Below codes are about a function that returning nil custom error in two variations. Expectantly, these codes should always exit successfully, but it didn’t happen on Code 2.
If we change operation() return signature to error interface instead, Code 2 able to exit again.
func operation() ([]byte, error) {
return nil, nil
}
Can someone please explain the mechanism behind it? And what’s the difference between these two exactly? Thanks.
In Code 2:
package main
import "fmt"
type customError struct{}
func (c *customError) Error() string {
return "custom error!"
}
func operation() ([]byte, *customError) {
return nil, nil
}
func main() {
var err error
fmt.Println(err) //here, err is a nil pointer to error interface
_, err = operation()
fmt.Println(err) //here, err is pointer to nil customError interface, err is not nil itself
if err != nil {
println("ERROR!!!! ARRG!")
return
}
println("peace out")
}
You can see the output here: https://play.golang.org/p/uvtktWe2MlS
You can fix Code 2 as following:
package main
type customError struct{}
func (c *customError) Error() string {
return "custom error!"
}
func operation() ([]byte, *customError) {
return nil, nil
}
func main() {
var err *customError
_, err = operation()
if err != nil {
println("ERROR!!!! ARRG!")
return
}
println("peace out")
}
You can see the fixed output here: https://play.golang.org/p/z4tVkriX30O
For better understanding please check following code:
package main
type customError struct{}
func (c *customError) Error() string {
return "custom error!"
}
func operation() ([]byte, *customError) {
return nil, nil
//return nil, &customError //how to return a custom error
}
func main() {
var err error
_, err = operation()
if err != (*customError)(nil) { //here, we are checking if err is not equal to a pointer to nil custom error
println("ERROR!!!! ARRG!")
return
}
println("peace out")
}
You can see the output here: https://play.golang.org/p/RhPkEOjl--p
For the original design for Go interfaces:
Go Data Structures: Interfaces
Posted on Tuesday, December 1, 2009.
For err == nil it has to have a type of nil and a value of nil, but err is set to a concrete type of *customError and a concrete value of nil and err != nil.
Instead, write:
package main
import "fmt"
type customError struct{}
func (c *customError) Error() string {
if c == nil {
return "custom nil!"
}
return "custom error!"
}
func operation() ([]byte, *customError) {
return nil, nil
}
func main() {
var err error
fmt.Printf("%[1]T %[1]v\n", err)
_, err = operation()
fmt.Printf("%[1]T %[1]v\n", err)
if v, ok := err.(*customError); ok && v != nil {
println("ERROR!!!! ARRG!")
return
}
println("peace out")
}
<nil> <nil>
*main.customError custom nil!
peace out
Thanks, Petrus! the FAQ hits the spot. The article about go interface is worth giving a read.
Appreciate that you spend effort to answer my newb question. 
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