Why can't I assign a *Struct to an *Interface?

I'm just working through the Go tour, and I'm confused about pointers and interfaces. Why doesn't this Go code compile?

package main

type Interface interface {}

type Struct struct {}

func main() {
	var ps *Struct
	var pi *Interface
	pi = ps
	
	_, _ = pi, ps
}

i.e. if Struct is an Interface, why wouldn't a *Struct be a *Interface?

The error message I get is:

prog.go:10: cannot use ps (type *Struct) as type *Interface in assignment:
	    *Interface is pointer to interface, not interface

When you have a struct implementing an interface, a pointer to that struct implements automatically that interface too. That's why you never have *SomeInterface in the prototype of functions, as this wouldn't add anything to SomeInterface, and you don't need such a type in variable declaration (see this related question).

An interface value isn't the value of the concrete struct (as it has a variable size, this wouldn't be possible), but it's a kind of pointer (to be more precise a pointer to the struct and a pointer to the type). Russ Cox describes it exactly here :

> Interface values are represented as a two-word pair giving a pointer
> to information about the type stored in the interface and a pointer to
> the associated data.

This is why Interface, and not *Interface is the correct type to hold a pointer to a struct implementing Interface.

So you must simply use

var pi Interface

This is perhaps what you meant:

package main

type Interface interface{}

type Struct struct{}

func main() {
        var ps *Struct
        var pi *Interface
        pi = new(Interface)
        *pi = ps

        _, _ = pi, ps
}

Compiles OK. See also here.

Here's a very simple way of assigning a struct to an interface:

package main

type Interface interface{}

type Struct struct{}

func main() {
    ps := new(Struct)
    pi := Interface(ps)

    _, _ = pi, ps
}

https://play.golang.org/p/BRTaTA5AG0S

Im using the following way of interface{} while im just consuming eventsI interface{} as arguments, im still able to send a Struct Pointers as you can see below.

func Wait(seconds float64) *WaitEvent {
	return WaitEventCreate(seconds)
}

main.go

var introScene = []interface{}{
		storyboard.Wait(5),
		storyboard.Wait(2),
	}

	var storyboardI = storyboard.Create(stack, introScene)
	stack.Push(&storyboardI)

Now inside storyboard.go file Create function

type Storyboard struct {
	Stack  *gui.StateStack
	Events []interface{} //always keep as last args
}

func Create(stack *gui.StateStack, eventsI interface{}) Storyboard {
	sb := Storyboard{
		Stack: stack,
	}

	if eventsI != nil {
		events := reflect.ValueOf(eventsI)
		if events.Len() > 0 {
			sb.Events = make([]interface{}, events.Len())
			for i := 0; i < events.Len(); i++ {
				sb.Events[i] = events.Index(i).Interface()
			}
		}
	}

	return sb
}

As you can see above the Storyboard.go is consuming just Events []interface{} but in-fact Im sending is a Struct pointer and it works fine.

another more example here