Golang error: interface conversion: interface {} is bool/float..., not string

Question

I am trying to decode an arbitrary JSON using Golang, so I unmarshal the incoming JSON in a map[string]interface{} as shown in the code below:

    func JsonHandler(jsonRequest []byte) {

      // Creating the maps for JSON
      var m interface{}

      // Parsing/Unmarshalling JSON encoding/json
      if err := json.Unmarshal([]byte(jsonRequest), &m); err != nil {
            panic(err)
       }

       //Creating an output file for writing
       f, err := os.OpenFile("/home/dorrahadrich/Desktop/output.txt", os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0600)
       if err != nil {
           panic(err)
       }

       defer f.Close()

       ParseJson(m, f, err)
   }

   func ParseJson(m interface{}, f *os.File, err error) {
       switch v := m.(interface{}).(type){
          case map[string]interface{}:
               ParseMap (m.(map[string]interface{}),f,err)
               fmt.Println(v)
          case []interface{}:
               ParseArray (m.([]interface{}),f,err)
               fmt.Println(v)
          default:
     }
 }

   func ParseMap(aMap map[string]interface{}, f *os.File, err error) {
    for key, val := range aMap {
        switch val.(type) {
        case map[string]interface{}:
            if _, err = f.WriteString(key + "={\n"); err != nil {
                panic(err)
            }

            ParseMap(val.(map[string]interface{}), f, err)

            //Close brackets
            if _, err = f.WriteString("};\n"); err != nil {
                panic(err)
            }

        case []interface{}:
            //Write to file
            if _, err = f.WriteString(key + "={\n"); err != nil {
                panic(err)
            }

            ParseArray(val.([]interface{}), f, err)

            //Close brackets
            if _, err = f.WriteString("};\n"); err != nil {
                panic(err)
            }
        default:
            otherValues(key, val.(interface{}), f , err)
        }
    }
}

func ParseArray(anArray []interface{}, f *os.File, err error) {
    for _, val := range anArray {
        switch val.(type) {
        case map[string]interface{}:
            ParseMap(val.(map[string]interface{}), f, err)
        case []interface{}:
            ParseArray(val.([]interface{}), f, err)
        default:
        }
    }

}
func otherValues(key string, other interface{}, f *os.File, err error) {
     if _, err = f.WriteString(key); err != nil {
           panic(err)
        }
    if _, err = f.WriteString("="); err != nil {
        panic(err)
        }

   switch other.(interface{}).(type) {
       case string:
           if _, err = f.WriteString(other.(string)); err != nil {
               panic(err)
            }
       case float64:
           if _, err = f.WriteString(strconv.FormatFloat(other.(float64), 'f', -1, 64)); err != nil {
                panic(err)
            }
       case bool:
    if _, err = f.WriteString(strconv.FormatBool(other.(bool))); err != nil {
                panic(err)
            }
       default:
     }  
   }

The problem is that whenever a JSON contains a bool/int/float or any not string value the program panics saying that it fails converting an interface to the given type! Please note that the JSON is arbitrary so I don't have any idea about the keys nor the values, I can't unmrashal into an interface nor access the values giving a path.

Unmarshal into a simple value of type interface{}. Does that work for you? Any valid JSON can be unmarshaled into that. — icza
– icza, Commented Mar 5, 2018 at 9:12
This is because When you are unmarshalling a json to interface{} it can also contain bool/float64/string. You have to add cases for the same too — Himanshu
– Himanshu, Commented Mar 5, 2018 at 9:12
I tried to switch the type of the "val" and convert it to string before passing it to the f.WriteString function, still the same error!! — Dorra Hadrich
– Dorra Hadrich, Commented Mar 5, 2018 at 9:12

Himanshu · Accepted Answer · 2018-03-05 11:22:46Z

2

The error says it all:

interface conversion: interface{} is bool/float64

when you are unmarshalling json the values for int and bool which are not of interface type. In your switch add case for bool/float64/string too. Since json is arbitrary unmarshal them using interface{}.

func otherValues(other interface{}, f *os.File, err error) {
    switch bb := other.(interface{}).(type) {
    case string:
        fmt.Println("This is a string")
    case float64:
        fmt.Println("this is a float")
    case bool:
        fmt.Println("this is a boolean")
    default:
        fmt.Printf("Default value is of type %v", bb)
    }
}

Use file.Write in place of file.WriteString

func (f *File) Write(b []byte) (n int, err error)

Write writes len(b) bytes to the File. It returns the number of bytes written and an error, if any. Write returns a non-nil error when n != len(b).

edited Mar 5, 2018 at 11:22

answered Mar 5, 2018 at 9:27

Himanshu

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10 Comments

Dorra Hadrich Over a year ago

I did unmarshal JSON into an interface{} and then did a switch of the real type of the interface{} and added the cases as you said but I am still getting the same error.

Himanshu Over a year ago

Show the json that you are unmarshalling and the code that you have tried after passing data. Are you getting same error

Himanshu Over a year ago

May be you are using file.WriteString that's why the string can be written. You can use file.Write

Himanshu Over a year ago

I have found a mistake in your code and edited it. We needs to convert other.(bool) to other.(interface{}).(bool)

Dorra Hadrich Over a year ago

But I am converting values to string before using the file.WriteString!

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