Decoding JSON within a wrapper using interface{}

jsonwrap.go

package main

import (
	"encoding/json"
	"fmt"
	"io"
	"strings"
)

type ResponseError struct {
	Code	int
	Message	string
}

func (e ResponseError) Error() string {
	return fmt.Sprintf("[%d] %s", e.Code, e.Message)
}

type Response struct {
	Result	interface{}
	Error	*ResponseError
}

// Decode decodes the response from r writing the result into the struct
// pointed to by want.
func Decode(r io.Reader, want interface{}) error {
	resp := &Response{Result: want}
	if err := json.NewDecoder(r).Decode(resp); err != nil {
		return err
	}
	if resp.Error != nil {
		return resp.Error
	}
	return nil
}

func main() {
	type a struct {
		Str string
	}
	type b struct {
		X, Y int
	}
	for js, resp := range map[string]interface{}{
		`{"Result":{"Str":"42"}}`:				&a{},
		`{"Result":{"X":6,"Y":9}}`:				&b{},
		`{"Error":{"Code":400,"Message":"Unauthorized"}}`:	&a{},
	} {
		if err := Decode(strings.NewReader(js), resp); err != nil {
			fmt.Printf("%q: error %q\n", js, err)
			continue
		}
		fmt.Printf("%q: %#v\n", js, resp)
	}
}