dynamic json

Apr 26, 2022

Golang really wants the json you parse to be of a known shape; but sometimes you get json of the form:

{ type: "email", data: { "subject": ..., "body": ...} }

Where you need to know the type before you know how to parse the data.

https://eagain.net/articles/go-dynamic-json/ is a good overview of how to handle that situation in go. The meat of it is to parse the top level once, then parse the rest based on the result:

type Envelope struct {
    Type string
    Data interface{}

type Email struct {
    Subject string
    Body    string

func main() {
    var msg json.RawMessage
    env := Envelope{
        Type: &msg,
    if err := json.Unmarshal([]byte(input), &env); err != nil {
    switch env.Type {
    case "email":
        var e Email
        if err := json.Unmarshal(msg, &e); err != nil {
        log.Fatalf("unknown message type: %q", env.Type)
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