Typed Nil in Go

Guess the output

Let’s take this method which is simple nil check

func IsErrNil(err error) {
    println(err == nil)
}

Let’s start directly with snippets and guess their output

func main() {
    var err error
    fmt.Println(err)
    fmt.Println(err == nil)
    IsErrNil(err)
}

What do you think the output is?

If you think that it’s

<nil>
true
true

Then right! Easy, cause in Go default value of error is nil

Let’s go to the next snippet, here I am defining my own custom error type that satisfies the error interface in Go

type CustomErr struct{}

func (c *CustomErr) Error() string {
	if c == nil {
		return "<nil>"
	}
	return "custom error"
}

func main() {
    var myCustomErr *CustomErr
    fmt.Println(myCustomErr)
    fmt.Println(myCustomErr == nil)
    IsErrNil(myCustomErr)
}

If you think that output is

<nil>
true
true

Then it’s wrong! The correct output is

<nil>
true
false

Why?

Let’s add this function that will print the type of the error

func printType(err error) {
	t := reflect.TypeOf(err)
	v := reflect.ValueOf(err)

	if t == nil {
		fmt.Println("type = <nil>, no value")
		return
	}

	if v.Kind() == reflect.Ptr && v.IsNil() {
		fmt.Printf("type = %v, value = <nil>\n", t)
	} else {
		fmt.Printf("type = %v, value = %v\n", t, v)
	}
}

If we call printType(myCustomErr), it will output the following

type = *main.CustomErr, value = <nil>

Calling printType(err), it will output

type = <nil>, no value

Here is the catch! In Go, an interface is nil only if it’s type and value are nil

The function IsErrNil takes an interface error so the err == nil will evaluate to false in case of myCustomErr cause it’s type is not nil

Conclusion

Bugs like these sometimes can be unintuitive, so keep your tests around :)

Here is final source code code