Golang base struct to define methods in substructs

I would like to create a base struct which have to method, I want to use these methods in the substructs. For example:

type Base struct {
  Type string `json:"$type"`
}

func (b Base) GetJSON() ([]byte, error) {
  return json.Marshal(b)
}

func (b Base) SetType(typeStr string) interface{} {
  b.Type = typeStr
  return b
}

In the new struct I want to use it like this:

type Auth struct {
  Base
  Username
  Password
}

and call these methods in the main:

func main() {
  a := Auth{
    Username: "Test",
    Password: "test",
  }
  a = a.SetType("testtype").(Auth) 
  j, _ := a.GetJSON()
}

In the SetType case I got a panic caused by interface{} is not Auth type, it is Base type.
In the GetJSON case I got a json about the Type, but only the Type.

Is there any solution for the problem what I want to solve?

As mentioned in the comments, embedding is not inheritance but composition, so you'll probably have to either:

• Re-think your design to use the tools Go has available
• Resort to extensive hacking to get the results you want

In the particular case you are showing (trying to get GetJSON() to include also the fields of the outer struct, here is a possible way of getting that to work that does not require many changes (just storing a pointer to the outer struct in Base when creating the struct):

package main

import (
    "encoding/json"
    "fmt"
)

type Base struct {
    Type  string      `json:"$type"`
    selfP interface{} // this will store a pointer to the actual sub struct
}

func (b *Base) SetSelfP(p interface{}) {
    b.selfP = p
}

func (b *Base) GetJSON() ([]byte, error) {
    return json.Marshal(b.selfP)
}

func (b *Base) SetType(typeStr string) {
    b.Type = typeStr
}

type Auth struct {
    Base
    Username string
    Password string
}

func main() {
    a := &Auth{
        Username: "Test",
        Password: "test",
    }
    a.SetSelfP(a) // this line does the trick
    a.SetType("testtype")
    j, _ := a.GetJSON()
    fmt.Println(string(j))
}

Playground link: https://play.golang.org/p/npuy6XMk_t