Demo code for http://pickardayune.com/blog/2018/01/05/nil-is-not-always-nil-in-go

is_nil_really_nil.go

package main

import (
	"fmt"
)

func main() {
	try(&Owner{})
	try(&Owner{org: &Organization{"group"}})
	try(&Owner{user: &User{"person"}})
}

func try(o *Owner) {
	defer func() {
		if rvr := recover(); rvr != nil {
			fmt.Printf(" => panic!! %#v\n", rvr)
		}
	}()

	fmt.Printf("Trying %#v\n", o)
	fmt.Printf(" => %q\n", o.DatabaseString())
}

type Owner struct {
	org  *Organization
	user *User
}

func (o Owner) DatabaseString() string {
	var owner DatabaseStringer
	fmt.Printf("  [0] %#v\n", owner)
	if owner == nil {
		owner = o.GetOrganization()
		fmt.Printf("  [1] %#v\n", owner)
	}
	if owner == nil {
		owner = o.GetUser()
		fmt.Printf("  [2] %#v\n", owner)
	}
	if owner == nil {
		return "NIL"
	}
	return owner.DatabaseString()
}

func (o Owner) GetOrganization() *Organization {
	return o.org
}

func (o Owner) GetUser() *User {
	return o.user
}

type DatabaseStringer interface {
	DatabaseString() string
}

type Organization struct {
	name string
}

func (o *Organization) DatabaseString() string {
	return "organization:" + o.name
}

type User struct {
	name string
}

func (u *User) DatabaseString() string {
	return "user:" + u.name
}