doubt about reflect.Type.String and empty interface

[Manlio Perillo]

Hello and happy new year.

I'm re-reading the Go Language Specification, and at https://golang.org/ref/spec#Variables I found this example:

  var x interface{} // x is nil and has static type interface{}

  var v *T               // v has value nil, static type *T

  x = 42                 // x has value 42 and dynamic type int

  x = v                   // x has value (*T)(nil) and dynamic type *T

Since the first comment is different for the others (`x is nil` against `x has value`) I decided to run a simple test

https://play.golang.org/p/QAX92NQDqO4.  It has an additional entry:

  var z *interface{}

The output of the first and last line is

x is nil and has static type interface{}

<invalid reflect.Value> <nil>

z is nil and has static type *interface{}

<nil> *interface {}

The question is: why reflect.Type.String() returns `<nil>` instead of  `interface{}` ?

Is the example printing the static or the dynamic type?

Thanks

Manlio Perillo

[Ian Lance Taylor]

https://blog.golang.org/laws-of-reflection

The reflect package takes value of type interface{}, so passing a
value of type interface{} and passing a value of type *interface{}
acts differently.

Ian

[Manlio Perillo]

On Sun, Jan 5, 2020 at 5:29 PM Ian Lance Taylor <ia...@golang.org> wrote:
>
> On Sun, Jan 5, 2020 at 8:23 AM Manlio Perillo <manlio....@gmail.com> wrote:
> >

>> [...]

> > I decided to run a simple test
> > https://play.golang.org/p/QAX92NQDqO4.
> >

> > [...]

> >
> > The question is: why reflect.Type.String() returns `<nil>` instead of `interface{}` ?
> > Is the example printing the static or the dynamic type?
>
> https://blog.golang.org/laws-of-reflection
>
> The reflect package takes value of type interface{}, so passing a
> value of type interface{} and passing a value of type *interface{}
> acts differently.
>
> Ian

Right, thanks. I was probably assuming TypeOf to be a generic function.

I wrote another test https://play.golang.org/p/hoTAnijCfg1.
Now it is clear why the first entry can only print nil for the type
and <invalid reflect.Value> for the value.

However printf %v verb prints nil for both an empty interface and an
interface with a dynamic value of nil:
https://play.golang.org/p/m_WfR2SPWQ3
Can this cause confusion?

Thanks
Manlio Perillo

[Marvin Renich]

* Manlio Perillo <manlio....@gmail.com> [200106 18:57]:

> I wrote another test https://play.golang.org/p/hoTAnijCfg1.
> Now it is clear why the first entry can only print nil for the type
> and <invalid reflect.Value> for the value.
>
> However printf %v verb prints nil for both an empty interface and an
> interface with a dynamic value of nil:
> https://play.golang.org/p/m_WfR2SPWQ3
> Can this cause confusion?

Yes, it definitely can (and probably does). However, if you change "%v"
to "%#v", it helps to clarify things:

https://play.golang.org/p/K4nPb27Btok

...Marvin