Go: Check Nil interface the right way

Mangat Rai Modi
3 min readDec 4, 2019

I had a simple problem. I have a function which takes a parameter of type interface{}. I need to check it for nil. Any seasoned Go developer will know that a simple i==nil check will not work because interfaces in Go contains both type and value. So you can have cases when —

  1. Type is null-able(like map, pointer etc) and value is nil
  2. Type itself is nil (of course value will be nil)

A nil will only work with option 2 above, as for option 1 a variable still have some type. As any good engineer I googled and found the following solution prominently suggested everywhere.

Popular Solution

func isNil(i interface{}) bool {                        
return i == nil || reflect.ValueOf(i).IsNil() }

This seems to be working for the following case -

type Animal interface {
MakeSound() string
type Dog struct{}

func (d *Dog) MakeSound() string {
return "Bark"
func main() {
var d *Dog = nil
var a Animal = d

We have a pointer to Dog type, which implements Animalinterface with the pointer receiver.

However the code fails when we give non pointer argument

type Cat struct{}

func (c Cat) MakeSound() string {
return "Meow"
func main() {
var c Cat
var a Animal = c

The isNil function will fail with a panic

panic: reflect: call of reflect.Value.IsNil on struct Value

It happens because we are trying to check if a struct value is nil, which is logically wrong for Go. One could argue that why would you give a struct value for nil check, but hey bugs happen! At least:

  1. I would like to fail gracefully.
  2. Or It shall not fail. If a function accepts an argument at compile time, it should be ready to give correct result for that argument(and not fail). This makes things more predictable.

If isNil takes an interface parameter, it should work for all types of interfaces — pointers, struct, channel etc..

Better Version

So I ended up writing following version

func isNilFixed(i interface{}) bool {
if i == nil {
return true
switch reflect.TypeOf(i).Kind() {
case reflect.Ptr, reflect.Map, reflect.Array, reflect.Chan, reflect.Slice:
return reflect.ValueOf(i).IsNil()
return false

Now, if the type is kind pointer, it checks for nil, else it is of course false(not nil)

Best Solution

However the best solution for our example would be to have following

func isNilBetter(i Animal) bool {
var ret bool
switch i.(type) {
case *Dog:
v := i.(*Dog)
ret = v == nil
case Cat:
ret = false
return ret

Here the compiler will not even allow us to compile if we gave any other case in above, not even Dog as struct. So it is type safe, more readable as we know which types we can get and no reflection! But it is not always possible to have this solution because

  1. We might have dozens of implementations of an interface.
  2. We might have accidentally implemented an interface (duck-typing), so we miss checks for that(I miss exhaustiveness of switch in Kotlin)
  3. We simply don’t know which interface we are going to use.

The complete example can be found here — https://gist.github.com/mangatmodi/06946f937cbff24788fa1d9f94b6b138



Mangat Rai Modi

mangatmodi.com | High scalable services, Low latency coding, and Concurrency! Get’s excited by any tough problem.