Golang Interface to struct conversion giving error - json

Question

I have an json as a string of following format:
{"add": [{"var": ["100"]}, "200"]}
Here the key 'add' is not a constant value for all the jsons. In some cases, it can be 'minus', 'multiply' etc.
The value of that key is an array. In this case [{"var": ["100"]}, "200"]. This means that the 100 should be added to existing value 200.
I am trying to parse this expression. Since the main key(in this case 'add') is not a constant, I cannot convert into a struct. So, I converted it to a json object by following way:
type mathExpVar struct {
valueVar []string `json:"var"`
}
var mathExpJson map[string][]interface{}
var input = "{\"add\": [{\"var\": [\"100\"]}, \"200\"]}"
err := json.Unmarshal([]byte(input), &mathExpJson)
for operator, values := range mathExpJson{
vals, ok := values[0].(mathExpVar) // here values[0] will be {"var": ["100"]}
if !ok{
return nil
}
}
Here 'ok' is always returning false. I am not sure why. There is no additional error message for me to check why this is failing. Could someone help me in resolving this?
Link to go playground for the same: https://go.dev/play/p/POfQmEoPbjD

Answer 1

A Working example: https://go.dev/play/p/02YzI5cv8vV
The whole reason the original response from Burak Serdar was not valid in my opinion is that it does not take into account the fact that you would need to handle the rest of the params as well. If you look closely enough, then you see that the expression is not an array of strings, its of varying type. This implementation handles the custom Unmarshalling and stores all the extra parameters in the Extra field.
Also code:
package main
import (
"encoding/json"
"log"
"reflect"
)
const jsonPayload = "{\"add\": [{\"var\": [\"100\"]}, \"200\"]}"
func main() {
data := MathExpressions{}
err := json.Unmarshal([]byte(jsonPayload), &data)
if err != nil {
log.Println("Failed to unmarshal json, error:", err)
return
}
log.Println(data)
for operation, expression := range data {
log.Print("Op:", operation, "Exp:", expression)
}
log.Println("Finished..")
}
/**
The sub definition for a specific expression in the object
*/
type ExpressionDefinition struct {
Vars []string `json:"var"`
Extra []string
}
func (e *ExpressionDefinition) UnmarshalJSON(data []byte) error {
tokens := make([]interface{}, 0)
err := json.Unmarshal(data, &tokens)
if err != nil {
return err
}
for _, token := range tokens {
log.Println("Processing token:", token, "type:", reflect.TypeOf(token))
switch token.(type) {
case map[string]interface{}:
for _, v := range token.(map[string]interface{})["var"].([]interface{}) {
e.Vars = append(e.Vars, v.(string))
}
case string:
e.Extra = append(e.Extra, token.(string))
}
}
log.Println(tokens)
return nil
}
/**
The main expressions object which contains all the sub-expressions.
*/
type MathExpressions map[string]ExpressionDefinition

Answer 2

Here the entire structure of the parsed json value will be stored in nested map[string]interface{}(json object) or []interface{}(json array) types.
In the line:
vals, ok := values[0].(mathExpVar)
values[0] would be of type map[string]interface{}, which cannot be asserted to mathExpVar, which is a struct, an entirely different datatype.
You need to type assert to map[string]interface{} first, then do this in each nested level as you go forward:
package main
import (
"encoding/json"
"fmt"
)
func main() {
type mathExpVar struct {
valueVar []string `json:"var"`
}
var mathExpJson map[string][]interface{}
var input = "{\"add\": [{\"var\": [\"100\"]}, \"200\"]}"
err := json.Unmarshal([]byte(input), &mathExpJson)
if err != nil {
fmt.Println("Error in unmarshalling")
}
for _, values := range mathExpJson {
var vals mathExpVar
valMap, ok := values[0].(map[string]interface{})
if ok {
varSlice, ok := valMap["var"].([]interface{})
if ok {
for _, v := range varSlice {
nv, ok := v.(string)
if ok {
vals.valueVar = append(vals.valueVar, nv)
} else {
fmt.Printf("%T\n", v)
}
}
} else {
fmt.Printf("%T\n", valMap["var"])
}
} else {
fmt.Printf("%T\n", values[0])
}
fmt.Printf("%+v\n", vals)
}
}
See: https://go.dev/play/p/Ot_9IZr4pwM
For more on interfaces and go reflection, check out: https://go.dev/blog/laws-of-reflection