Custom json decoding to check for error or other data

main.go

// play link: https://play.golang.org/p/xf43Ta309p
// golan-nuts ref: https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/gg4T4jMuz0U
package main

import (
	"encoding/json"
	"errors"
	"fmt"
)

type ErrResponse struct {
	Error string `json:"error"`
}

func isErrResponse(resp string) error {
	var any interface{}
	err := json.Unmarshal([]byte(resp), &any)
	if err != nil {
		return err
	}

	switch v := any.(type) {
	case map[string]interface{}:
		if msg, ok := v["error"]; ok {
			s, isString := msg.(string) // only if the value is of string type
			if isString && s != "" {
				return errors.New(s)
			}
			return nil
		}
		fmt.Printf("JSON data is an object: %v\n", v)
		return nil
		/*
			case []interface{}:
				fmt.Printf("JSON data is an array: %v\n", v)
				return nil
			default:
				fmt.Printf("JSON data is of type %T: %#v:\n", v, v)
				return nil
		*/
	}

	return nil
}


func main() {
	err := isErrResponse(`
	{
		"error": "there was an error"
	}
	`)
	fmt.Printf("1. %+v\n", err)
	
	err = isErrResponse(`
	[
		{"name": "carrie"}	
	]
	`)
	fmt.Printf("2. %+v\n", err)
	
	err = isErrResponse(`
	{
		"error": 5
	}
	`)
	fmt.Printf("3. %+v\n", err)
}