golang get a struct from an interface via reflection

I'm trying to do some deserialization and I'm a little confused about how to access a struct when passing in an interface.

package main

import (
    "fmt"
    "reflect"
)

type Robot struct {
    Id int
}

func f(i interface{}) {
    v := reflect.ValueOf(i).Elem().FieldByName("Id")
    fmt.Println("fields: ", reflect.ValueOf(i).Elem().NumField()) 
    ptr := v.Addr().Interface().(*int)
    *ptr = 100
}

func main() {
    robot := Robot{}

    var iface interface{} = robot // if I omit this line and pass in robot this works
    f(&iface)
    fmt.Println(robot.Id) //I want to get here 100

}

http://ift.tt/1jYwF6N

The play example works if you just pass in the struct directly, however as it's possible for anything to be passed in that implements a specific interface (in my example case I'm just using the empty interface). However I can't figure out how to then treat it as a struct underneath.