unmarshalling arbitrary data

Is there a way to marshall JSON data in such a way that it can be unmarshalled in parts / sections?

Let's say that the top half of data is a "code" which would signal what to do with the bottom half ... such as unmarshall the bottom half into a specific struct depending on the "code".

There are two structs that may be sent as the bottom half ...

type Range Struct {
    Start int
    End   int

}

type User struct {
    ID    int
    Pass  int
}

PSEUDO CODE EXAMPLE

It may look like this ...

message := &Message{
    Code: 4,
    &Range {
        Start: 1,
        End: 10,
    }
}

Itt may look like this ...

message := &Message{
    Code: 3,
    &User {
        ID: 1,
        Pass: 1234,
    }
}

So, when unmarshalling that data I could ...

// get code from top half
m := Message{}
err = json.UnMarshallTopHalf(byteArray, &m)
if m.Code == 4 {
    // ok, the code was four, lets unmarshall into type Range
    r := Range{}
    json.UnmarshalBottomHalf(byteArray, &r)
}

I have looked at JSON & Go to learn how to marshall and unmarshall defined structs. I can do this, but I cannot figure out a way for arbitrary data as in the example above ...

type Message struct  {
    Code int `json:"cc"`
    Range *Range `json:"vvv,omitempty"`
    User *User `json:"fff,omitempty"`
}

then given code == x, use range, if Y, use User.

You can unmarshall bottom half in json.RawMessage first, something like

package main

import (
	"encoding/json"
	"fmt"
)

type Message struct {
	Code    int
	Payload json.RawMessage // delay parsing until we know the code
}
type Range struct {
	Start int
	End   int
}
type User struct {
	ID   int
	Pass int
}

func MyUnmarshall(m []byte) {
	var message Message
	var payload interface{}
	json.Unmarshal(m, &message) // delay parsing until we know the color space
	switch message.Code {
	case 3:
		payload = new(User)
	case 4:
		payload = new(Range)
	}
	json.Unmarshal(message.Payload, payload) //err check ommited for readability
	fmt.Printf("\n%v%+v", message.Code, payload) //do something with data
}

func main() {
	json := []byte(`{"Code": 4, "Payload": {"Start": 1, "End": 10}}`)
	MyUnmarshall(json)
	json = []byte(`{"Code": 3, "Payload": {"ID": 1, "Pass": 1234}}`)
	MyUnmarshall(json)
}