",
}
SendNotification(&admin)
admin.User.notify()
admin.notify()
}
Here function SendNotification recognises admin struct as Notifier, because admin struct has access to embedded user struct which implements the interface via pointer receiver. Ok.
Why then the code below doesn't work. Why norgateMathError needs to implement the interface and not use implementation from err error (for me it is the same situation):
package main
import (
"fmt"
"log"
)
type norgateMathError struct {
lat string
long string
err error
}
// func (n norgateMathError) Error() string {
// return fmt.Sprintf("a norgate math error occured: %v %v %v", n.lat, n.long, n.err)
// }
func main() {
_, err := sqrt(-10.23)
if err != nil {
log.Println(err)
}
}
func sqrt(f float64) (float64, error) {
if f < 0 {
nme := fmt.Errorf("norgate math redux: square root of negative number: %v", f)
return 0, &norgateMathError{"50.2289 N", "99.4656 W", nme}
}
return 42, nil
}
.\custom_error.go:28:13: cannot use &norgateMathError{...} (type *norgateMathError) as type error in return argument:
*norgateMathError does not implement error (missing Error method)

Answer 1

In the first case User is embedded inside Admin, thus Admin gain access to all methods defined on the User type.
In the second case, norgateMathError has a field err of type Error, thus do not automatically gain access to it's methods.
If you want norgateMathError to have an Error() method you have to define it manually
func (n norgateMathError) Error() string {
return n.err.Error()
}
There is a different between embedding a field and just having a field. More information can be found in the reference