How to unmarshal JSON into interface{} in Go?

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Define a struct type for the fixed part of the message with a json.RawMessage field to capture the variant part of the message. Define struct types for each of the variant types and decode to them based on the the command.

type Message struct {
  Cmd string `json:"cmd"`
  Data      json.RawMessage
}  

type CreateMessage struct {
    Conf map[string]int `json:"conf"`
    Info map[string]int `json:"info"`
}  

func main() {
    var m Message
    if err := json.Unmarshal(data, &m); err != nil {
        log.Fatal(err)
    }
    switch m.Cmd {
    case "create":
        var cm CreateMessage
        if err := json.Unmarshal([]byte(m.Data), &cm); err != nil {
            log.Fatal(err)
        }
        fmt.Println(m.Cmd, cm.Conf, cm.Info)
    default:
        log.Fatal("bad command")
    }
}

playground example

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