Unmarshaling a dynamic with golang encoding/json

dynamic_json.go

package main

import (
	"encoding/json"
	"fmt"
)

type Envelope struct {
	Type    string
	Payload interface{}
}

type ReadEnvelope struct {
	Type    string
	Payload json.RawMessage
}

type Person struct {
	Name string
}

type Dog struct {
	Coat string
}

type Fish struct {
	Length float64
}

var typeMap = map[string]func() interface{}{
	"person": func() interface{} { return &Person{} },
	"dog":    func() interface{} { return &Dog{} },
	"fish":   func() interface{} { return &Fish{} },
}

func main() {
	d := []Envelope{
		{Type: "person", Payload: Person{Name: "Ben"}},
		{Type: "dog", Payload: Dog{Coat: "black"}},
		{Type: "fish", Payload: Fish{Length: 20.0}},
		{Type: "porgy", Payload: Fish{Length: 20.0}},
	}

	j, _ := json.Marshal(d)
	var lst []ReadEnvelope
	json.Unmarshal(j, &lst)
	payloads := make([]interface{}, len(lst))
	for i, env := range lst {
		fn, f := typeMap[env.Type]
		if !f {
			fmt.Printf("Received unhandled type %s\n", env.Type)
			continue
		}
		v := fn()
		json.Unmarshal(env.Payload, v)
		payloads[i] = v
	}

	for _, e := range payloads {
		switch v := e.(type) {
		case *Person:
			fmt.Printf("Got a person and know its type %+v\n", v)
		case *Dog:
			fmt.Printf("Got a dog and know its type %+v\n", v)
		case *Fish:
			fmt.Printf("Got a fish and know its type %+v\n", v)
		}
	}
}