Comment by gnfargbl
As a long-time Go programmer I didn't understand the comment about two types of nil because I have never experienced that issue, so I dug into it.
It turns out to be nothing but a misunderstanding of what the fmt.Println() statement is actually doing. If we use a more advanced print statement then everything becomes extremely clear:
package main
import (
"fmt"
"github.com/k0kubun/pp/v3"
)
type I interface{}
type S struct{}
func main() {
var i I
var s *S
pp.Println(s, i) // (*main.S)(nil) nil
fmt.Println(s == nil, i == nil, s == i) // true true false
i = s
pp.Println(s, i) // (*main.S)(nil) (*main.S)(nil)
fmt.Println(s == nil, i == nil, s == i) // true false true
}
Being charitable, I guess the author could actually be complaining that putting a nil pointer inside a nil interface is confusing. It is indeed confusing, but it doesn't mean there are "two types" of nil. Nil just means empty.
The author is showing the result of s==nil and i==nil, which are checks that you would have to do almost everywhere (the so called "billion dollar mistake")
It's not about Printf. It's about how these two different kind of nil values sometimes compare equal to nil, sometimes compare equal to each other, and sometimes not
Yes there is a real internal difference between the two that you can print. But that is the point the author is making.