main.go

package main

import (
	"fmt"
	"encoding/json"
)

type Thing1 struct {}
func (Thing1) DoThing() {}

type Thing2 struct {
	Number int `json:"number"`
	String string `json:"string"`
}
func (Thing2) DoThing() {}

type Bleh interface {
	DoThing()
}

type Holder struct {
	Type string `json:"type"`
	Doer Bleh
}

func (h *Holder) UnmarshalJSON(data []byte) error {
	type Alias Holder
	aux := &struct {
		*Alias
	}{
		Alias: (*Alias)(h),
	}
	
	if err := json.Unmarshal(data, &aux); err != nil {
		return err
	}
	
	switch aux.Type {
	case "Thing1":
		aux.Doer = &Thing1{}
	case "Thing2":
		aux.Doer = &Thing2{}
	}
	
	envelope := &struct {
		Payload Bleh
	}{
		Payload: aux.Doer,
	}
	
	if err := json.Unmarshal(data, &envelope); err != nil {
		return err
	}
	
	h.Doer = envelope.Payload
	return nil
}

func main() {
	payload := `{
		"type": "Thing2",
		"payload": {
			"number": 42,
			"string": "once upon a time in a galaxy far far away"
		}
	}`
	
	h := &Holder{}
	if err := json.Unmarshal([]byte(payload), h); err != nil {
		fmt.Println(err)
	}
	fmt.Println(h)
	fmt.Println(h.Doer)
}