Golang pass nil value as an interface through reflection - go

Question

I have a function with interface argument:
func f(e error) {
if e == nil {
fmt.Println("YEY! NIL") // how to get here?
} else {
fmt.Println("NOT NIL :(")
}
}
How do I pass it a nil value via reflect so that it passes == nil check?
Approach 1:
func main() {
rf := reflect.ValueOf(f)
nilArg := reflect.Zero(reflect.TypeOf((error)(nil))) // panic: reflect: Zero(nil)
rf.Call([]reflect.Value{nilArg})
}
Approach 2:
type MyError struct{}
func (e MyError) Error() string {
return ""
}
func main() {
rf := reflect.ValueOf(f)
nilArg := reflect.Zero(reflect.TypeOf(&MyError{})) // NOT NIL :(
rf.Call([]reflect.Value{nilArg})
}
Second approach doesn't work due to https://golang.org/doc/faq#nil_error
Playground: https://play.golang.org/p/V0bMSPcCKI

Answer 1

Use the expression reflect.TypeOf((*error)(nil)).Elem() to get the reflect.Type for interface error.
The first approach in the question does not work because the expression reflect.TypeOf((error)(nil)) returns nil. The concrete type of a nil interface value is nil.
The trick is to pass a non-interface value to reflect.TypeOf() and use reflect methods to get the desired reflect.Type from there. In this answer, I pass a *error to reflect.TypeOf() and call Elem on the result to get the reflect.Type for error.
Use the following to create nilArg:
nilArg := reflect.Zero(reflect.TypeOf((*error)(nil)).Elem())
playground example

Answer 2

You have two options, basically e in the arguments to function f is (*MyError)(nil) not nil. So either assert the interface back to type MyError or use more reflection to check if it's nil.
e.(*MyError) == nil
or
reflect.ValueOf(e).IsNil()
Runnable Example: https://play.golang.org/p/2KWguSx618
useful links:
Check for nil and nil interface in Go
https://groups.google.com/forum/#!topic/golang-nuts/wnH302gBa4I/discussion