zlacker

>>ustad+(OP)
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
    }

The author of this post has noted a convenience feature, namely that fmt.Println() tells you the state of the thing in the interface and not the state of the interface, mistaken it as a fundamental design issue and written a screed about a language issue that literally doesn't exist.

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.

>>gnfarg+8i
Author here. No, I didn't misunderstand it. Interface variables have two types of nil. Untyped, which does compare to nil, and typed, which does not.

What are you trying to clarify by printing the types? I know what the types are, and that's why I could provide the succinct weird example. I know what the result of the comparisons are, and why.

And the "why" is "because there are two types of nil, because it's a bad language choice".

I've seen this in real code. Someone compares a variable to nil, it's not, and then they call a method (receiver), and it crashes with nil dereference.

Edit, according to this comment this two-types-of-null bites other people in production: >>44983576